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.
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.
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.
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 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.
Servo Piston and Actuating Rod.
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.
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 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.
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 1/2-3 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.
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 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.
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.
Typical Valve Body Gasket.
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.
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.
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.
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. Modern electronic-shift transmissions will use speed sensors and other 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.) 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.
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:
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
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)
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.
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.