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Old 09-02-2007, 12:40 AM   #1
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Default Automatic Transmissions and Torque Converters Explained

Please add comments or suggestions. If there is anything you would like to see added/reworded, post or PM me. I update this thread on a regular basis.

One area that a lot of people seem to fear and mostly misunderstand, is the automatic transmission. I will explain the basic construction and operation of these mystery boxes to the general base of Honda-Tech. This is a lot of information to swallow at first, so beware. You may have to read it a few times AND ask questions.

Any basic automatic transmission contains many conventional parts and many which are design specific. (planetary gear type, helical gear type, CVT) A planetary gearset, conventional helical gears, or a variable pulley system can be used in various combinations to create all the required speed ranges for the transmission. I will focus primarily on a basic hydraulically-controlled transmission. More modern electronically-controlled transmissions are much easier to understand, as solenoids and sensors control virtually everything. This makes design and understanding of electronic-shift transmissions excessively simple.


The Torque Converter


The first area of any automatic to be discussed, is where power enters the transmission: the torque converter. This piece of equipment is what allows an automatic transmission to function without stalling the engine. The torque converter also provides additional torque input. This torque increase can range from 1:1 at full lockup to ~2:1 times the engines torque output. We are going to discuss how this power transfer occurs shortly.

Click the image to open in full size.

As you can see from the picture above, there are five major parts in a modern torque converter. They are from top to bottom, the impeller/cover assembly (1) (also called a pump), stator (2), turbine (3), torque converter clutch (4), and the TCC apply housing/rear cover (5). The stator component will be explained a bit later. For now we will focus on a very basic 2 element (impeller/turbine only) torque converter. If you can imagine two fans side by side, and one of the fans blowing air into the other, this is how a torque converter works, except it does this with an actual fluid medium (ATF) within a contained housing. The air pushed off the blades of one fan, will strike the blades of the fan next to it, powering that fan. This would also be analogous to a viscous coupling without the multiplied torque.

The impeller (first piece) is attached to the donut shape on the inside of the converter front cover and is driven by the engine. It is welded to the inside of the converter. The impeller is not shown in the picture above, but is identical to the turbine which is visible. The job this part has is to propel ATF by using centrifugal force from engine rotation to strike the turbine blades. The faster the impeller spins, the more fluid and force it applies.

The turbine is the other set of blades opposite of the impeller and is driven by the displaced oil from the impeller. This component splines to the transmission's input shaft, and operates similar to a slipping manual transmission clutch disc. The torque converter clutch is also affixed/splined to the turbine. Its function is to input power to the transmission during torque converter lockup.

The stator is the third and smallest piece. This component is is what allows the torque converter to multiply engine torque. It's basic job is to redirect fluid from the turbine exhaust back to the impeller intake. The stator is splined to a support shaft, item I in the picture below. It also has a one-way clutch (or a sprag) to keep it stationary during torque multiplication and free wheel during cruising.

Click the image to open in full size.


The flex plate (or drive plate) flexes during torque multiplication. This movement is the result of the impeller pushing oil on the turbine and the stator pushing the same oil back into the impeller. This causes the two parts to repel sightly, and the flex plate to flex in turn. It helps dampen the transfer of power by allowing this movement to occur. Vibration would result if this flex plate was too rigid, or if the pilot were to become seized.

Now how does this all function as a whole unit. The transmission's internal oil pump is always driven by the engine. Either through the torque converter tangs, (Red 6 in the torque converter picture) or through an auxiliary shaft. (which would be III in the picture if it were shaft driven) The shaft is not present on this one, but this is where it would be located. Pump oil will fill up the torque converter up and the fluid will be forced through the blades of the impeller due to centrifugal rotation of the converter. This fluid force is curved by the impeller blades to strike the blades of the turbine at a specific angle to provide sufficient power to turn the transmission. When you order a performance converter, these angles will change to better suit power transfer. This increases the stall speed of the converter, and allows an engine to be closer to a higher average peak torque range.

Now its time to go full circle. Once the oil forces the turbine to rotate it must exhaust this oil somewhere. Since the turbine is basically the opposite in design, it flows back inward towards the center of the unit. This is just opposite of the impeller, which brings oil in from the center and throws it outward. If the turbine were allowed to exhaust its oil into the center uncontrolled like this, it would create a huge turbulence inside along with vibrations and lots of heat. This is where the stator plays its part. It's sole job is to redirect the fluid from the turbine back into the impeller at the right angle which creates pressure. Any time pressure exists within the torque converter, additional torque is created. The stator is only functional when the impeller and turbine are not turning the same speed. The speed difference is what causes the turbulence in the first place. When the shafts are turning close to the same speed as one another, the stator freewheels as both blades are turning similar enough for the turbine fluid to enter the impeller again at the correct angle without the need for the stator redirect it.

Click the image to open in full size.


The tags in this picture are REVERSED, the turbine in this picture is the impeller, and the impeller in this picture is the turbine. This picture happens to illustrates fluid flow well.

This video should help visualize the construction and operation of a torque converter.


Now lets make more sense out of all of this fluid flow. There are two types of oil flow that occur in a torque converter: vortex flow and rotary flow.

Vortex Flow is the flow of fluid through the blades when the two blade speeds are different. Say the engine is turning 4500rpm (which is the speed of the impeller as well) and the transmission input shaft (turbine) is only turning 3000rpm. This difference causes turbulence and is where the stator does its job by redirecting the fluid back to the source. This multiplies engine torque by creating pressure which could be though of like a gear reduction but it happens before power actually gets into the transmission.

