Transmission fluid and filter changes are a regular part of good automotive maintenance. And if you have recently purchased your new to you Corvette its hard to tell the last time preventative maintenance occurred. Depending on which transmission your C4 Corvette has will dictate which process to follow. The process to change the transmission fluid is pretty easy really except for the second transmission (overdrive) on the 4+3. That one is a little more involved.
The best thing you can do for this process is to get your C4 Corvette as high off the ground as possible. See my page here if you need a review of how and where to jack up your C4 Corvette. Try to give your self as much room to work under your car as possible. Always remember safety first.
Automatic
The automatic transmission does not have a drain plug. To change the fluid you must remove the pan.
You should be able to get a filter and gasket from any car parts store. This should save you some money instead of buying it from a Corvette specific store online and avoid the Corvette markup.
A large drain pan should be placed under the transmission pan to catch the oil as it drains. Start by removing every other bolt from the transmission pan. Similar to the instructions below on the overdrive transmission, remove the front and side bolts then loosen up the back ones to slowly let the old transmission fluid flow out. After that is complete make sure to remove and old stuff from the transmission so the new gasket will connect well. Also, now is a good time to wipe everything down and clean off any old fluid that spilled during the pan removal. Clean off old sealant or gasket material and make sure things are dried completely.
Now remove the old filter. The filter will still have fluid in it so be careful not to get it all over. Slightly move the old filter back and forth while pulling down on it. Be sure the old O ring comes off with the old filter. Another quick check to see if anything needs to be cleaned.
Now put the new filter on. Be sure to get the filter attached completely. It might need a little tap or two with your hand or a rubber mallet. Now put the new gasket on the pan. They sometimes get a little warped in the box. Using the screws line up the gasket and reattach the pan. Start by putting in every other bold and tightening them up slightly. Go all the way around and put the others in as well. Make sure the gasket is in the correct place. The plan is to get all the bolts even, 9foot lbs is the specification. Go around a couple of times to make sure the pressure is consistent.
You can now lower your C4 Corvette to add transmission fluid. It’s best to put in an amount of fluid that is less than required and check as you go adding the amount of fluid that is needed. Now is a good time to check for leaks to make sure everything was put back together correctly. It’s then a good idea to start the car and let it warm up and recheck the fluid levels.
Doug Nash 4 speed (1984-1988)
The first thing to make sure is that you can remove the fill plug. if you can’t remove that you don’t want to drain the current fluid out. Kind of like the oil filter, I always check to make sure that comes off before draining the oil, so same process here.
Remove the drain plug and let all the fluid flow out. Once you are satisfied the fluid has drained as much as possible. Replace the drain plug. Using a pump, refill the transmission at the filler plug. Continue to fill until excess fluid begins to overflow and come out of the filler opening. Let the excess drain until it is even with the bottom of the filler plug. You can now replace the filler plug and you are all done. Be sure to check for any leaks.
4+3 overdrive (1984-1988)
As I mentioned before, make sure that you can remove the fill plug. if you can’t remove that you don’t want to drain the current fluid out.
The OD filter is the same one that is used in the Ford FMX series transmissions from the late 60s to early 70s Fords. You should be able to get the filter from any car parts store. This should save you some money instead of buying it from a Corvette-specific store online and avoid the Corvette markup.
A large drain pan should be placed under the overdrive oil pan to catch the oil as it drains. The oil pan attaching bolts at the front and sides should be removed first. Then the rear bolts should be loosened enough to pry the oil pan loose, allowing the fluid to drain; the remaining bolts should then be removed to remove the oil pan. Carefully drop the pan straight down here. Pull straight down to remove the filter. The rubber seal may come off with the filter, so simply reinstall it on the new filter once it has been cleaned with solvent and dried. To remove the old fluid from the cooler lines, you can use very light air pressure (5 psi, for example). Remove one line and blow out the fluid. Install a new pickup tube grommet on the new filter. Make sure the magnet is still installed in the oil pan. The best thing I’ve found is to use RTV sealant to secure the pan, and use a bead of black all around the pan. Some people choose to use a gasket here instead.
Make sure the pan rails and transmission case flange are completely clean of old sealant or gasket material and dried thoroughly. Ensure that the pan rails and transmission case flange are aligned properly. Install pan bolts and torque to approximately 8-10 ft-lbs and equally tight. Fill the overdrive unit with Dexron II automatic transmission fluid. When you refill the pan, add fluid until some spill out the fill hole. Then spin one rear wheel by hand for a couple of turns; that will force fluid into the empty coolant lines if you have drained them. Then top off the pan and re-install the fill plug. Make sure to check for any leaks.
6 Speed ZF (1989-1996)
To open the plugs you need an 11/16 Allen key. I’ll mention it again, make sure you can get the fill plug out before removing the drain plug. Plus it does help to drain easier.
Remove the drain plug and let all the fluid flow out. Once you are satisfied the fluid has drained as much as needed. Replace the drain plug. Using a pump, refill the transmission at the filler plug. Continue to fill until excess fluid begins to overflow and come out of the filler opening. Let the excess drain until it is even with the bottom of the filler plug. You can now replace the filler plug and you are all done. Make sure to test for any leaks.
Conclusion
I hope the above instructions helped you change your transmission fluid on whatever transmission your C4 Corvette has. Always check for leaks after you have completed the process and done a short test drive.
