Time: 1 hour
Tools: Drain pain, sockets and 3/8” torque wrench
Talent: (3)
Applicable Years: 1984 to 1988
Tab: $45.00
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.
The Filter
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.
I have an 84 with the Nash 4+3, my problem is that the OD will work until the coolant temp is above 172 degrees, any hotter than that, it does not engage….Anyone know what mat cause this? Fluid and filter have been changed with no difference. What exactly is the ‘Jones’ plug and where is its location?
What are the part #’s for the switches on the left side of the 4+3 transmission that control the overdrive unit?
A simpler method to get the OD to work (assuming that your 4+3 is mechanically functional) is to:
1) Clip the Tan/Black wire coming from the ECM.
2) Leave the end that points to the ECM dangling.
3) Attach the other end of the wire that you just clipped to a toggle switch.
4) Attach the other terminal on the toggle switch to ground.
This lets you toggle the OD relay when your switch is closed. Use it wisely.
I’d you engage the clutch, flip the toggle and then release the clutch. just as if you’re shifting it into 5th 🙂
I never saw an answer to the question I asked in September 2020 about the issue with sensors and reverse gear and would an inline fuse cause the problem . also when testing the overdrive unit on the hwy after the shop visit it was not working?
What size are the sensors on the overdrive unit from 1985 corvette 4+3 doug Nash transmissiion
I have a 85 C4 with 4+3 . I am having an issue with revers gear also found my overdrive would not engage on hwy when I got my car out of the shop. is it possible that a wire was disturbed in the harness to cause these two issues? The car worked in all gears including reverse before the long visit to the shop?
I would put the jumper on the harness coming from the ecm to the 2nd gear switch. Some one pointed out earlier that jumping a switch not connected to any electronics will do nothing. Some one also pointed out the oil pressure problem of using the OD engaged while in first as there is not enough oil pressure in the unit, my shop manual says the car must be going over 15-20 mph (depending on temp/quality of oil and filter) for there to be adequate oil pressure. The engine should also be warmed up. I have read several different numbers as to how hot the engine needs to be for the unit to work, over 140, 160, 170, 180… basically the car shouldn’t go into overdrive until it has warmed up some.
My plan using info received from both this page/comments and the shop manual is to jump the harness (jones plug I’ve seen it called) for second gear, and use it basically as a 5th gear for cruising highway speeds ( 45 and up).
Would like to know where to place the jumper since I just bought a 4+3 87. Switch or harness? I replaced the relay and oil/filter. Light comes on when I hit the O/D button but no O/D. Not liking this transmission so far….
I need some exact information about your project.
First, what year?
Second, it is unclear what you are trying to do here. Are you replacing an existing transmission? Changing from an automatic to this manual transmission.
Third, Not sure what jumper you are referring to.
Gus Gustafson
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Sorry I have an 87 corvette i recently purchased with the 4+3 . 53k original miles. Fluid/ filter changed, it was low due to a leak in pan but filled. Nothing out of the ordinary in there. Replaced relay. Car IS NOT hard to get in reverse. Shifts fine. I have no OD. The light comes on as it should between gears and when I hit the button. I just found out mine being a later design only has a 1st gear switch. If the first gear switch fails can I jump it like in the article above for the second gear switch to get OD to engage? I don’t know what has failed.
Pressure switch, solenoid, 1st gear switch?
Ryan, did you ever get your vette overdrive working? Mine works great in 3rd only. Can’t figure it out. It’s a newer one with only 2 switches on the side of my trans. Steve
Thought the same thing. How can a switch not connected to anything make a difference jumping it? Jumping the harness makes much more sence to me, but I haven’t looked at a wiring diagram to see how it operates.
I have a 1987 z51 with a Doug Nash, very interested in how to do this mod, as well as the pros and cons of the mod. Does this effect gas consumption? Does it effect the top speed? wear and tear on OD unit / 4 speed? ease of shifting?
