Time: 1 hour
Tools: Drain pain, sockets and 3/8” torque wrench
Applicable Years: 1984 to 1988
Tip: Treat this job the same way you would treat changing the fluid and filter with an automatic transmission. You’ll probably end up making just as big a mess on your garage floor.
Complementary Work: As long as you have a mess on your garage floor you might as well change the gear oil inside the 4-speed part of the transmission.
The overdrive unit was simply hung off the back of the 4-speed transmission. In a great many ways the overdrive was very similar to the old two-speed Powerglide. This particular overdrive unit was originally designed for the Jeep CJ-7 and CJ-5. Chevrolet was working on such a tight deadline for the fourth generation Corvette that there was no time to develop a new unit specifically for the Corvette.
Delco Electronics solved the computer problems and made sure that the manual section and the overdrive were truly integrated units. All of this was really done to meet the EPA fuel mileage regulations, but at the same time the overdrive fourth gear provided a high speed cruising capability that easily exceeded all the earlier Corvettes. Thanks to the EPA the Corvette was a 150 mile per hour car.
The electronics necessary to create this dual transmission weren’t all that complicated but it did make for some interesting driving. There are really three unique, and different, algorithms programmed into the overdrive ECM, one each for second, third and fourth gears. The overdrive ECM logic is incorporated in into the main ECM, or the chip, as it’s commonly known.
This program monitors miles per hour, engine temperature, and throttle position in an effort to properly engage the overdrive. The program will normally not engage under 184°, although my ’85 always liked 176°. I guess all computers are not created equal. The real trick, however, is to get rid of all the complicated electronics and turn this transmission into an 8-speed.
A Quick Easy Modification
I discovered how to do this by accident. Well, not quite by accident. Actually Chris Petris, who used to build transmissions for the Corvette Challenge racing series, helped me with this. There’s a switch on the side of the transmission that tells the computer which gear is engaged. This 2nd gear switch is one of the first things to go bad on the 4+3 . The switch is engaged every single time you shift the transmission, whether the overdrive is engaged or not. It’s no wonder that this is usually the first item to wear out.
When my 2nd gear switch went bad the car simply wouldn’t engage overdrive properly. I could hit the switch on the console to turn on the overdrive and it might not actually engage until I drove some twenty miles down the road. You can imagine how aggravating this was. When Chris and I couldn’t locate a replacement switch right away we simply grounded the switch by running a very short shunt from one terminal to the other. This effectively made the OD a completely mechanical unit, operated only by the interior switch, which in my case is on the console.
Eliminating this switch made all the difference in the world. This was like giving me a new Corvette. While Chevrolet designed all the electronics to pass the EPA fuel mileage cycle, I simply wanted a useful transmission. The 4+3 actually works better if you take some of the sophistication out of the system.
What we did was fool the computer into thinking that I’m always in 2nd gear or higher. This switch is on the left side of the transmission, and can be seen easily if you have the car on a lift. This switch moved around a little during the years, but it was always the switch towards the rear.
Remove the wiring harness from the switch and hook it out of the way with a cable tie. You won’t be using this harness. Now make a little jumper wire that fits into the two terminals. You can remove the switch from the transmission and make the jumper wire on your workbench if you prefer. If it takes more than fifteen minutes you’re goofing off.
Since the overdrive unit in the 4+3 transmission is really a version of the old Powerglide automatic transmission is shouldn’t be a surprise that it has a transmission filter. You should treat this situation just as you would any automatic transmission. You’ll even create the same type of mess on your garage floor.
The technique is to simply drop the oil pan on the bottom of the overdrive unit and hope that most of the fluid goes into the large drain pan you’ve place under the pan. Be careful not to lose any of the bolts.
The best way to clean the pan in your home garage is to use spray cans of brake clean and a roll of paper towels. Pay particular attention to the sealing edges of the pan. At one time GM used RTV sealant on this surface. That wasn’t such a great idea and now you can get the filter kits with gaskets. If you don’t find a gasket in your car you can assume that the fluid hasn’t been changed in a few years.
Make sure that you also clean the transmission surface as well. These overdrive units are notorious leakers. Using a gasket on the surface will solve most of the sealing problems. Once everything is clean you can install a couple of bolts to hold the oil pan back in place and carefully tighten the bolts using a cross pattern to get it nice and even.
I would use Dexron III in the unit and probably opt for a synthetic fluid. Mobil 1 is the obvious choice but it seems everyone has a different preference here. Just ask some questions and see what other owners are using.
These units have proven pretty reliable once they got past the 1984 model year. There were some internal changes made and the clutch material is different on the later cars. The best thing you can do for this overdrive is change the fluid and filter once a year, or at least every 15,000 miles.
7-1.tif: GM placed a magnet in the oil pan to capture any metallic particles. Don’t be shocked if your magnet is covered with steel shavings. That simply means the magnet is doing what is designed to do. You can remove the magnet for cleaning and then place it back in the correct location.
7-2.tif: The overdrive gave the C4 an incredible top speed for the mid-‘80s. You’ll notice that the difference between fourth gear and fourth gear overdrive is almost forty miles an hour at 4,000 rpm. Since most of us don’t drive much over 80 mph on the highway it simply means we get pretty incredible gas mileage. I’m still wondering where you can actually use all that gear ratio. Even at Sebring, which is a very fast course, I only get into 3rd overdrive. Maybe I need to drive the banking at Daytona. Yep – that ought to get me into 4th overdrive.
IMG004: This little jumper switch did wonders for my driving enjoyment. Placing the jumper wire in this switch turned my overdrive into a manual overdrive. All of the stuff that was designed to meet fuel mileage requirements is now gone. I even have overdrive in 1st gear now – not that I really need it. The switch is easy to modify, but if you have any questions about your abilities just call the Corvette Clinic in Sanford Florida.
7-4.tif: This is an interesting combination of parts. We have the traditional small block Chevrolet engine bolted to the very traditional T-10 transmission. Then that’s hooked to yet another transmission which looks strangely similar to the old Powerglide from the fifties. Then that second transmission (or overdrive) is bolted to a huge length of aluminum U-channel (the Driveline Support), which in turn is bolted to a differential carrier in the rear of the car. What’s amazing is that it works so well.
This is an adaptation from Richard Newton’s most recent book 101 Projects for Your Corvette 1984 – 1996. He has also written two other best selling Corvette One deals with the 1968 to 1982 Corvettes, How to Restore and Modify Your Corvette 1968-82 , while another deals with the Sting Rays from 1963 to 1967, Corvette Restoration Guide 1963-1967 All of these books are available from Corvette Central.