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The Basics: Automatic Transmission Servicing

I have had a few people asking for help on servicing their transmission. There have been many questions concerning service intervals, and what is actually involved when a transmission shop services their transmission. This article is written to give you a basic overview of what is involved. There are many different types of cars, and so there are also many different procedures, as well, depending on the vehicle. This article is not intended to replace your shop manual, but simply to act as a basic guide to use in addition to your shop manual. Where the this article differs from the shop manual recommendations, it is better to follow the shop manual and ignore my recommendations.

When and Why Service an Automatic Transmission

Transmission fluid, like any other fluid, such as motor oil, antifreeze, and other fluids, tends to "break down" with heat and time. As the operating temperature rises, the fluid breaks down at a quicker rate, requiring replacement at much shorter intervals. This is not a direct proportional rate. If the transmission fluid lasts a certain amount of time at 200 degrees farenheit, the fluid may only last half as long at 230 degrees and half, again, as long at 250 degrees. These numbers are off the top of my head and are used, only as an example. The actual breakdown may (and probably is) be much different than these numbers. I only mention this to illustrate the importance of keeping the temperature under control within an automatic transmission. Most vehicles utilize a heat exchanger within the radiator, to keep the transmission cool. This is the best way to cool a transmission. Water is a much better heat-transfer medium than is air. This is why 70 degree air does not feel, to you, as cool as 70 degree water does. Most after-market coolers are hung in front of the radiator and use air as a heat-transfer medium, rather than water; for this reason, after-market coolers are not capable of getting rid of as much heat as the original radiator cooler, and should be used, only, in addition to the original radiator cooler; Not by themselves. The transmission runs much hotter than an engine, so there is no danger, as some people believe, of overheating the transmission with the engine coolant. Actually, having a heat exchanger within the radiator, helps the transmission in another way, by initially heating up the transmission and helping to act as a thermostat to keep the transmission at a more consistent operating temperature.

I have seen many recommendations, in terms of automatic transmission service intervals. Most, generally agree that a good "rule of thumb" is to service the transmission every 25,000 miles. Of course this will depend upon how the vehicle is used and what type of vehicle. Most newer cars, and trucks have lockup torque converters (What's a lockup torque converter?) which tend to create much less heat than older, traditional automatics of the '70's and early '80's, so for those that do, you may want to extend the service period, if you use the vehicle primarily on the highway, and do not tow, fourwheel, or use the vehicle in any other "severe use condition".

The term "fluid break down" refers to the destruction of additives within automatic transmission fluid, due to the combination of heat and time. The higher the heat, the less amount of time the fluid will last. When the fluid "breaks down" it turns dark in color, and begins having an unpleasant odor. The fluid becomes more acidic and, as a result, hardens the rubber seals within the transmission. When these seals harden up, they can no longer perform their job efficiently, and they begin to leak. When the seals begin to leak, they tend to allow clutches to slip. The clutches within an automatic transmission are not able to withstand much heat, so they eventually burn, which makes them slip more, which makes them burn more......until the transmission developes severe shifting problems or may, in some cases, quit moving at all. The only alternative, when the problem becomes this advanced is a complete overhaul, which, among other things, includes replacement all of the clutches and rubber seals.

What is Involved When Servicing an Automatic Transmission?

Obviously, draining and refilling the transmission, as well as replacing the filter (in most models) is part of a transmission service. In many cases, a transmission cannot be completely drained. Many transmissions do not allow the technician to drain the torque converter, so, in some cases, only 1/3 of the fluid can actually be changed easily. It is wise, in these instances, to service the transmission more often, since you are actually just diluting the old fluid with new fluid, rather than replacing the old fluid. In cases where you cannot drain the torque converter, and the transmission fluid is very "stale", many technicians "flush" the transmission by removing a cooler line, after performing the "service" and allowing the transmission to pump out the old fluid, with the engine running, while you are pouring new fluid into the transmission, until the "old" fluid, escaping from the cooler line, is bright red, like the new fluid you are pouring. Plan on having plenty of fluid on hand to completely flush the transmission, with this procedure. This can usually be done at a transmission shop, at extra cost, when you have the transmission serviced.