Rotary Flow is achieved when the two blades are turning at a similar speed to one another. At this point the job of the stator is done and is simply pushed along it's one-way clutch until a speed difference between the turbine and impeller occurs again. Rotary flow produces almost no torque multiplication because the engine and transmission are turning at near the same speed, which would indicate cruising speeds and light throttle. The overall goal of the torque converter is to achieve rotary flow, by using vortex flow to get there. Usually in each gear, there is about 500rpm variance from when you let off the pedal, to full throttle. The RPM will change but no gear change occurs. This is the torque converter attempting to make extra torque to get the turbine (and the transmission) up to the engine's current speed faster. Rotary flow achieves a near 1:1 ratio, where more speed is required and torque is not. Once the blades are of similar speeds, they can no longer work to produce extra torque since an object cannot turn faster with the same power applied to it than the source that supplied it without physical gearing. Even if it could, this would be counter productive since the impeller would lose its apply pressure, similar to starving an engine bearing of oil at high RPM. This concept does have a function though: To decelerate the vehicle after letting off the gas.

You can also think of the two flows in a gear type manner, Vortex being a reduction gear, like 2:1, and rotary being a direct drive gear like 1:1 or close to it. It is a change-off: If rotary flow is high, vortex is low. If vortex flow is high, rotary is low. One of the common misconceptions with automatics is that the gearing is significantly taller (and thus undesirable) than a typical manual transmission. But when the torque converter also acts as a gear reduction this makes complete sense.

A typical manual transmission has a 2.8-3.75 first gear. A typical automatic has a 2.25-2.75 first gear plus the 1.5-2.0 reduction in the converter. The ratios vary, but don't be fooled. The torque converter is never figured into the gearing on a spec sheet.


The torque converter clutch serves to provide the 1:1 ratio at steady speeds. This is essentially a wet clutch within the converter that is applied to eliminate any speed variances between the impeller and turbine. This provides a boost to fuel economy because waste heat is no longer generated. This makes the converter more or less a 35 pound flywheel. The converter clutch can be pulse-width controlled or simply turned on or anything in between. In order to apply the clutch, two methods can be employed. Either the oil behind the converter can be exhausted, or oil pressure can be applied to the front of the clutch. Either method will work but tends to be design dependent. This results in the clutch moving towards the rear cover, and pressing against it, locking the turbine to the converter housing.


Common Questions:

Why does my vehicle move itself while in gear without any brakes?

There is enough torque multiplication at idle to propel the car forward. If you have a heavy load or are towing, it may not creep at all or creep at a reduced rate. An excessive idle speed tends to cause creep as well, since the converter is now operating faster. The pitch of the stator blades influences creep and stall speed to a high degree. Most vehicles will creep more when they are cold. Very viscous fluids can also cause creeping to become more apparent.

Why do automatic transmissions get poorer fuel economy?

Automatic transmissions use more fuel because the impeller and turbine can never turn the same speed since they are not mechanically connected to one another. This results in wasted power due to the speed differences between the them. The lack of a 1:1 mechanical connection is the heart of the problem. This is where the torque converter clutch comes in. Most manufacturers today engage the converter clutch as early as second gear to maximize fuel economy. This will explain to a greater degree why automatic transmission equipped vehicles have a negligible impact of fuel economy. Even so, automatics strive for everything you want automatically: power, comfort, ease of use. With a manual you must tailor yourself to the transmission if you ever expect to get the mileage of any modern automatic transmission.

What does a torque converter clutch do?

This is the mechanical link used when rotary flow is high, to provide a direct 1:1 ratio for the best possible fuel mileage, without sacrificing acceleration and torque multiplication. Many people believe their automatic is a 5 speed, when in fact it is a 4 speed with a converter clutch. This can feel like a 5th gear shift when it locks. Simply pressing down the gas or the brake will usually disengage it, and engine rpm will jump up around 200-400rpm when it disengages. Looking at the picture above, the TCC solenoid will apply oil pressure between the turbine and the TCC face, pressing the clutch onto the machined surface in the apply housing. This locks the turbine to the converter housing making it turn at engine speed. If you would like, you can think of it as slipping 4th gear in a manual transmission, and then dumping the clutch to prevent any further slippage (or in this case, waste).

What is converter stall speed or stall RPM?

Stall speed is a basic test that stresses the torque converter and clutches in a transmission to determine if there is a problem with either of them. The test gets its name by preventing the turbine from rotating while at the same time rotating the impeller as fast as possible. The stall speed is also the point of maximum torque multiplication, and there is a specification every converter and engine arrangement by a manufacturer. If stall RPM is low, an engine output problem could be indicated. If it is too high, a clutch may be slipping, (if engine RPM continues to rise or the transmission shudders and doesn't move) or there may be too much engine power. Just like a clutch disc for a performance car, torque converters must be changed to suit the engines output and RPM range. A stall test produces enormous amounts of fluid heat, and can only be done safely for a handful of seconds. Testing all positions requires fluid change-over, where enough time has passed to ensure you are not overheating the converter oil.

What are some common problems with a torque converter?

Some common transmission problems can be caused by the torque converter itself. These problems can be hard to diagnose without the proper knowledge of how they operate. If a car has a hard time accelerating from a stop but will cruise normally, the stator one-way clutch has most likely failed. This means that the stator is free to rotate in both directions. If the one-way clutch seizes, meaning it physically cannot turn either direction, the vehicle will take off normally, but the top speed of the vehicle will be severely impaired. Usually this problem will not allow even the biggest vehicles to go any faster than 40mph. This can quickly destroy the transmission oil due to overheating, and possibly ruin the transmission. The torque converter's lockup clutch is the most common point of failure. The friction material used may degrade due to excessive heating and stop-and-go driving. This wears the material to the point where it can no longer hold the converter assembly at a 1:1 ratio. This can be felt like a clutch slipping, which produces a shudder most of the time. When this clutch deteriorates, it can damage the oil pump gears with clutch debris. If it cannot lockup correctly when commanded, it will also produce large amounts of heat even at cruising speed. This in turn, destroys most of the components within the transmission: Clutches, bearings, seals, etc.


Next is an in-depth explanation of a modern automatic transmission, its components, and how they all work to get you down the road.
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Old 09-02-2007, 12:43 AM   #2
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Default Re: Automatic Transmissions and Torque Converters Explained (slowcivic2k)

Click the image to open in full size. Click the image to open in full size.
General Motors 4T60E Transaxle. Honda M4RA FWD Automatic Transaxle.


