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 of the unit. On both the inside and outside of the case, there are numerous devices that may not look so friendly, but allow the transmission to perform its job without you constantly having to control or command 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 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 Transmission
is any device that solely converts torque into speed and vis-versa. It is not designed to power the wheels directly. A Transaxle
is the similar to a transmission, but contains a final drive and differential which can then be connected directly to the drive wheels with a shaft or some other mechanical connection.
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. The style of pump used is dependent on the fluid flow and pressure requirements, the package size, and the location within the transmission.
4T65E Oil Pump and Housing.
Shaft Speed Sensors:
These are 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 tolerance, to determine if the transmission has shifted correctly and can maintain the gear position. If a clutch slips, these sensors will detect the slippage and may take appropriate action to prevent additional damage if possible. This can include placing the powertrain in limp-in mode to prevent severe damage to components. A diagnostic trouble code would usually be stored to help diagnose the failure. Many transmission codes are OEM dependent due to the proprietary design of the transmission and its parts, but there is a standard code set for all automatic transmissions within the United States.
Reluctor Type (VRS) Magnetic Speed Sensor.
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 mechanical action. Some servos 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 back side 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.
Servo Piston and Actuating Rod.
Bands are used to hold something stationary by affixing a band that is physically attached to one part (normally the transmission case) to the object to be held. They can be of various sizes and styles but their operation remains the same. These bands 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.
Friction Bands for Various Transmissions.
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 to a shaft as long as pressure is applied. The steel plates spline to a 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.
Clutch Disc (1), Steel Friction Surface (2), Return Spring/Wave Plate (3), Clutch Piston (4), Clutch Drum (5).
This is a shock absorbing device. Accumulators are round in shape and usually 1/2-2 inches in size. It's job is to cushion and delay the apply of a gearshift by allowing hydraulic pressure to press downward on it allowing the gearshift to feel more seamless or linear. It resists the oil apply pressure with spring pressure on the opposing side of the piston. The picture below shows an OE recoil spring, and an aftermarket replacement metal rod to make gear changes quicker and reduce gear slippage by not allowing any movement at all. 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 also aids in engaging the gear faster/harder during high load acceleration, and slower/softer when throttle position is lower.
Accumulator Piston, apply Spring, and aftermarket Rod.
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 repair 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 commonly available. These can turn a traditional econo-box transmission into a high-performance unit. There are also correction kits available that can correct deficiencies or annoyances for the average user during a rebuild situation. Valve bodies are very specific to the year and model of transmission you have. The overwhelming majority of valve bodies can only be replaced with the same part number as the original. A select few can be adapted to fit different transmissions. This usually requires intense modification to the valve body and/or the casing itself.
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 itself are precisely located to allow the correct amount of fluid to cross over at the right points. These gaskets are incredibly specific, as the architecture also tends to change many times during production runs. These gaskets can also be modified for performance or to correct a problem by over-boring, or choking a passage with a plug.
Typical Valve Body Gasket.
These can be normally open, normally closed, or duty cycle solenoids depending on whether the pressure is normally exhausted, or normally applied, or is pulse-width modulated. These typically control the clutches that apply gears, control main line oil pressure, transmission cooler oil 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 to control these devices.
Generic Shift/Pressure Control Solenoid Valve.
Check *****/Check Valves:
A simple metal/composite ball or spring loaded valve. There can be many of these located within many parts of the 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.
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 on hydraulic shift transmissions, and primitive electronically-shifted transmissions that still use mechanical pressure controls. Nearly all modern automatic transmissions use solenoids to duty cycle the line pressure, eliminating the need for this valve altogether.
General Motors Valve Body Mounted Throttle 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. The throttle valve will also apply oil pressure to oppose this valve to inhibit or shorten a shift depending on engine load or throttle position. Modern electronic-shift transmissions will use speed sensors and other sensor parameters to shift the car at the desired time.
A General Motors 200R Governor Valve.
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.
This is the center gear of the set, and provides the smallest gear within the gearset.
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.
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.
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.
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
Sun is Drive, Carrier is Driven, Ring is Held.
(74 + 31) / 31 = 3.38:1
Ring is Drive, Carrier is Driven, Sun is Held.
(74 + 31) / 74 = 1.42:1
Sun and Ring are Drive, Carrier is Forced Driven, no Held member required.
(31 + 105) / (31 + 105) = 1 or 1:1
Ring is Drive, Sun is Driven, Carrier is Held. (Reaction Carrier)
31 / 105 = 0.29:1
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.) To attain a usable 4th gear (0.29:1 is useless!), and any additional gear ratios, planetary 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 reversed rotation to the reaction carrier. 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 are powered at the same time, they bind up on the pinion gears and carrier, which would ordinarily destroy the unit. The reason it works in this situation is because there is no held member, so the two gears have nothing in the transmission to bind against. The pinions lock the sun and ring gears together, and the entire planetary gearset rotates at a 1:1 ratio. This will get more complex and situational because to get all the desirable functions out of an automatic transmission, such as engine braking, a minimum of two planetary gearsets (a compound gearset) must be used. Most domestic manufacturers use planetary gearsets because the input and output lay on the same plane. This makes the design simple and more 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 transmissions use an "L" shape configuration.
