One of the more interesting aspects of rebuilding an automatic transmission is that its success or failure often rests on the final step: the installation. It doesn’t matter that you purchased quality parts, installed all the upgrades, and taken your time to produce a high quality rebuild. If the installer gets it wrong, you’ll see that unit again.
One of the major risks during installation is the transmission fluid. Not only do you need to make sure you have the correct type, but you may also need to follow a specific procedure to set the level.
Back in the old days, life was a lot simpler; there were two major types of fluid and, quite frankly, if you got it wrong, it really didn’t cause too much grief. There was a dipstick and a tube that was large enough for you to add fluid almost as fast as the pump could suck it from the pan.
The procedure was simple: You added enough fluid to bring the level to just above pan, fired the engine up, and added the remainder. Setting the level was just a matter of reading the dipstick, usually using your fingers to detect the correct temperature: comfortable to hold = too cool; uncomfortable = correct temperature.
Today things have changed. With the advent of regulated torque converter lockup and computer controls, the fluid became a much more important part of the transmission’s operation. These day’s it’s common for a shop to have six or more different fluids; and even then you’ll sometimes have to purchase a special type for that odd unit.
The other major change in many cases is how you get the fluid into the unit. Somewhere along the line, the dipstick disappeared, whether as a cost saving measure or to stop the do-it-yourselfer from adding the wrong fluid. Now checking fluid level is no longer a matter of just popping the hood; often there’s a detailed procedure involved.
The first few seconds at startup are the most critical: This is when you risk damaging the pump and torque converter. The problem is that you can’t get the fluid into the unit quickly enough to make sure the pump or converter won’t run dry.
Very often you need to fill the unit through a bung with a very small opening, and even if you do have a dipstick, it may fit through a fill tube with a tiny diameter. And that restriction can cause the torque converter hub or internal bearings to run dry. To avoid damage, your best bet is to follow this procedure:
- Make sure the pump is well lubricated, either with transmission fluid or an approved lubricant.
- Prefill the converter with at least 1 pint (about 0.5L) of the specified fluid; 2-3 pints (about 1–1.5L) for larger units.
- Before starting the engine, add fluid until it’s around 1 quart (1 liter) overfull. But no more than that; you don’t want it to blow oil out the vent.
- Start the engine and run it for 10 seconds.
- Shut the engine off and add oil to about the same level as before.
- Start the engine again for 10 seconds and repeat the procedure until you’ve reached the transmission’s expected capacity.
- Correct the fluid level using the manufacturer’s specifications for method and temperature.
It’s important to follow the manufacturer’s procedure carefully, especially when it comes to the gear range. There are many units that won’t fill the torque converter correctly in park. If in doubt, try checking the level with the selector in neutral, taking the appropriate safety precautions.
If the unit is a carry-out, make sure the person fitting the unit is aware of this fill procedure. That way you’re less likely to see that unit again with an unexplained pump or torque converter failure.