Diagnosing & Servicing Automatic Transmissions
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Diagnosing & Servicing Automatic Transmissions

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To prevent having any service from being over sold or under sold, it is important to not only know that a particular service should be performed, but also why it should be performed. That knowledge can also help you spot when to recommend that service, and when not to. When speaking about transmission fluid, the short version of the explanation is that it gets dirty and wears out.

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In a typical automatic transmission, the teeth of the gears are in full contact at all times. These gear sets are commonly made up of a ring gear, a sun gear and planetary gears. They provide the various gear ratios by stopping one gear from turning, applying engine torque to a second gear, then sending the power from the remaining gear to the wheels. (Figure 1) In Figure 1, number one is the ring gear, number two are the planetary gears, and number three is the sun gear. From this one gear set, multiple speeds are achieved by holding any one gear, applying engine torque to another, and linking the 3rd with the output shaft. “Direct” speed, often times also 3rd gear, is achieved by locking the entire assembly together and rotating all gears as one solid unit.

In order to be able lock and unlock these different spinning gears, clutches and bands are needed. Clutches are used to link and separate the spinning gears with the input and output shafts. Bands are used as a type of brake to prevent a gear from rotating.

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It is within these gear clusters, clutches and bands that the bulk of the wear and contamination occurs to the fluid. As the friction material of a clutch or band begins to apply, there is a brief slipping period. It is during that slipping period that heat is generated that cooks the fluid, friction material wears off and enters the fluid, and the fluid’s molecules are being sheared. The gears also heat the fluid and grind at the fluid’s molecules, as well as contaminate it with metal.

Eventually, the fluid is so ground up, cooked and contaminated that it should be replaced with fresh fluid. It is important for the fluid to be in good condition because it serves to lubricate the components.

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Not only do the gears require adequate lubrication, but so do the clutches and bands. To produce a comfortable and smooth transition that the driver associates with a healthy transmission, the manufacturer will prolong the clutch and band slipping period as needed. During this period of time, the transmission could be said to be running in two speeds at once. The lubricating ability of the fluid is tested the hardest during that time. The fluid becomes an integral part of the friction material’s ability smoothly glide into a holding position. If the fluid can not adequately lubricate during this slipping period, the friction material can rapidly and repeatedly produce a grab-slip-grab-slip effect that is felt as a shudder. The most popular clutch in the transmission to produce this shudder, is not technically in the transmission, but rather in the torque converter. The torque converter contains a clutch that is appropriately referred to as the torque converter clutch (TCC).

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The purpose of the TCC is to eliminate the 200 to 300 RPM loss that a torque converter, without a lock up clutch, would have when comparing its input to output speeds. The TCC locks the shell of the torque converter to the input shaft or “reaction shaft” entering the transmission. This provides the same direct-through drive at the torque converter that the clutch for a manual transmission provides. This is done for fuel economy gains.

When looking at the vehicles in terms of model years, the newer they are, the more likely they will have the TCC controlled by the PCM through Pulse Width Modulation (PWM). PWM is simply a way for the PCM to turn something on gradually, rather than all at once. The PCM will prolong the TCC slip during engagement for a unnoticeable engagement.

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It may, at times, choose to partially unlock the clutch to provide some slip, then lock it back up as opposed to completely unlocking it. Fluid that is no longer able to lubricate the TCC properly during all of this slipping in and out of the locked position, can produce a shudder that can mimic that of an ignition misfire in its feel to the driver.

Usually when the shudder is light, completely changing the fluid out for fresh fluid of the proper type will stop the shuddering. However, that is assuming that no damage has occurred to the clutches. The shudder feeling, if left alone long enough, can cause damage to the already worn friction material. Also, the shudder can be produced by damaged friction material as well as faulty solenoids and valves.

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The clutches and bands that provide the various speeds from the gear sets are engaged by use of hydraulic pressure. The pump in the transmission draws the fluid up though the filter, fills the torque converter and sends the fluid through the valve body to be directed to clutches and servos. Anything left over is sent through the cooler lines to cool the transmission. The valve body is a set of valves and solenoids in a housing that direct the fluid to the proper clutch pack or servo (to engage bands) at the proper time to engage and release them as needed. A sticking valve, or a loss in hydraulic pressure can cause a gear engagement, slipping, shudders, wrong gear, lack of up shift or down shift, total lack of engagement and more. It is all too often that a customer will notice a shifting or engagement problem, then take their car in to have the transmission serviced thinking that will fix it. Just about the only transmission operating concern that stands a chance of being corrected with fluid replacement is a brief light shudder during TCC and/or certain speed engagements.

