Automatic transmissions are a mystery to most motorists. All they know is that they put their gear selector in "D" for Drive and their vehicle goes forward, they put it in "R" for Reverse and it goes backward, and they put it in "P" for Park when they shut the engine off. They may know the transmission requires some kind of fluid, but have probably never checked the level or even looked for the dipstick unless they drive an older vehicle with a leaker. So if the transmission starts to slip, shift oddly, makes noise or misbehaves, most motorists don’t have a clue as to what might be causing their problem — nor do many technicians who are not transmission experts.

Automatic transmissions are a complex and wonderful thing when they are working properly. But when internal problems cause the transmission to act up, it takes a fair amount of expertise and experience to diagnose the fault. If the problem is not low fluid, a faulty solenoid, valve body or controller, the transmission usually has to come out for a tear down and inspection. This can be a time-consuming process, so many shops simply replace the transmission when there’s a major internal problem rather than trying to repair or rebuild it themselves.

Rebuilding automatic transmissions is an exact science that requires familiarity with the unit being rebuilt as well as the the correct parts, tools and specifications. If you don’t get it right the first time, you’ll certainly get the opportunity to do it over again. That’s why many transmission shops today no longer rebuild transmissions themselves. There are too many different makes and models. The shop can make more money replacing transmissions with a reman transmission sourced from an outside remanufacturer than rebuilding the units themselves. A typical FWD transmission job today can easily run $1,500 to $2,000, and the lion’s share of that is labor.

Brake and alignment shops that don’t normally do transmission repairs usually refer customers who are having transmission problems to a transmission specialist. Even so, there are a number of common transmission problems that don’t require the skills of a specialist.

FLUID LEVEL & LEAKS
One of the most common complaints with automatics is fluid leaks. Fluid can leak out of the driveshaft seals, the input shaft seal, the transmission pan gasket, the torque converter or the ATF cooler and line connections. If the fluid level gets low, the transmission may be slow to engage when it is shifted into drive. Gear shifts may be sloppy or delayed, or the transmission may slip between shifts. If the fluid level is really low, the transmission may cause the vehicle to not go at all.

On most vehicles, the fluid level should be checked when the fluid is hot with the engine idling, the parking brake set and the transmission in Park. If fluid is needed, add only enough ATF to bring the level up to the full mark. Do not overfill because doing so can cause the fluid to become aerated, which may affect transmission operation.

If the dipstick reads low, the transmission is probably leaking. So look underneath to see where the fluid is going. If there are no visible leaks, check the radiator for ATF in the coolant. The ATF cooler inside the radiator may be leaking and cross-contaminating the fluids.

You should also check the condition of the fluid. Some discoloration and darkening is normal as the fluid ages, but if the ATF is brown or has a burnt smell, it is badly oxidized and needs to be changed. Varnish on the dipstick is another indication of worn out fluid.

You can also do a "blotter test" to check for worn fluid. Place a few drops of ATF on a paper towel and wait 30 seconds. If the spot is widely dispersed and red or light brown in color, the fluid is in satisfactory condition. But if the spot does not spread out and is dark in color, the ATF is oxidized and should be changed.

Many transmission experts say most transmission problems can be prevented by changing the ATF and filter regularly for preventive maintenance. How often depends on how the vehicle is driven. For some vehicles, this might be every 30,000 miles or two years.

The harder the transmission works, the hotter the fluid runs. The life of the fluid drops quickly once its temperature gets up above about 200º F. Installing an aftermarket auxiliary ATF cooler that is parallel to the OEM ATF cooler is recommended to keep fluid temperatures down on vehicles that are used for towing or are driven hard.

ATF also becomes contaminated with normal wear particles from the clutch plates, bushings and gears. The filter will trap most of this debris before it can cause problems. But many older Asian transmissions only have a plastic or metal screen that does little to protect the transmission against internal contaminants and nothing to keep the fluid clean. On these vehicles, changing the fluid is the only way to get rid of these contaminants.

When adding or replacing ATF, use the type specified by the vehicle manufacturer. GM, Ford, Chrysler, Honda, Mercedes and others all have their own specs for ATF. There’s no such thing as a "universal" ATF that works in all transmissions. Some fluids meet a variety of specifications, but cannot meet them all because of the different friction additives that are required.

Ford has three automatic transmission fluid specifications: Type F (a non-friction modified formula for most 1964-81 transmissions), Mercon (a friction modified ATF similar to Dexron II for 1988-97 transmissions), and Mercon V (Ford’s latest friction-modified formula, introduced in 1997).

General Motors has two specifications: Dexron II and III. Both are friction modified formulas and Dexron III can be used in the older GM transmissions that originally required Dexron II.

Chrysler has a number of different ATFs: MS-7176D (also known as ATF+2) is Chrysler’s version of a friction-modified ATF that’s similar to Dexron II. But Chrysler’s fluid is more slippery than GM’s, so Chrysler recommends using only ATF that meets their specs in Chrysler transmissions. In other words, do not use Dexron or Mercon in a Chrysler transmission.

Chrysler MS-7176E (also known as ATF+3) was introduced in 1998 and supersedes ATF+2. It should only be used in 1998 and newer Chrysler transmissions, but can also be used in earlier Chrysler transmissions.

Chrysler ATF+4 is for 2000-01 model year applications, and their newest fluid ATF+5 is for 2002 and newer models.

TRANSMISSION FAULT CODES
If a vehicle’s Check Engine light is on, it means the computer has detected a fault and has set a diagnostic trouble code. There’s no way to know if it’s an engine code, transmission code or body code, so you’ll have to plug a scan tool or code reader into the diagnostic connector to extract the code.