The Automatic Transmission


The case of the transmission serves to house all the internal and external components. On both inside and the outside, there are numerous devices that may not look so friendly, but nonetheless allow the transmission to do its job without you constantly having to control it. The oil pan, side pan, servo covers (2 pictured), shaft speed sensors, the PRNDL (Gear Position) switch, cooler lines, and many other external and internal components specific to that transmission allow all of this to occur seamlessly. The above GM 4T60E transmission is designed for large engine FWD passenger cars. Space limitations of these particular vehicles require that the actual transmission assembly be placed behind the engine and connected with 2 gears and a chain to transfer power back to the actual transmission. Most import vehicles with smaller engines and transaxles can fit multiple shaft transaxles within the same plane as the engine itself. The layout places the differential towards the rear of the unit to connect axle shafts to the drive wheels.

For clarification, a vehicle Transmission is any device that solely converts torque into speed and vis-versa. It is not meant to power the wheels directly. A Transaxle is the similar to a transmission but contains a final drive and differential, which can power the wheels by connecting the transaxle directly to the wheels.

Click the image to open in full size.

The Parts


Oil Pump: The transmission oil pump provides hydraulic oil flow for all the circuits in the valve body: Apply oil for the clutches and bands in the transmission, oil for the torque converter, and lubrication oil. The pictured pump is a variable displacement vane pump. It has the ability to change its output based on engine load, by using a variable-pitch housing that can be controlled by a throttle valve, boost valve, or a solenoid valve if its electronically-shifted. The transmission oil pump is always driven directly by the engine. Some oil pumps use a Gerotor type design or some other architecture, similar to a Honda oil pump. Irregardless of the style used, they all fulfill the job at hand.

Click the image to open in full size.
4T65E Oil Pump and Housing.


Shaft Speed Sensors: Very similar to vehicle speed sensors in design and function, these sensors are used to determine if the incoming shaft RPM and outgoing shaft RPM are within a reasonable window, to determine if the transmission is operating correctly. If a clutch slips these sensors will detect the slippage, and take appropriate action to prevent damage if possible. This can include placing the powertrain in limp-in mode to prevent severe damage to components. A diagnostic trouble code would be stored to help diagnose the failure. Many transmission codes are OEM dependent due to the proprietary design of the transmission and its parts. There is however a standard code set for all automatic transmissions within the United States.

Click the image to open in full size.
Reluctor Type (VRS) Magnetic Speed Sensor.


Servos: These are almost only found on planetary type gearboxes. Honda does use servo shafts and a reverse gear fork to produce reverse gear in it's automatics. They are normally round in shape, and usually 1-3 inches in diameter with a pin installed to perform some action. Some are serviceable from the engine bay but can be found anywhere on or inside the transmission. The job of the servo is to convert hydraulic force into a mechanical action, by using a large round piston and apply rod to push some device (normally a band or fork) onto a planetary member, much like the way a brake wheel cylinder works. Fluid is placed on the backside of the piston, forcing the rod and piston to move and engage the device at a hydraulic advantage depending on the size and pressure of the fluid applied to it.

Click the image to open in full size.
Servo Piston and Actuating Rod.


Bands: Bands are used to hold something stationary by affixing a band that is physically attached to the case to the object to be held. They can be of various sizes and styles but their operation is basically the same. They are usually made out of steel lined with friction paper and other friction enhancing compounds. Common uses for bands is a forward gear band, and reverse band. These are found predominantly in transmissions using planetary gearsets.

Click the image to open in full size.
Friction Bands for Various Transmissions.


Clutch Packs/Drums: These are used to drive members of a planetary gearset. They can be used in conjunction with one-way clutches (or sprags), or other devices to complete this task. The assembly itself consist of a drum, which is affixed to the one end of the object to be driven, steel plates and clutches, and a piston in the back of the assembly to force the clutches into the steel plates. This engages a gear or a planetary member depending on design. The steel plates spline to the drum, and the clutches spline to a shaft, or planetary member. If the outside of the drum is precision machined, a band could also hold this member stationary for certain gears to make the overall assembly much smaller.

Click the image to open in full size.
Clutch Disc (1), Steel Friction Surface (2), Return Spring/Wave Plate (3), Clutch Piston (4), Clutch Drum (5).


Accumulators: This is a shock absorbing device. Accumulators are round in shape and usually 1/2-3 inches in size. It's job is to cushion the apply of a gearshift by allowing hydraulic pressure to press downward on it allowing the gearshift to feel more seamless. It resists the oil apply pressure with spring pressure on the opposing side of the piston. The picture above shows the stock spring, and an aftermarket replacement metal rod to make gear changes quicker and reduce gear slippage. In some designs the throttle valve or a solenoid valve assembly can feed oil to the spring side of this piston, opposing transmission main line pressure. This helps engage the gear faster/harder during high load acceleration, and slower/softer when throttle is lower.

Click the image to open in full size.
Accumulator Piston, apply Spring, and aftermarket Rod.


Valve Body: This is the complex hydraulic PCM of the transmission. A series of hydraulic circuits that allow transmission fluid to flow to all the various circuits required by the transmission. In a factory setting, the valve body should not be field-serviced only replaced as a complete unit. Aftermarket kits that upgrade the spool valves and springs and other components such as accumulators to enhance performance are available. These can turn a traditional econo-box transmission into a performance unit. These valve bodies can be updated many times during the course of a production run, and are very specific to the transmission.

Click the image to open in full size.
Valve Body from a GM Transmission.


Valve Body Gasket/Spacer Plate: These gaskets interface the valve body to the transmission case and to other auxiliary valve bodies. It is equipped with two sealing surfaces, and in some cases a steel spacer plate in between, making it look and behave like a Multi-Layer Steel head gasket. The passages within the gasket and the plate can be modified to enhance performance of the unit. These gaskets are incredibly specific, as the architecture also tends to change many times during production runs.

Click the image to open in full size.
Typical Valve Body Gasket.