Most import transmissions/transaxles do not use planetary gearsets. These instead utilize 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. This style of clutch is also used on many of the planetary gear members as well. The only real difference between to two is the bands, and the planetary assemblies.
Hydraulic and Electronic Automatic Transmission Controls:
Now that the gearset portion is done, it's 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 itself. 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:
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. (Additional circuitry would obviously be needed to disable the previously applied gears, or else a lock-up occurs, but this is good enough graphical interpretation to understand automated shifting.) 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 converter lockup and a direct connection between the engine and transmission. This will normally feel like an additional gear shift in some cases, dropping engine speed by up to 500rpm. This lockup helps improve fuel economy on the highway to levels comparable to a manual. Once the brakes are pressed or the gas pedal is moved significantly, the converter clutch will normally disengage. Although this clutch is quite large, it cannot handle the full capacity of the engine's power for very long without damaging it.
In a true hydraulic schematic, auxiliary circuits will accompany the shift circuits to prevent certain problems (like gear clashing and timing), 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 in the valve body to prevent these conditions from occurring. These circuit designs are manufacturer dependent, as there is obviously more than one way to solve a problem like this. In many cases the transmission aftermarket will engineer new methods to improve the design, longevity, and performance 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). Gear clashes can be programmed out of the transmission simply by using software! This allows for a much simpler 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
Differential with integrated notches for a parking device.
The park position/differential lock is achieved by using a reluctor style cam wheel, and a parking pawl that engages the reluctor by using the manual valve rod. When park is selected, this rod pushes a lever down into the groove on the parking pawl which moves it and then locks it's tab 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. This device can only prevent vehicle movement so long as both wheels turn at the same speed while parked. If one wheel is off the ground, the vehicle WILL roll!!
Most vehicle owners with automatic transmissions neglect to use the parking brake for one reason or another. Yes, it IS a parking brake, is it not an emergency brake. Not using the parking brake places the load of the entire vehicle on the parking pawl, rod, differential and related bearings, engine mounts, axles, and suspension bushings and other related driveline parts. Parking uphill or downhill magnifies the chances for damage and deformation to occur on these parts. Many RWD automatic transmission owners often complain about excessive clunking when shifting out of park, or from reverse to drive. The added stress by not using the parking brake will cause wear which will manifest itself as slop in these situations. This is why the parking brake must be applied first to keep the vehicle stationary. After years of neglected use, the cables and other related parking brake parts (including the park brake lever or handle!) will seize up along with the parking shoes. In many cases the shoes may completely fall off the vehicle! This damage usually requires an additional 80-140 dollar cost to a standard brake job at a minimum to correct, not including any other non-wearable parking brake components (like cables) that may require service as well.
There are a number of modifications and transmission parts that are available to improve transmission performance for some units. 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 performance transmission build. In some cases they can be cheaper as well.
The type of transmission fluid used is a very important consideration, and especially so for automatic transmissions. 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 special-application fluids. The specifications for these mentioned fluids are not met by most, if any, "Universal ATF" brands. Pay close attention when you have your transmission fluid serviced, and ensure that manufacturer-approved fluids are used during service. Even with "Universal ATF" that may meet the requirements for your vehicle, most manufacturers will clearly state in the service and owner's manual that "Using anything but genuine fluid from the manufacturer may affect longevity, performance, and noise/vibration/harshness of the unit." (or NVH for short)
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. 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. Towing and excessive loads on the engine will significantly shorten the life of the fluid, and thus, the transmission itself. Most service schedules post a "Normal" and "Severe" service schedule. Normal service is a difficult quantity to define, but severe service is much easier.
Engine warm up of 15-20 seconds after the first start in average climates, moderate acceleration for short periods of time (less than 10 seconds), relatively consistent low throttle conditions. High sustained highway speeds without severe service limitations listed below.
Extended towing, uphill driving, taxi/delivery/ferry services, extended stop and go driving, excessive idling, excessive engine starting and stopping (heat plume effect), operation in extreme heat or cold, are all examples for severe service.
There will always be a great debate over "flushing", "exchanging", or "draining and refilling" a transmission during service. A "flush" is generally a fluid exchange that involves a "flushing agent" or solvents to aid in removal of debris or to "clean" the interior of the transmission. This service uses a specialized fluid machine hooked up to the transmission cooler lines and will generally use an on-board pump to pump new fluid and this "solvent" into the transmission. A fluid "exchange" service uses a similar type of machine. This machine is usually a passive machine that uses the transmission pump itself to expel the old oil, and force the same quantity of new oil into the transmission. Type type of machine will normally use a double acting piston. As the transmission pump moves 1 quart out, the piston in the machine will move, and force 1 quart of new oil into the sump. A "drain and fill" is a service where the drain plug is removed, and all fluid is drained from the unit. It is then refilled with the correct fluid, operated for a few minutes, and then the process is repeated. This process requires 2-4 cycles of this procedure to ensure the most complete fluid turnover to remove as much of the oil fluid from the transmission as possible.
