Diagnosing Automatic Transmission Problems
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 do not 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 is 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 do not get it right the first time, you wil certainly get the opportunity to do it over again. That is 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 $2,000 to $2,800 or more for parts and labor.
Some independent repair garages that do not normally do transmission repairs usually refer customers who are having transmission problems to a transmission shop. Other shops may replace the transmission, but do not actually overhaul the transmission themselves. They buy a remanufactured transmission from a transmission supplier.
TRANSMISSION 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 your transmission works, the hotter the fluid runs. The life of the fluid drops quickly once its temperature gets up above about 200 degres 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.
USE THE TYPE OF TRANSMISSION FLUID SPECIFIED FOR YOUR TRANSMISSION
When adding or replacing ATF, use the type specified by the vehicle manufacturer. GM, Ford, Chrysler, Honda, Mercedes and others all have their own specifications for ATF. There is no such thing as a “universal” ATF that works in all transmissions. Many universal fluids do meet a variety of specifications, but no one product can meet all OEm requirements 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 (Fords latest friction-modified formula, introduced in 1997).
General Motors has produced Dexron II, III and VI. These are friction modified formulas. Dexron III can be used in the older GM transmissions that originally required Dexron II. Dexron VI was introduced in 2006 for GM Hydra-Matic 6L80 6-speed rear-wheel-drive transmissions. Dexron VI now replaces Dexron III and II, and can be used in GM or import transmissions that formerly specified Dexron III or 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 is similar to Dexron II. But Chrysler’s fluid is more slippery than GMs, 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.
For more information about fluids, see:Automatic Transmission Fluid (types and applications)
TRANSMISSION FAULT CODES
If a vehicles Check Engine light is on, it means the computer has detected a fault and has set a diagnostic trouble code. There is no way to know if it is an engine code, transmission code or body code, so you will have to plug a scan tool or code reader into the diagnostic connector to extract the code.
If the transmission OD (overdrive) light is on or flashing, it means the transmission controller has diagnosed an internal transmission fault. To diagnose the problem, a scan tool that can read transmission codes must be plugged into the vehicle diagnostic connector (usually located under the dash near the steering column). The scan tool will then display the transmission code(s) that turned the warning light on. 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, a bad sensor or solenoid, you may be able to fix the problem without having to take your vehicle to a transmission shop.
Electrical fault codes are set when the transmission controller or PCM detects an open or a short in a shift solenoid, shaft speed sensor or other device. Performance codes are set when the computer sends out a command, such as a 2-3 shift, but the transmission does not respond properly.
With electrical codes, you can use a DVOM to test a solenoids 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 is going on inside the transmission. When things do not match up properly, a “ratio error” fault code may be set indicating something is wrong with the way the transmission is shifting gears. This may cause the transmission to go into the default or limp-in mode, which typically turns all the solenoids off and leaves the transmission in 2nd or 3rd gear.
The only way to isolate these kinds of transmission 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 input or output speed sensor.
Regardless of what the code says, it is a good idea to check for any OEM technical service bulletins that might relate to the code or the complaint. Many times you will 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 is 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.
- Disconnect the battery to erase the computer’s memory.
- Reconnect the battery and start the engine.
- 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.
- 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 are going less than 25 mph.
- 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.