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Automatic Transmission/Transaxle Auto Service Tech Tips
These Tech Tips are provided courtesy of autoclassroom.com, an online training resource for automotive instructors, students and technicians.
Change the bolts and washers
Whenever disassembling an automatic transmission, always replace the pump bolts and washers to avoid the possibility of a leak. A leak in the area of the pump can cause quite a headache for the technician because the entire assembly must be removed from the vehicle to fix it later.
The paper towel test
New ATF will penetrate a paper towel better than used oxidized ATF. To compare old fluid with new fluid, place 3 drops of new fluid on a paper towel and 3 drops of used ATF on the paper towel about 3 inches from the first sample. Wait for 30 minutes.
The new ATF will have expanded (penetrated through the paper towel) much farther than the old oxidized fluid. This test can be used to convince a customer that the ATF should be changed even though, to the naked eye, the fluid looks OK.
Smooth is in -- rough is out
It used to be common practice in the automatic transmission business for the rebuilder to use sandpaper to "roughen up" the surface of the steel discs to achieve a good clutch apply. This is not true now. Sanding creates grooves and sharp peaks that decrease the oil film between the paper on the friction plate and the steel plates. There should be a thin oil film between the paper and the steel, creating a holding bond and making it possible for a clutch pack to apply and release thousands of times without wear. Therefore, for consistent shifts and long-lasting transmissions/transaxles, do not sand the steel discs. Used steel discs are often smoother than new steel discs due to normal wear. As long as the steel discs are not discolored, heat checked, or warped, used steel discs should be reused.
Assembly lube is used during the reassembly of automatic transmissions. If red assembly lube is used on seals, it may look like an automatic transmission fluid leak when the transmission gets hot and the lube melts. However, if you use blue, green, brown, or clear assembly lube, the color will immediately identify it as assembly lube. Many transmissions have been disassembled because the service technician thought that the red liquid dripping from parts of the transmission was automatic transmission fluid, when, in fact, it was just assembly lube that melted and ran when the transmission reached normal operating temperature.
What's leaking?
The color of the leaks observed under a vehicle can help the technician determine and correct the cause. Some leaks such as condensate (water) from the air conditioning system are normal, whereas a brake fluid leak is very dangerous. The following are colors of common leaks:
Sooty black -- engine oil
Yellow -- antifreeze (engine coolant)
Green -- antifreeze (engine coolant)
Blue -- antifreeze (engine coolant)
Orange-- antifreeze (engine coolant)
Red -- automatic transmission fluid (ATF)
Murky brown -- brake or power steering fluid or very neglected antifreeze (coolant)
Clear -- air conditioning condensate (water) (normal)
Quick and easy automatic transmission/transaxle diagnosis
An experienced technician told the beginning technician that automatic transmission/transaxle diagnosis is often very easy. For example:
If the vehicle does not move in drive or reverse: Remove the transmission/transaxle for service because the problem is likely mechanical rather than hydraulic (valve body) or electrical (computer, solenoids, or sensors). If the unit will not power the vehicle, the unit will more than likely require removal for a thorough mechanical inspection and repair.
If the vehicle moves, but does not shift correctly: The problem is likely electrical related (computer, solenoid, or sensor) or hydraulic (valve body) rather than mechanical. This is usually true because if the vehicle is able to move forward and backward, the major mechanical components, while not in like-new condition, are at least able to function. Therefore, correcting the non-mechanical problem should be the technician's first consideration.
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