Solenoids: These can be on/off or duty cycle solenoids that can both be either the normally open or normally closed type, depending on whether the pressure is normally exhausted, or normally applied. They can control the clutches that apply gears, control main line oil pressure, transmission cooler flow, etc. The Powertrain Control Module or Transmission Control Module if equipped uses engine and transmission sensors to control these solenoids. Throttle angle, manifold pressure, engine RPM, and vehicle speed are just a few that are used.

Click the image to open in full size.
Generic Shift/Pressure Control Solenoid Valve.


Check *****/Check Valves: A simple metal/composite ball or spring loaded valve. There can be many of these within a transmission. Their purpose is to momentarily delay or control fluid flow to a certain part of the valve body or other component or to block oil flow completely. Shift circuits normally have check ***** located within them for accumulator fill-up to prevent clutch pressure from being lost on apply and allow pressure to escape once a clutch is released.

Click the image to open in full size.
General Motors 4T60E Check Ball Locations.


Throttle (TV) Valve: This valve is controlled directly by accelerator pedal movement, and is used to determine shift timing and line pressure. It is traditionally operated with a separate cable attaching directly to the valve's lever which is then connected to the throttle body. A vacuum diaphragm called a modulator can also be used in place of a cable system to control the valve using manifold pressure. These are found only on hydraulic shift transmissions. Modern transmissions use solenoids to duty cycle the line pressure, eliminating the need for this valve.


Click the image to open in full size.
General Motors Valve Body Mounted Throttle Valve


Governor Valve: Also customarily used in hydraulically-shifted transmissions, this valve is responsible for shifting the car once the correct hydraulic pressure is produced. It accomplishes this by producing an oil pressure based on engine speed. Once the correct speed is reached, this pressure will act to shift gears at the right speed. Modern electronic-shift transmissions will use speed sensors and other parameters to shift the car at the desired time.

Click the image to open in full size.
A General Motors 200R Governor Valve.


The Gearset


Planetary gearsets are rather ingenious and are used more often than most might believe. There are at least 3 members to any planetary gearset, and 4 component parts.

Sun Gear: This is the center gear of the set, and provides the smallest gear within the gearset.

Carrier: The carrier provides the largest numerical gear in the gearset, and we will discuss how in a minute. The job of the carrier is to provide a mounting point for the pinion gears to rotate on. It is the second member of the gearset.

Planets/Pinion Gears: The planet gears simply interface the last member, the ring gear, to the sun gear. These pinion gears will rotate along the two other gears, rotate stationary, or hold the sun and ring gear as one unit.

Ring Gear: Sometimes called an annulus or internal gear. This is the last member of the gearset. It's numerical gear ratio puts it between the sun gear, and the carrier.

Click the image to open in full size.


Solving Planetary Gear Ratios


This can sound a bit confusing to perform at first but its not very hard to solve the ratios. With some practice and help from here it will make solving basic planetary gear ratios very simple. The carrier's gear ratio is the only ratio that we cannot determine by counting its teeth because the carrier itself has no teeth to count. But we can determine the ratio based on the other two gears which have teeth that can be counted. In this case, we will use this gearset for example:

Sun: 31 Teeth
Ring: 74 Teeth
Carrier: S + R = C

From here we now have enough information to calculate the ratios for the gearset. First we must understand how the planetary gearset transmits power at reduction-drive, direct-drive, and over-drive. The three members must occupy one state each. The three states being Drive, Driven, and Held. If we did not hold a planetary member, the power would simply rotate on the pinions in the opposite direction resulting in no output. Therefore, all planetary gearsets must have a input gear (Drive), a reaction gear (Held), and an output gear (Driven). The pinions are a critical component in allowing this action to take place without breaking the whole gearset during shifts.

Figuring the gear ratios are the same as for any manual transmission:

Driven / Drive = Ratio

1st Gear: Sun is Drive, Carrier is Driven, Ring is Held.

(74 + 31) / 31 = 3.38:1

2nd Gear: Ring is Drive, Carrier is Driven, Sun is Held.

(74 + 31) / 74 = 1.42:1

3rd Gear: Sun and Ring are Drive, Carrier is Forced Driven, no Held member required.

(31 + 105) / (31 + 105) = 1 or 1:1

4th Gear: Ring is Drive, Sun is Driven, Carrier is Held. (Reaction Carrier)

31 / 105 = 0.29:1

Reverse Gear: Sun is Drive, Ring is Driven, Carrier is Held. (Input Carrier)

105 / (-31) = -3.38


As you can see here, there are many ratios available out of this single gearset. But there are some considerations that must be taken to calculate the gears. You will notice I have inserted a negative into the reverse equation above. Holding the pinion carrier will produce the opposite ratio because the pinions are held stationary. Because of this, if the ring gear rotates clockwise with the pinion carrier held, the only way the sun gear can turn is opposite. Imagine a gear in your hand that spins, the top goes left, so the bottom must go right.

As you look at 4th, we also run into another situation. You may have noticed that I specified two different carriers, an input and a reaction. From my first statement you would imagine 4th gear would actually be overdrive in reverse! (If you put the negative in there like I did for reverse.) Automatics use either a compound (two combined planetary gearsets) or Ravigneux gearsets (two sun gears, two sets of pinions on a common carrier and one ring gear). To get 4th gear in most planetary automatics, the input sun gear is held stationary with a band/clutch and a one way clutch, and the input carrier is the drive member, and the reaction carrier is the driven member. Reverse is always achieved by holding the input carrier stationary, this allows the sun and ring gears to rotate opposite of each other, and produce reverse. The compound arrangement is the most common to see, and easiest to digest.

3rd gear is attained by driving any two members of the gearset. When this happens the sun gear spins slower because of its smaller tooth count while the larger ring spins faster because of its higher tooth count. When the two engage at the same time, they bind up on the pinion gears and carrier. The reason this works is due to not having a held member, so the two gears have nothing in the transmission to bind against. The pinions force the entire planetary gearset to rotate at a 1:1 ratio. This will only get more complex because to get all the desirable functions out of an automatic transmission, such as engine braking, two planetary gearsets (a compound gearset) must be used. This makes things a bit more complicated, and will make understanding a bit harder, because the design changes with every transmission. Most domestic manufacturers use planetary gearsets because the input and output lay on the same plane. This makes the design very simple and compact. Due to the larger transverse mounted engines used in front wheel drive domestic cars, a transfer assembly (usually 2 gears and a chain) is used to move power from the torque converter, to the actual transmission. This is why most domestic automatic transmission have an "L" shape configuration.