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Normally, any other symptom is a sign that something is failing inside the transmission. Of course, like all things that begin to fail and cause a few symptoms here and there, the symptoms will become more exaggerated as the failing parts inside get worse.

Naturally, when the vehicle becomes not drivable, they are screaming that it wasn’t like that before you worked on it. On all accounts, that may be true; it wasn’t like that… they could still drive it before. However, it didn’t become “like that” because you “worked on it.” It became “like that” because whatever it was that was wearing out got worse.

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In this day of electronic transmissions, worn parts in the transmission can stay hidden until the day they completely give up all together. The PCM will do things like raise line pressure and change a clutch’s engagement timing to compensate for worn parts until it can no longer compensate for them.

Sometimes, just disconnecting a battery so that the PCM has to start over learning how to hide these problems can reveal just how bad off some people’s transmissions really are.

To help protect against that, preparation to service a transmission begins at the write-up of the repair order. Should a customer come into the shop requesting a transmission service, the question of why they would like the service to be performed should be asked in a diplomatic manner. Listen to your customer’s response. They don’t always say directly that they have noticed a shifting problem. Sometimes they tell you that they’ve noticed a problem, while trying to tell you that they don’t have a problem; “It works fine, I can just tell it’s time to service it.” Wait a minute… how can you “tell” it’s time to service it? Always explain to your customer that a transmission service is like an oil change for the transmission. Explain to them that replacing the oil in the transmission has about the same chance of fixing a rattling noise in an engine; the chances are slim to none. Add to any final paperwork that the customer has been advised to the effect that there is no promises from your shop that it will correct any shifting problem. If any shifting issues have been reported, be sure to list them on the final paperwork.

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After the write up, especially if no problems were mentioned by the customer, the vehicle should be test driven before the service. The technician should note engagement time into drive and reverse from the park and neutral positions. Listen for whining noises (gear sets making noise). Also, delayed shifts, shudders and check engine light and flashing overdrive lights should be noted on the repair order.

Inspect speedometer operation. Pay attention during the shift for a “flare shift.” (It’s helpful if it has a tachometer to see the engine RPM rev up in the shift.) Notice if it has any other issues that may be engine performance related, such as misfires or loss of power. A severe loss of power at low speeds that seems to correct itself at higher speeds can also be a torque converter issue — the stator can be faulty.

It is ideal if these inspections can begin with a cold vehicle. Then repeat the park to drive and reverse engagement inspection again after the test drive on a hot transmission. Valves and solenoids do stick at certain temps and not at others.

Once the test drive is completed, an in-bay inspection should be performed. Note any fluid leaks at all, not just transmission leaks, to protect against the customer later playing the “ever-since-you” card. Inspect the fluid level and quality. Look for signs of water showing up as pink swirls in the fluid (possible leaking transmission cooler). Smell the fluid to make sure it doesn’t smell burnt like a slipping clutch. Also, if the fluid is red and fresh looking, it is time to talk to the customer again about their request for a trans service. They might be trying to fix something or leave you holding the bag over something. The same can also be true if the fluid level is a quart or more over full. It is a natural tendency for a customer to automatically add fluid to a troublesome transmission as a first-aid attempt to correct a shifting problem.

Traditionally, there are two types of transmission services, though some designs don’t follow tradition. Typically, there is the pan and filter service and then there is the total fluid exchange; each have their pros and cons.

Typical pan and filter service:

  1. Remove pan bolts on two sides, loosen the others slightly, and then wait for the draining to stop. Or, use a fluid extractor down the dipstick tube to pre-drain the pan. Note: Some transmissions may have a drain plug. Dipstick-less transmissions may have something that resembles a drain plug, but it may not be one. Some dipstick-less transmissions, such as those found in newer model Ford Explorers, can have the transmission pan severely damaged if the outer hex of the fill port is mistaken for a drain plug and removed. (Photo 1)

  2. Remove more bolts until there are only some on one side. Leave one side with loose bolts so that the pan will tilt to further drain. (Photo 2)

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  • After the draining has stopped, push the pan straight back up against the transmission with one hand and remove the remaining bolts. Lower the pan in a level fashion.

  • Remove the filter assembly. (Photo 4)

  • Inspect the bottom of the pan for excessive metal and clutch fragments. Normally, there will be some metal stuck to the magnet in the pan. However, there shouldn’t be any piles of metal in the pan. Any clutch fragments should not be large. If the transmission has a plug laying in the pan (common for first service of a Ford transmission), discard the plug. The plug (seen in the upper right corner of the picture) was only used for the factory’s purpose. The trans was pre-filled without a dipstick and the plug was inserted to prevent fluid loss. On the assembly line, the dipstick tube was shoved into the trans case knocking the plug down into the pan. The presence of this plug suggests that this transmission has never had the pan lowered on it before. (Photo 5)

  • Compare the filter that was removed to the filter supplied by the part’s supplier. Carefully examine both sides. (See Photo 6 and 7)

  • Install the new filter.

