Transmission fluid should be changed periodically. Your owner’s manual should give you the recommended intervals which could be anywhere from 15,000 miles to 100,000 miles. Most transmission experts recommend changing the fluid and filter every 25,000 miles.
Few transmissions have drain plugs to drain the old fluid. In order to get the fluid out, the technician removes the transmission oil pan. This is quite a messy job and generally not recommended for the casual do-it-yourselfer. Even if the transmission has a drain plug, the only way to also change the transmission filter is to remove the pan. When the pan is down, the technician can check for metal shavings and other debris which are indicators of impending transmission problems.
In most cases during these transmission services, only about half the oil is able to be removed from the unit. This is because much of the oil is in the torque converter and cooler lines and cannot be drained without major disassembly. The fluid change intervals are based on the fact that some old fluid remains in the system.
When the transmission is serviced, make sure that the correct fluid is used to re-fill it. Each transmission manufacturer has their own recommendation for the proper fluid to use and the internal components are designed for that specific formula. GM usually uses Dexron, Fords prior to 1983 use Type F while later models use Mercon. Late model Chrysler products use ATF +3 +4 (Not using the correct fluid for Chrysler transmissions is the most common reason for their transmission problems.) Toyota sometimes uses Type T which is only available through Toyota and Lexus Parts departments. Honda also specs out their own formula which is available from Honda or Acura parts departments. A transmission will not work properly or may even slip or shudder with the incorrect fluid, so make sure that you double check. Your owner’s manual will tell you which fluid is required. Naturally, the owner’s manual will try to convince you to only use the manufacturer’s branded fluid, but they will also provide you with the specs for the oil. If the aftermarket product indicates on its container that they meet or exceed the specs for a particular type of transmission fluid, it is generally ok to use that product.
- Adjustments and In-Car Repairs
There are several problems that can be resolved with an adjustment (A simple adjustment is one that can be made without removing the transmission from the vehicle.) or minor repair. If a late model transmission (computer-controlled transmissions started becoming popular in the early ’90s) is not shifting properly, it is often the result of a computer sending incorrect signals due to a faulty sensor, or the transmission is not reacting to the computer command because of a bad connection or defective solenoid pack. These problems can be corrected while the transmission is in the car for considerably less money then a complete overhaul.
If a non computer-controlled transmission is shifting too early or too late, it may require an adjustment to the throttle cable. Since throttle cables rarely go out of adjustment on their own or due to wear and tear, these mis-adjustments are usually due to other repair work or damage from an accident.
If the vehicle has a vacuum modulator instead of a throttle cable, there is an adjustment that can be made using an adjustment screw in some modulator designs. In vehicles with modulators, however, it is very important that there are no vacuum leaks and the engine is running at peak efficiency. Engine vacuum is very sensitive to how well the engine is running. In fact, many technicians use a vacuum gauge to diagnose performance problems and state-of-tune. Many problems that seem to be transmission problems disappear after a tune-up or engine performance related repair was completed.
In some older transmissions, bands can be adjusted to resolve “slipping” conditions. Slipping is when an engine races briefly when the transmission shifts from one gear to the next. There are no adjustments for clutch packs however.
- Reseal job
A transmission is resealed in order to repair external transmission fluid leaks. If you see spots of red oil on the ground under the car, your transmission may be a candidate for a reseal job. In order to check a transmission for leaks, a technician will put the car on a lift and examine the unit for signs of oil leaks. If a leak is spotted at any of the external seals or gaskets and the transmission otherwise performs well, the technician will most likely recommend that the transmission be resealed. Most of the external seals can be replaced while the transmission is still in the car but, if the front seal must be replaced, the transmission must first be removed from the vehicle in order to gain access to it, making it a much costlier job.
- Replace accessible parts
There are a number of parts that are accessible without requiring the removal of the complete transmission. many of the control parts including most of the electrical parts are serviceable by simply removing the oil pan. The parts that are accessible, however, vary from transmission to transmission and most transmission repair facilities would hesitate to provide meaningful warrantees on external repairs for the simple reason that they cannot see if there are any additional internal problems in the components that are only accessible by transmission removal.
- Complete Overhaul
In a complete overhaul (also known as rebuilding a transmission), the transmission is removed from the vehicle and completely disassembled with the parts laid out on a workbench. Each part is inspected for wear and damage and then either cleaned in a special cleaning solution, or replaced with another part depending on its condition. Parts that have friction surfaces, such as bands and clutches are replaced as are all seals and gaskets. The torque converter is also replaced, usually with a remanufactured one. Technical service bulletins are checked to see if the auto manufacturer recommends any modifications to correct design defects that were discovered after the transmission was built. Automobile manufacturers often make upgrade kits available to transmission shops to resolve these design defects
- Replacement unit vs. overhaul existing unit
When a transmission requires an overhaul, there are generally two options that you may have. The first is to remove your existing transmission and overhaul it, then put the same, newly rebuilt unit back in your car. The second option is to replace your existing unit with another unit that has already been rebuilt or remanufactured. The second option will get you out of the shop and on your way much faster but may cause you problems down the road. The reason for this is that, in some but not all cases, a particular transmission model can have dozens of variations depending on which model car, which engine, which axle ratio, even which tire size. The problems you could experience could be as simple as a speedometer that reads too high or too low (the speedometer is usually connected by cable to a gear in the transmission output shaft.) You may also experience incorrect shift points or even complete transmission failure because your engine may be more powerful then the one the replacement unit was originally designed for. This is not the case with all transmission models so voice your concerns with your technician. Most shops will rebuild your existing unit if you request it as long as they can afford to have a lift tied up with your car while the transmission is being rebuilt. Of course this is only important if you are sure that the transmission you have is the original one and has never previously been replaced.