Click the image to open in full size.

Most import transmissions/transaxles do not use planetary gearsets, but instead use a set of standard helical gears as in a manual transmission/transaxle. In place of the synchronizer hub, slider, ring, and spring, is a clutch drum that contains friction clutches and steels that spline to the shaft and gear respectfully. These are very simple to rebuild, and truly take no more additional time versus a manual transmission to overhaul.


Hydraulic and/or Electronic Controls


Now that the gearset portion is done, it will be time to explain how a transmission actually shifts gears automatically. All automatic transmissions are controlled with two basic methodologies: Hydraulically-controlled shift or Electronically-controlled shift. In either case they shift gears in much the same manner but use different components to control the shift. The following diagram is for demonstration purposes only, and is a great over-simplification of how a valve body would really function. Refer to the picture below:


Click the image to open in full size.


This is the most basic hydraulic transmission, and it has 5 forward speeds, one reverse, and a TCC (torque converter clutch) in 4th and 5th gears. The 4 valves inline with each other from top to bottom are the 1-2, 2-3, 3-4, 4-5 shift valves. The shifter moves the manual valve, and uncovers oil passages to the desired position. (what you see on the console) It does this by moving the manual spool valve when the gearshift is moved, and allows oil pressure through the specified point. The manual spool valve is staggered so that it cannot be in two positions at once. For example, in reverse, the shifter moves one spot to the left, and uncovers the passage from the oil pump, and to the reverse bands/clutches. But when OD is selected, the spool valve moves two more clicks, past neutral and into overdrive, and as it moves, it covers up the reverse passages, and uncovers the OD passages. For the transmission to shift 1-5 automatically, we need devices to tell it when to shift, since it is not electronic. In this case I chose to use a Throttle Valve, and Governor Valve.

The Governor valve in this case is centrifugal valve that is driven like a speedometer gear, and has weights attached to it that fly out as it goes faster, and allows more oil pump pressure to pass through it. As speed increases, so does this oil pressure. Some types of governors operate like small oil pumps, and achieve the same result. The TV valve is a throttle position valve, plain and simple. As you step on the gas, this valve moves and helps delay the shift (as acceleration is desired, so higher RPM is needed). By stepping on the pedal, the valve moves, and increases oil pressure in this track. This pressure is fed to the back side of each shift valve, and assists the pink springs in keeping the respective valve closed, until governor (speed) pressure is too great, and the valve moves to the open position, allowing the oil pump to apply the next clutch.

Each of the shift valves have a pink spring that is calibrated to a certain pressure. In this case, we will say that the governor valve makes 1psi of pressure for every mph of car speed, and the 1-2 shift valve spring is calibrated to 12psi. In first gear, with little throttle pressure, the shift comes sooner, at around 12mph because the TV valve is not helping the spring, so the minute the governor valve gets up to 13psi, it can overcome the 12psi spring, and move the shift valve to shift to the next gear. If you mash the pedal in 1st gear, it will rev out to its max, and around 40 or so mph, it will shift the car. It shifts late because the TV valve is providing its maximum 28psi of pressure to help the 12psi spring, resulting in 40psi of closing force. So the minute the governor valve reaches 41psi (40mph) it will shift.

Each shift valve stays open to apply oil pump (mainline) pressure to the next shift valve when it opens. When the car shifts to 4th and 5th gears, oil pressure is also routed to the TCC (or PWM) solenoid, allowing the TCC to apply when the solenoid opens. (or closes) This allows a direct connection to the engine, and will feel like a 6th gear shift in some cases, dropping engine speed by up to 500rpm. This helps improve fuel economy on the highway to levels comparable to a manual. The minute the brakes are pressed or the gas pedal is moved significantly, the converter clutch will disengage as it alone cannot handle the full load of the engine.

In a true hydraulic schematic, auxiliary circuits will accompany the shift circuits to prevent certain problems, and to enhance performance. A kick-down circuit could be used to prevent the transmission from hunting between two gears when conditions are right for both gears. For example, say you keep your speed at a constant 40mph. If your 2-3 shift point is 40mph, the car would in theory shift from 2 to 3 and from 3 to 2 every time you pass over or under 40mph. This is obviously annoying, and hydraulic circuits exist to prevent these conditions from causing problems. These circuit designs are manufacturer dependent, as there is obviously more than one way to skin a cat. In many cases the aftermarket will engineer methods to improve the design to increase longevity of the unit.

In an electronic-shift transmission, hydraulically controlled throttle pressure and governor pressure are eliminated entirely. Electronic solenoid valves are used to create throttle pressure, governor pressure (or its equivalent), and to shift gears. These transmissions will have a TCM that determines shift timing, based on vehicle speed, engine load, and any number of additional inputs (MAP or MAF input, CTS, TPS, RPM, etc). This simplifies valve body design, and diagnostics can be greatly improved with the addition is sensors and solenoids that can be tested with a scan device or other external connections.


Parking, Maintenance, and Options


Click the image to open in full size.
Differential with integrated notches for a parking device.

The park position/differential lock is achieved by using a reluctor style wheel and a parking pawl that is engaged by the manual valve rod. When park is selected, the rod pushes a lever down into the groove on the pawl which locks it into the reluctor wheel. This prevents the reaction/differential side of the transmission from turning. This is standard equipment on most automatic transmissions. This prevents the vehicle from rolling if the parking brake failed to engage/hold the vehicle stationary.

Most vehicle owners with automatics refuse to use the parking brake for one reason or another. Yes, it IS a parking brake, not an emergency brake. Not using the parking brake places the load of the entire vehicle on the pawl, rod, differential and related bearings, engine mounts, axles, and suspension. Parking uphill or downhill only magnifies the chances for damage to occur. This is why the parking brake should be applied first to keep the vehicle stationary. After years of neglect these cables will seize up along with the parking shoes and related parts. Sometimes the shoes may completely fall off the car. This usually provides an additional 80-140 dollars to a standard brake job, not including any other hardware or parts that may require service.