How is overdrive filled? And how much does it hold and how do u check level
What shafts it in and out of OD? I have the unit out and want to install it in a 57 chevy. What do I need to do?
where do I find the filter for the OD. Chevrolet has discontinued it
We sell a Transmission filter kit – 4+3 overdrive pan gasket and filter for 1984-1988 which can be found by clicking this link here to our website. https://https://www.corvettecentral.com/search?CurrentSearchCategoryId=C4&q=transmission+filter+kit
Jumping the 1st gear circuit to enable overdrive operation in 1st is a good way to bust the overdrive in a couple of different ways. These parts are NLA. Break it, you’re in deep, expensive, ca-ca.
The unit relies on pressure from an internal pump to shift it into overdrive. This pump is driven by the rear wheels. Until a certain road speed is reached, there will not be enough pressure to safely engage the overdrive clutch without allowing it to slip. There is a pressure switch inside the unit that doesn’t allow the OD to engage until a minimum adequate pressure is reached. If the overdrive circuit is energized in first gear, and the car is accelerated from a stop, at some point, while still in first, the switch will close and the OD will engage. Probably violently. Many owners like to use the clutch when shifting into OD to lessen the forces on the components. You can’t do this when it engages on its own, because you can’t anticipate it.
The other thing is that the overdrive clutch relies on adequate pressure to apply it. The faster the car is going, the higher the output of the pump, and the higher the pressure. This is INVERSE to the amount of torque the clutch has to absorb and transmit. In first gear the transmission is multiplying say 300 ft lbs of engine output by 2.88 is 865 ft lbs going through the overdrive clutch to the wheels.1,91 2nd is 573 ft lbs, 1,33 3rd is 399 ft lbs, and 1:1 4th is 300 ft lbs. Note the very large step between the torque in first and second. The original design wasn’t intended to absorb the higher torque applied in first gear. And the pressure to make the clutch hold is the lowest in first gear. Exactly inverse of what needs to happen.
I won’t do it to my car. I wonder how many OD clutches have been burned up by this mod, and the owner never even considered that this mod may have been a contributing factor.
My OD no longer works, and I wanted to try jumping the 2nd gear switch as suggested. Are the potential problems you caution about only related to jumping the 1st gear switch? I can’t find a replacement solenoid, or I would just change it…haven’t tried cleaning the magnet, someone suggested that would help the solenoid work…Any advice welcome! thanks,
Randy
Just to be clear, you only need to jump the 2nd gear switch to make this work? And the jumper wire goes on the harness back to the ECM?
I have an ’85 Corvette with the 4+3 transmission. It has one position switch for 1st/2nd. What’s the point in bypassing the switch? The console mounted rocker switch works as designed so I have full control of overdrive engagement or downshift once out of 1st gear and after min engine temp is met (145F). The only point would be to “lock” the overdrive in for top end? Otherwise my car dowshifts out of overdrive shortly after breaking into the triple digits, a speed limiter? Would bypassing the switch keep the overdrive engaged in 4th regardless of rpm or throttle position?
Technically if you jumped the plug for 1st gear, 2nd gear and left 4th gear plugged in wouldn’t you have a 4 speed transmission, and engage overdrive in 4th gear only; thus it would be a 5 speed tranny. Since the picture 7-4 has no 3rd gear plug you can’t make it a 6 speed tranny can you?
Which plug is the jones plug?
An article in 2007, about the C4 4+3 overdrive. It stated to put a jumper on the switch. I don’t follow this, because with the harness removed from the switch the switch doesn’t attach to anything, other than being screwed into the transmission case. Could it be that just unplugging the harness did the trick? Just confused as I can understand what good a jumper wire across a switch not electrically hooked to anything will do??? Thanks for the help..?
Thanks did it work?
this artical is wrong. the jumper wire goes on the jones plug not the switch. thanks for the artical though, once i figgered out you got it backwards it worked great
Not having gotten under the car yet, what are you calling the jones plug? Electricsl plug connected to second gear? Or on 4 th gear?