In practice, my prime gripe being with the "flushing solvents" the same amount of used fluid can be removed. Its really about how much fluid do you want to pump in, and spit out. Even a drain and refill is guaranteed to waste new fluid. As a general rule for myself, I will use the entire system capacity as a fluid target. If you system is 8 quarts, drain and refill as many times as you can with 8 quarts, and end with a full system. With a "fluid exchange machine" it will turn over fluid as long as the car runs. This is much faster, and don't not damaged the system in any way.
Some machines are active exchangers, meaning they use a machine mounted pump to force fluid in. These machines should be avoided, because it is possible to hook the pressure side of the machine up to the sump, and overfill the unit to the point where the transmission is so full of fluid it pukes out of the dip stick, vent hole, transmission seals, etc. This type of servie will not harm the transmission internals, but can make a huge mess out of the seals and other service points.
Because of the ease of service, I use a passive exchanger. If you hook it up backwards, it wont move anything because the pump will fight itself trying to expel fluid from the return lines. This is why most passive exchange machines have a pinwheel or flow direction indicator. If you neglect it, you'll be standing around for hours wondering why nothing has happened.
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 as completely as possible. 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 be adequate for 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. If a DTC exists, always start with correcting it first along with basic checks.
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? Does the problem occur only under certain circumstances? Cold start? Only after 15 minutes of driving? Believe it or not, humidity? 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. You may start poking and prodding only to fix the problem, but you have no real clue as to what corrected it. This is not fixing the problem. In due time, most owners will come back for the same issue.
Step 2: Initial Inspection
This should be done after the initial test drive so that any complaint that does exist 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. Use probing questions to gain additional information. This step may 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 based on customer data. 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. The customer needs to be aware of this information as it may alter their willingness to repair the vehicle. DO NOT corner a client into a repair promising them the world. Machines and humans both make mistakes. Don't bet your reputation unless you can offer a guarantee!
Step 4: Teardown Inspection
At this point you should be able to determine if the failure is internal to the unit and thus require a complete teardown inspection of the unit to verify the diagnosis. This process will require a seal overhaul kit at the least to restore the unit to an operational state after the teardown. Depending on your test drive results, customer communication, and inspection results, this may warrant a total overhaul of the unit. At this step, 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 the end of this step, a point of failure should be identified and an estimate of the repairs to correct the problem can be created and discussed with the customer.
Step 5: Rebuild and Reassembly
If the customer agrees to have the unit rebuilt, you can now rebuild the unit to an operational state. This would involve repairing or replacing all of the required parts and also those requested by the customer to his/her order. In some rare instances, a teardown inspection result in a tow-away scenario. I can count a handful of cases where an insurance company would cover teardown and inspection but refuse to repair a unit due to it being modified from it's original condition for some reason. Vehicle history is absolutely essential for 3rd party warranty customers. Make sure they are aware of the limitations of their warranty, or have them discuss this with their guarantor.
If the unit does not require an overhaul, a reconditioning kit can be used to restore the state of the unit. This simply involves replacing all sealing components within the unit. In a few instances, this can correct the problem. However, it is very important to relay to your customer that this unit is not rebuilt. All wearable items within the transmission have not been serviced and thus do not qualify. Replacing a torque converter seal does not qualify as a rebuild simply because the unit was removed.
Step 6: Install and Test Drive
This would cover returning the vehicle to service, performing a test drive, break-in, or bed-in procedure based on the repairs completed. 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. They should also be made aware that the unit may shift differently as a result of seals being replaced, or the overhaul being completed. This will be an instantaneous change for the owner. Even if the unit is repaired correctly, some owners may complain because they have adjusted to an improperly operating transmission.
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 repair transmission 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 concerns. Most of these problems 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. Eventually the content must become complex.
Using Transmission or Powertrain PID's to Diagnose Problems:
The first thing any technician 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. The lazy gear can be felt during a basic test drive. 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 based on TSB's or service information.
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 and tolerances in which the gears will finally reside. As such, they must be broken-in in the new environment correctly.
This should also explain to those who are curious 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 known as bed-in. Most rolling element bearings use a very tight or interference-fit to the components they are installed on. This tight fit will relax as the part does work, and the pre-load on these parts will decrease as the bearing wears.
Use this information as a guide: New bearings require a bed-in when used with used parts. The bearings will require seating, and the working parts will need to adapt the the change in position that occurs with any bearing change. New gears, shafts, working parts, etc, will ALWAYS require a break-in procedure. As a result of this change, the working parts will mesh in slightly differing locations which will need to be optimized and require 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 post-installation.
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
Used Gear + New Bearing = Bed-in
Used 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 given the new clearances between them. This may discharge a lot of material initially as the two parts lap to each other, so a pre-mature fluid service is highly advised to remove this contamination. 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.
In a bed-in situation, the working parts have previously refinished themselves to one another. Replacing a used bearing with a new one on these parts should only require a short amount of time for the bearing to bed-in, and for the re-used parts to perform minor relapping to one another.