There are a number of modifications that can be made to improve transmission performance. High friction-coefficient clutches, high stall converters, modified valve bodies/accumulators, wider bands and more powerful servos. Completely redesigned gearsets can also be found for some transmissions. These performance upgrades are in many instances comparable in price to a manual transmission build.

Fluid usage is very important in any transmission, and especially so for automatic transmisions. Using the wrong type of fluid, too much or too little, can produce devastating effects on the longevity or performance of a transmission. Fluids such as GM Dexron VI, Ford type F, Ford Mercon V and SP, and most CVT transmission fluids are specialty oils. The specifications for these mentioned fluids are not met with most "Universal ATF" brands. Pay close attention when having your transmission serviced and ensure manufacturer-approved fluids are used during service. Even with "Universal ATF" that may meet the requirements for your vehicle, most manufacturers state in the service manual that using anything but genuine fluid from your particular manufacturer may affect longevity, performance, and noise/vibration/harshness. (or NVH for short)


Click the image to open in full size.
Used, and New Automatic Transmission Fluid (ATF).

The condition of the fluid is also vital to its long-lasting operation. ATF should not change in color with normal use unless otherwise specified. Therefore a service interval is generally defined by most manufacturers to maintain the quality of the oil and remove contamination. This fluid must be able to lubricate and also allow sufficient friction under pressure so that clutches and bands work properly. A change in color can indicate contamination from a variety different sources. A failing clutch assembly, busted radiator cooler (allows ATF and engine coolant to mix), or oxidized fluid are just a few modes of contamination. The scent of the oil can also indicate to some level neglect, burned clutches, or other contamination.

There will always be the great debate over flushing or draining and refilling a transmission during service. A "flush" is generally a fluid exchange. Sometimes a "flushing agent" is used to help remove debris or "clean" the interior of the transmission. This service generally hooks a machine to the transmission cooler and pumps new fluid as the transmission's oil pump expels old fluid into a waste container. I professionally only perform a fluid exchange without any additives as this only uses the specified oil and nothing else that could potentially damage the system. Some technicians claim that damage may occur during a flush service. This is a simply untrue, as only improper procedure/fluid or defective equipment could have the potential to cause any damage at all. Any other problems would relate to a pre-existing transmission problem that requires correction.


Click the image to open in full size.
Transmission Cooler kit for a Jeep Cherokee

Automatic transmissions normally have a cooling unit specifically for the transmission. Even with the stator eliminating most of the turbulence within the torque converter, the torque converter and transmission still produce a great deal of heat. This loss is caused by the lack of a true mechanical connection from the engine to the transmission and losses due to friction. These effects are increased with towing and stalling (pressing the gas while in gear and not moving, or heavy loads). Sometimes the torque converter can get so hot that the outside of it turns blue, indicating severe overheating.

Temperatures can range from 140F to 450F, depending on the cooling system and construction of the transmission. ATF, like any other oil, will thin with increases in temperature. Over the fluid change interval the fluid may fall out of grade if the transmission is subjected to extreme service. In most cases with towing or other severe duty, the manufacturer may recommend fluid services sooner to decrease the likelihood of damage due to poor oil viscosity.


Diagnosis and Reconditioning


All mechanical parts have a defined service life. I have seen even the worst transmissions put on over 200,000 miles without incident, and the best units fail before they even approach 80,000. Depending on design tolerances, build quality, and driving habits, your particular unit may experience a longer or shorter lifespan accordingly. Before we go bashing on certain transaxles, it is very important to understand the design and intended function. Without understanding these aspects you may personally feel like you have a junk transmission. When in reality the transmission was designed for a purpose other than what you actually use it for.

For instance, an economy transaxle regardless if it is a manual or automatic will behave a certain way. The feel of the unit may feel positive or negative to you. This feeling is relative and will change for every individual. I would certainly not pay $40,000 for a Cadillac that bump-shifted or was otherwise erratic to me. This type of shifting may be deemed acceptable in a compact car but certainly not for a higher-end luxury vehicle. This does not mean the design is faulty, it just means it does not fit your personal expectations. Rebuilding enough transmissions like I have and you will learn many tricks to increase your shift feel, reliability, and predictability to levels closer to those you desire. This is very important in the transmission industry to prevent what would otherwise be a "useless comeback": A customer that desires a Cadillac-style shift feel that is not attainable in a Toyota Corolla. There are limitations to every implemented design: A Honda M4RA will never shift as good as a 4L85E GM unit.

Does this mean all is lost? Of course not. But you must plan your repairs and modifications accordingly to realistically achieve your goals. For instance, every automatic transmission I rebuild is audited. This includes talking to the primary driver and gauging his/her intent and inspecting the unit using a test drive or teardown inspection to determine the best course of action. Using this information can help adjust your rebuild procedure to fit the needs of the owner. I have also rebuilt many units with OEM parts specifically by request, only to get complaints months later about poor shift quality or shudder. This is why communication to your customer or yourself is paramount to any successful rebuild. Not all OEM parts are bad and in many cases they are superior in fit and function. However these parts may not fit the driver.

Diagnosis begins with understanding the problem and what is causing it. A detailed account of the events leading up to the failure are critical to help easily identify the problem be it internal or external to the transmission, or something else entirely. A complete vehicle ownership and maintenance history is ideal. This helps determine if a rebuild or any other repairs were made to the unit prior to you diagnosing it. Diagnostic trouble codes are also incredibly helpful in diagnosing pattern-failures and anything else for that matter.

Step 1: The Test Drive

Driving the vehicle to gauge the severity and extent of the problem is essential. This should allow you to feel what the driver is feeling to some degree. Does it miss a gear completely? Does it shift or move at all? Does the torque converter clutch lock up when commanded? Is there a DTC stored? All of these user-level identifiers must be checked before anything else can be done to diagnose or correct the problem.