  • Clean old trans fluid and debris from the pan including any from the magnet. Do not spray the valve body off with any cleaning agent and do not wipe the valve body off with a rag.

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  • Many transmissions are equipped with a reusable gasket. If it does have the reusable gasket, inspect it for nicks and wipe it dry. If the gasket is not damaged, or was not previously leaking, it is best to reuse it. Many times, if the gasket is reusable, the pan will have a sticker or be embossed with a message stating so. (Photo 8)

  • Install the pan, and hand torque with a 1/4” drive ratchet. Generally, the bolts only need to be tightened approximately 1/4 to 1/2 of a turn after the heads of the bolt have reached the pan. However, you should find torque specs in your repair information source for the pan bolts.

  • Some transmissions may have a drain bolt for the torque converter. You should rotate the engine slowly to inspect for one. If the transmission is equipped with one, the torque converter should be drained. (Photo 9)

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  • Fill the transmission through the dipstick tube, or fill through the designated fill port with the specified tool on dipstick-less trans. Be sure to follow level adjusting instructions.

  • After filling is complete, be sure to allow the vehicle to run on the lift and inspect for leakage.

    Pros of filter service:

    • Being able to remove all debris from pan area.

    • Being able to inspect for excessive metal or clutch fragments.

    • Being able to replace the pan gasket if it is leaking.

    • Having a new filter installed.

    Cons of Filter Service:

    • Only changing about half of the total capacity (assuming the T/C has no drain method).

    • The fresh and clean fluid may appear as dirty as the old fluid before the service.

    • Potential liability to future pan gasket leakage.

    • The possibility of allowing debris to enter the valve body during filter replacement.

    • The possibility of a worn front pump not being able to prime back up after pan and valve body drainage allows air into the system.

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  • Risk of a large mess from the “butter fingers” effect.

  • Sometimes having to deal with removal of an extremely hardened and stuck gasket.

    Cooler line hook-up trans flush:

    Another type of service is a fluid exchange service. Some people call this a “flush.” Very inappropriately, some people go so far as to call it a “back” flush. There is no “back” flushing occurring in this service. The term “back” flushing suggests that the machine used will push fluid backwards through its normal path. That is not true. What simply happens is the transmission pumps the fluid out by use of its pump, and the machine refills the transmission at the same rate as what comes out. There are two main styles of flush machines. Some are used with the pan off (direct valve body hook-up), and some are used with the pan on (cooler line hook-up).

    1. Fill the machine as directed with about two extra quarts of fluid than the transmission and torque converter’s total capacity.

    2. Select the easiest way to access transmission cooler line. (Photo 10)

    3. Disconnect the line and install the appropriate adapters. (Photo 11)

    4. Connect the machine’s hoses to the adapters. (Photo 12)

    5. Supply power to the machine, start the engine and let it idle, and turn the machine on. Wait for the exchange process to complete.

    Some transmissions will need to be completely warmed up before a fluid exchange service can take place. That is because some transmission’s cooler line flow is thermostatically controlled, such as Ford’s five- and six-speed automatic transmissions commonly found in newer Explorers. Typically, a flush machine will have some type of flow indicator. The fluid can only properly exchange if flow is present through the cooler lines. Driving the trans before hand and light brake torqueing can be used to keep the transmission warm during the fluid exchange process.

    Pros to fluid exchange:

    • Virtually total fluid replacement, only a minute amount of old fluid will remain.

    • Fluid will usually appear very clean on the dipstick afterwards.

    • The transmission is never drained, therefore no air enters the hydraulic system.

    • Fewer steps than pan service.

    • Some machines allow the tech to be freed up to perform other services while the fluid is being replaced.

    Cons to fluid exchange:

    • The filter is not replaced.

    • The debris is not removed from the pan.

    • The technician is not able to inspect the type and quantity of the debris to spot pending failure.

    • Risk of damage to cooler lines.

    • Having to maintain a very large number of adapters on hand for the various makes of vehicles.

    Always use the manufacturer’s recommended fluid type. Some transmissions can be severely damaged from use of the wrong type of fluid. One such example is the Continuously Variable Transmission (CVT) found in the newer model Ford, like the Five Hundred and the Freestyle.

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    Other makes also have vary specific fluid requirements. Use of the wrong type can cause anything from some light shuddering to complete transmission destruction. Be sure to check fluid type and filling procedures.

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