If your scan tool or code reader is capable of reading transmission codes (some are not or require extra software) and you find a transmission code, what happens next depends on the code. If the code indicates an internal performance problem, the transmission will probably need the attention of a specialist. But if the code indicates an electrical fault, you may be able to fix the problem without having to refer the job to a specialist.

Electrical fault codes are set when the transmission controller or PCM detects an open or a short in a shift solenoid or other device. Performance codes are set when the computer sends out a command, such as a 2-3 shift, but the transmission doesn’t respond properly.

With electrical codes, you can use a DVOM to test a solenoid’s resistance. If the solenoid is open, shorted or out of specifications, it needs to be replaced. Performance codes, on the other hand, require further diagnosis and can themselves be caused by electrical faults in sensors.

Electronic transmissions use speed sensors to monitor shifts and what’s going on inside the transmission. When things don’t match up properly, a "ratio error" fault code may be set indicating something is wrong with the way the transmission is shifting gears. The only way to isolate these kinds of faults is to follow the diagnostic charts for the particular code(s). Ratio error codes often turn out to be caused by a fault in a shaft rpm sensor.

Regardless of what the code says, it’s a good idea to check for any OEM technical service bulletins that might relate to the code or the complaint. Many times you’ll find the fix in the TSB, which may require replacing a certain component or even reprogramming the transmission computer.

Some Chrysler transmissions, for example, can experience a "bump shift" condition. There’s nothing wrong with the transmission, but the computer needs to be reprogrammed to recalibrate the shift points. In Chrysler three-speed automatics, changing the fluid to ATF+3 can also help eliminate harsh shifts.

Some transmission problems may require "retraining" the computer. This is also necessary if an electronic transmission or computer has been replaced. Chrysler TSB 18-24-95 describes the retraining procedure that allows the computer to relearn the correct shift points.

1. Disconnect the battery to erase the computer’s memory.
2. Reconnect the battery and start the engine.
3. Drive he vehicle while trying to maintain a constant throttle position as it accelerates up through all four gears. If the transmission is shifting properly, it should be in 4th gear by the time you reach 45 to 50 mph. Repeat this procedure from a standing start 15 to 20 times.
4. With the vehicle traveling at less than 25 mph, do five to eight wide open throttle kickdowns to get the transmission to downshift to 1st from 2nd or 3rd gear. Drive in 2nd or 3rd gear for at least five seconds between kickdowns and remember to kick it down only when you’re going less than 25 mph.
5. While driving at 45 to 50 mph, do five to eight part to wide open throttle kickdowns to either 2nd or 3rd from 4th gear. Again, continue driving for at least five seconds in 4th gear between kickdowns.

TORQUE CONVERTER PROBLEMS
The torque converter is the fluid coupling between the engine and transmission. If the transmission is making noise in gear, but the noise goes away when it is shifted into neutral, the problem may be worn needle bearings in the torque converter. Needle bearings are used inside the torque converter to separate the stator from the impeller, the stator from the turbine, and the turbine from the converter housing.

Torque converters contain a one-way clutch. If the clutch jams and locks the stator (which normally keeps the stator tuning only one way), the converter can’t circulate the fluid properly between the impeller and turbine. The resulting turbulence inside the unit can cause a lack of power at highway speeds and may cause the engine to overheat during cruise conditions. A locked one-way clutch will also cause the fluid to run dangerously hot, which can damage the transmission and lead to transmission failure. If the one-way clutch fails to hold its position and freewheels in both directions, the torque converter can’t multiply torque normally and the vehicle will accelerate slowly (like starting out in 2nd gear). The only cure for either condition is to replace the torque converter.

A "stall test" can be used on many older transmissions to check its ability to hold torque and the operation of the converter one-way clutch. Some manufacturers do not recommend using a stall test because it stresses the transmission, so if you use this procedure, do not do it for more than five seconds at a time. Do not perform this test on vehicles that are equipped with a traction control and/or ABS systems. The inputs of a partially opened throttle, brakes applied, and no detectable movement by the wheel speed sensors will at best set a code, and at worst cause damage to the vehicle. Also, on electronically controlled transmissions that use vehicle speed sensor to determine shift points be prepared for a code to be set if you perform a stall test.

Before conducting a stall test, check the fluid level and condition. Chock the wheels and set the parking brake. Start the engine and place the transmission in drive while holding the brake pedal under firm pressure so the vehicle doesn’t move. Then push the accelerator to the floor while holding the brakes on. Note the maximum rpm the engine reaches. This is the stall speed. If it is lower than specifications, the torque converter one-way clutch is slipping. If the stall speed is higher than specifications, the transmission is slipping. Possible causes include a low fluid level, restricted fluid filter, a sticking pressure regulator valve, slipping clutches, bands, shaft splines or one-way clutch.

Most late-model automatics have some type of lockup torque converter to improve fuel economy. If the lockup fails to engage, there will be some slippage and fuel economy will drop. Causes here include a bad lockup solenoid, incorrect sensor input information to the transmission controller or PCM (typically a speed sensor) and hydraulic control problems. If the lockup fails to release, the engine may shudder and die when coming to a stop. Causes here include a faulty lockup solenoid, a sticking lockup valve, grounded lockup solenoid wiring or a missing lockup solenoid spacer plate screen.

Lockup shudder is another complaint that may be encountered. This refers to a vibration that’s felt just before or after lockup occurs. This kind of problem can be hard to diagnose because the cause may be the torque converter, transmission or engine. Bad motor mounts, engine misfire, a bad CV-joint or U-joint, etc. can all cause vibrations that may be felt as a shudder throughout the drivetrain.