Step 2: Initial Inspection

This should be done after the test drive so that any complaint that does exists is unmodified by you. Is the unit full of the correct fluid? Does the unit leak fluid? If so, from where? Now would be the correct time to question the customer regarding the problem's history and when it started. This step will also help you identify and isolate engine related problems from transmission problems. After the test drive you can use the previous data to check clutch pressure points, main line pressure, and others if they are available. This would also be a good time to reference any DTC's, service bulletins, and pattern failures that may be available for the application in question.

Step 3: Basic Corrections

If during step two you noticed the fluid was low or that the engine had a bad engine mount, correct these deficiencies and perform a second test drive. In some cases simple fixes like this will remedy the concern to the customer. However, it is vital that you relay this information to the owner. A chronically low fluid level can manifest itself as damage internal to the unit that cannot be seen, and still require premature future service.

Step 4: Teardown Inspection

At this point you should be able to determine if the failure is internal to the unit and thus requires a complete teardown inspection of the unit to diagnose the issue. This usually entails a seal overhaul kit at the least, and depending on your test drive, customer communication, and inspection results may require a total overhaul of the unit. At this phase the unit is completely torn down and all bearings, clutches, seals, bolts and other hard transmission parts are scrutinized for the root of the problem. At this point a detailed estimate and point of failure can be delivered and discussed with the customer.

Step 5: Rebuild and Reassembly

If the customer agrees to have the unit rebuilt, now would be the time to rebuild the unit to an operational state. This would involve repairing or replacing all of the parts requested by the customer to his/her order. Rarely does a teardown inspection result in a tow-away scenario, but I can count a handful of cases where an insurance company would cover inspection but refuse to repair a unit due to it being modified from it's original condition.

A recondition involves gaskets and seals only. A rebuild consists of clutches, steel plates (if required), torque converter, external cooler and filter assembly, and hard parts such as bearings, gears, or valve body parts. At this point it should be ready for reinstallation. There are very few instances where a torque converter should be reused.

Step 6: Install and Test Drive

This would cover returning the vehicle to service, and a test drive, break-in, or bed-in period to follow that. All rebuilt parts require a bed-in or break-in period depending on what was replaced or repaired in Step 5. After final checks for leaks, clutch pressures if required, and fluid level, it should be returned to the owner.


These steps should be referenced regardless if you are a shop or an individual owner looking to rebuild yourself. Technical information regarding rebuilding that I have acquired over the years will follow for all of those interested in taking on a project like this.

For many technicians, rebuilding a transmission is a rare event or an everyday job. The vast majority of technicians do not repairs problems much beyond unit level, with a few exceptions. In most cases this is because of a lack of tooling, shop space, expertise, and shop liability. Most of these are present in most automotive shops today in some quantity. I will share my experience in the field so that others may learn from my past failures and experiences. Most of these techniques are very technical in nature, and will only be explained from that point of view. If you do not understand this information or how to apply it, consult a local professional.

Using Transmission or Powertrain PID's to Diagnose Problems

The first thing any tech should perform is a DTC and PID inspection during the test drive. This can help you determine clutch wear or delay between shifts. In most rebuilds I perform a gear shift malfunction is detectable before the PCM will trigger a slippage DTC for the indicated gear. Some transmission malfunctions are totally unrelated to the transmission itself. DTC's such as the CTS or TAC (Throttle Angle Control), may prohibit the transmission from operating as designed. Each code must be referenced for a diagnostic procedure. The technician should be able to tell you if the DTC in question may cause a shifting problem or not.

Understanding Bed-in and Break-in Differences

A bed-in and break-in procedure are similar but to different levels of magnitude. Anytime load-bearing or working parts are installed, a break-in is required to lap the new components to each other. Even though many parts are lapped or matched from the factory, this is no substitute for the environment in which the gears will finally reside. As such, they must be broken-in in the new environment.

This also explains why re-used bearings require much less pre-load versus a new set of bearings: New bearings have not worked in any environment and require what is know as bed-in. Use this information as a guide: New bearings require a bed-in (no working parts, only bearing parts), New gears, shafts, working parts, etc require a break-in procedure. Anytime a bearing is replaced on a shaft, a bed-in procedure is required. As a result, the working parts will mesh in slightly differing locations which will need to be optimized and require only a short amount of operational time to establish a "broken-in" condition. Even if you replace a single gear, like 2nd or 5th from another transmission, a break-in is required to seat the "new" gears into their "new" environment. Failing to do so may result in premature gear wear or galling, scoring, or other damage which will become permanent on the gears which is irreversible.

If I was to formulate it in order of severity, from most ideal to least:

New Gear + New Bearing = Break-in

New Gear + Used Bearing = Break-in

Same Gear + New Bearing = Bed-in

Same Gear + Used Bearing = Bed-in

Another Used Gear + New Bearing = Break-in

Another Used Gear + Used Bearing = Break-in

So what the heck are the "technical" differences between Bed-in and Break-in? Simply put, a break-in requires a low load operating environment so that the replacement working parts have a chance to refinish themselves to one another. This discharges a lot more material initially as the two parts lap to each other, so a pre-mature fluid service is highly advised. Using a magnetic drain plug allows the technician to determine what parts are wearing, and if the particles that are seen are normal or abnormal.
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Old 09-02-2007, 01:01 PM   #3
 
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Default Re: Automatic Transmissions and Torque Converters Explained (slowcivic2k)

Wow, that is a great post. Thank you for this great explanation of Automatic Transmissions. I currently drive a '97 Civic with an Automatic transmission. In my Factory Service Manual it says that my "automatic transmission is a 3-element torque converter and a dual-shaft electronically controlled unit which provides 4 speeds forward and 1 reverse." Now what I would really like is to make a PCM that I can control. A switch would control whether it uses the OEM programming or my own programming. And then I would like to electronically control when it switchs with two switches on my steering wheel.... But I guess, now I'm just day dreaming.
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Old 09-02-2007, 01:19 PM   #4
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Default Re: Automatic Transmissions and Torque Converters Explained (cabofixe)

Yep, has the turbine stator and inpeller like it should, and 4 forward speeds, attained by toggling the A/B shift solenoids on and off.