In addition to changing the fluid by removing the pan, installing a new filter (make sure that you get a good quality filter; I use Sealed Power brand filters. Using a cheap filter can be much worse than reusing the old filter. Make sure to remove any previous filter seals/gaskets before installing a new filter and seal or gasket on models that use them.) , adjusting bands, and reinstalling the pan (do not use any sealer on the pan gasket. Tap the pan back, where the bolts tend to distort the pan, with a small ball peen hammer, and make sure the gasket surfaces are clean and dry), a road test, both before, and after is essential. Make sure that the transmission shifts properly, through all of the gears, and at the right time or speed. Make sure that the transmission has "kick down" (shifts to "passing gear" during full-throttle at approximately 50 mph....Oh, and UH....make sure that it is safe and well within the law to travel at that speed where you are testdriving). Make sure that the linkage is adjusted properly(there is usually a "drive stop" that the shifter stops at when shifting to drive; this stop must make the linkage stop at exactly the drive position, not half way between drive and neutral, etc.), and that the shift indicator reads properly (when the car is in drive, the needle should say "Drive", or "D" or something like that.), and last, but maybe, most important, make sure that the engine is running properly; a poorly running engine can simulate some really bizarre transmission problems; I wish I had a one-dollar bill for every time I had to diagnose a poorly shifting transmission that turned out to be a vacuum problem, or a clogged fuel filter!

When refilling the transmission, check the fluid several times, and when reasonably assured that the transmission is full, take a test drive, and check the fluid again, after the test drive. Some vans, and trucks, and even some cars can give some misleading dip stick readings, so be careful. Turn the dipstick over and read both sides; The proper fluid level is never the higher of the two readings. Be more inclined to believe the side of the stick that reads the lowest, but always check the fluid at least two times to help avoid a false reading.

Under the vehicle, check all of the U-joints, engine and transmission mounts, constant velocity joints (front wheel drive cars), exaust hangers, cooler lines (make sure that the steel lines are not rubbing each other, or any other component on the vehicle, where they may rub a hole in themselves), vacuum lines, wiring (make sure, if your transmission has any wiring to it, that the wires are tucked out of the way, where they will not get caught by brush). Check for leaks. This is where I have a problem with most "quick lube" places. Sure they change the oil and service transmissions (although I have seen very few "quick lube" technicians that can service a transmission properly without messing something up; take it to a transmission shop), but an oil change is much more than that. It is an opportunity to check over you car, closely, so you won't be stuck somewhere, along side the road, in the rain, wishing your car wasn't broken.

During the service, take care of any problems that you identified during the testdrive, or while working on the car. If the transmission shifted too early, or too late, adjust the TV pressure, or vacuum modulator. If the drive stop was a little "off", or the if needle read "neutral" when the transmission was it "drive, adjust it. If the transmission mount was all compressed, or oil-soaked, replace it. If the transmission was leaking, replace the seal, or do whatever it takes to Properly fix the leak. Whatever needs to be corrected, take the time to perform the necessary repairs. A new truck can be $35,000, or more, a new car can be $15,000 and up; These are more than merely transportation; They are investments, so take care of them; Fix what needs fixing, and maintain what needs maintaining, again, thoroughly, and properly.

What a Service Will Not Fix

A service will not cure a slipping transmission, unless the slipping was caused by too low a fluid level, in which case, adding some fluid would have cured the result of the problem; the cause of the problem is a leak, or else the transmission would not have had a low fluid level. Transmissions do not "burn" fluid like an engine can. In some instances a bad vacuum modulator will allow the engine to "suck" fluid and burn it, but the transmission cannot "use"(or consume) transmission fluid on it's own. A transmission service is merely changing the fluid and filter, just like an engine oil change merely changes the oil and filter; Changing the transmission fluid will not cure a transmission problem any more than changing the engine oil will cure an engine problem.