This company makes one for diagnostic purposes. It's basic job allows you to hook it up to the transmission, and shift the car manually with a button without PCM control, just like your day dream. Wake up, its already here. Click the image to open in full size. The Schaffer Shifter.

http://www.schaffertest.com/whyuse.htm
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Old 09-02-2007, 02:05 PM   #5
 
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Default Re: Automatic Transmissions and Torque Converters Explained (slowcivic2k)

Thanks for the link! I have tried asking people if anyone could do it and they treat me like I'm an idiot. Thanks!
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Old 09-02-2007, 02:24 PM   #6
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Default Re: Automatic Transmissions and Torque Converters Explained (cabofixe)

nice post Click the image to open in full size. this should be in the Transmission Topic Index/GOOD LINKS/ETC. sticky
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Old 09-02-2007, 06:36 PM   #7
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Default Re: Automatic Transmissions and Torque Converters Explained (Throwdown)

Nice post man. Pretty good information, but for the most part, Honda automatic transmissions do not have planetary gears.
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Old 09-02-2007, 08:45 PM   #8
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Default Re: Automatic Transmissions and Torque Converters Explained (Blown90hatcH)

Yeah, great post, lots of good info. A good way I read to describe autos is a desk fan blowing into another one.

I am not really scared of autos because I don't know how they work, it is just because they are less efficient, and more complexity pretty much always guarantees more failures. Semi trucks use meshed gears for a reason. Click the image to open in full size.
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Old 09-03-2007, 04:14 PM   #9
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Default Re: Automatic Transmissions and Torque Converters Explained (Blown90hatcH)

Quote:
Originally Posted by Blown90hatcH
Nice post man. Pretty good information, but for the most part, Honda automatic transmissions do not have planetary gears.
That was part of the second post, that Honda's (and most imports for that matter) use a set of standard helical gears, instead of a planetary gearset, and the clutches in such a transmission can be thought of as an automatic synchronizer. (As the drum (or hub) in most cases affixes to the gear, which contains the steels and clutches)
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Old 10-21-2007, 08:59 PM   #10
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Default Re: Automatic Transmissions and Torque Converters Explained (slowcivic2k)

It's a lot to take in at once..I'm not gonna lie :-D.

I'll add it to the FAQ too.
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Old 10-21-2007, 11:43 PM   #11
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Excellent...Thanks for the great write up, it's a good review for me.
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Old 10-22-2007, 01:37 AM   #12
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good info!
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Old 07-26-2008, 05:23 PM   #13
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Default Re: (vteczone)

wow great info!
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Old 07-27-2008, 10:21 PM   #14
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Glad to see people still read it. Thanks.
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Old 07-28-2008, 10:05 AM   #15
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Default Re: (slowcivic2k)

thanks for this post.. the torque converter in particular has always been a box of mysteries to me.. I figured it was something like a centrifugal clutch on a go cart, but turns out thats not really the case. good info.
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Old 07-31-2008, 08:27 PM   #16
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Glad to be of help, I've modified the posts and added more info and clarity.
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Old 08-08-2008, 07:23 AM   #17
 
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WOW! Thats a lot more info than when i study, I will read it to see if i can find why my 92 auto civic shifts by itself at 5k rpms from 2 to 3 even when i have the stick in second.
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Old 08-21-2008, 07:24 PM   #18
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Thats an odd problem. Was it ever apart before?
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Old 09-03-2008, 03:13 AM   #19
 
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Default Re: Automatic Transmissions and Torque Converters Explained (slowcivic2k)

Thats great man, only thing I feel it could use is a list of Honda Trannys and USDM models with shift functions ie. Electronic, Hydraulic. And Converter types for ease of diagnosics! Thanks Bro for the hard work!
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Old 09-04-2008, 04:49 PM   #20
 
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Default Re: Automatic Transmissions and Torque Converters Explained (BlazinSlowHonda)

Are asking about my tranny problem? 2 to 3 at 5k rpm's? Sorry but, what did you mean with your question; "Was it ever apart before?". I don't have a perfect english!!! Click the image to open in full size.
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Old 09-04-2008, 09:42 PM   #21
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Default Re: Automatic Transmissions and Torque Converters Explained (PRCIVIC)

Good thread.

Automatic transmissions are an enemy of mine!!! I've had to rebuild a couple out of 05+ Odysseys due to insurance policies(wouldn't pay for replacement, only rebuilding)...not tooo bad, but Hondas are definitely not as easy as a GM TH350 Click the image to open in full size.


It's always fun testing clutch packs though Click the image to open in full size.

A lot of Honda auto issues can be fixed by cleaning out the valve bodies and screens.

Oh yeah, Honda autos suck badly for the most part and cannot handle much power- someone needs to design hi-po clutch packs Click the image to open in full size.
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Old 09-03-2009, 07:55 AM   #22
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Default Re: Automatic Transmissions and Torque Converters Explained

What fluid do you use to fill the torque converter??
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Old 09-04-2009, 06:56 AM   #23
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Default Re: Automatic Transmissions and Torque Converters Explained

Very nice write up. I never thought someone would do a whole research on auto transmission. Auto car have no fun driving. = (
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Old 09-02-2012, 01:29 AM   #24
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Default Re: Automatic Transmissions and Torque Converters Explained

Updated to add information, I'll bring CVT's in here or another post?
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Old 09-03-2012, 01:34 AM   #25
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Default Re: Automatic Transmissions and Torque Converters Explained

Might I ask about partial TC lockup in 2-3-4 then full TC lockup above 50 mph in a CRV tranny.

My new to me CRV needed a rebuild, the reman. The TC acted like a race/high stall converter until it got hot as started BANGING in low gear. Once the 1-2 shift was over and partial TC lockup, it drove like normal.

The CRV acted like an old Buick TH400 with a switch pitch converter, 3200-3500 stall in low gear but 2400 rpm stall in 2-4 until 50 mpg when full TC lockup occurred.
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