Help with my 4+3
 

Looking to get some ideas on how to get my overdrive unit working properly. First off the switch on the shifter does not do anything. I have not diagnosed whether it is it bad switch or if something is not hooked up properly at the shift knob or down below at the switch. The overdrive unit does work and will shift into overdrive as if the switch is pressed at normal operating temperature. Pressing the switch does not turn it on or off however. If I floor it in second through 4th gears, the overdrive unit will kick off and I will get direct drive for those gears. However if I floor the accelerator in first gear I cannot access the overdrive unit until the car has sat and cooled down for a bit. When I got the car the overdrive unit did not work. I replaced the overdrive relay and this is what is currently happening. Looking for some ideas on where to go next.

The car is an 86. It's a 4/86 build with the aluminum heads. As far as I know the ecu is original. I do know that tjere is not supposed to be overdrive in first gear. The overdrive engagement does in fact follow the light.

The odd thing is that when I floor the accelerator in first gear the overdrive unit will not engage in 2nd 3rd or 4th gear and will remain in direct drive until the car has sat for a short period. If I floor it in any other gear other than first, the overdrive disengages as it should and goes into direct drive. Once I let up the overdrive unit will engage once again, and the overdrive light does follow each way.

It sounds like someone has bypassed the switch and/or relay and just hooked +12V to the OD connector (white plastic by drain plug).

BTW, if you floor it too many times (especially in 1st) you won't have an overdrive to worry about. They don't stand up to abuse well and the pump is driven by the output shaft (first gear = low pressure).

Without actually getting under the car, I don't think any of the wiring has been tampered with. I've been under the car a lot in the last year and have made sure all connections at the overdrive unit were tight and clean. I found one picture on my phone of when I first got the car. I laid under it and was taking a some pictures. I found this one showing one of the connectors. Not sure which one it is though but figured I'd post it.

Attachment 48353760

I don't beat on the car really. No dumping the clutch or hard shifting though the gears. I just happened to figure out the issue when I was doing some spirited driving.

I'd put a test light on the wire going to the white connector. That will tell you when it's being commanded to go into OD.

Here's a comment from quite some time ago and if you use the search for users mentioned at the conclusion you could possibly stumble upon some very good information.

This was a modification done inside the the car. I don't have a 4+DNE so I doubt I'll research it much but might be an interesting read and maybe easily followed up with others.

https://www.corvetteforum.com/articl...rdrive-bypass/

Using the 'advanced search' for the C4 section and 'scorp508' turned up threads back to '01. I used that user id because I was familiar with many of comments/information he's passed on through the years.. For the search term I just used 'overdrive'. I'd think it easy to tweak the search term.

From this link use the 'SEARCH THIS FORUM' button above 'THREADS/POSTS' columns.

https://www.corvetteforum.com/forums...-1984-1996-19/

I'll look into those links, thank you. I don't want to modify the system, moreover, just getting it working right is my goal. I think I'll disassemble the shifter and inspect and test the switch. If its bad I'll replace it and go from there. Its hard to diagnose faults if there are faulty components and I have not inspected the switch yet. Its just odd that the car has to sit for a short period of time before the overdrive unit will function after being run with wide open throttle in first gear.

I didn't intend to point you towards a 'modify' but I'd think some very good reading. I would think the information IHBD mentioned is maybe contained in there in some fashion. There's certainly functional/control changes year to year.

Do you believe yours to be original?

I suspect that you are dealing with modified programming. Someone may have swapped the original memcal or reprogrammed it with a newer bin from an 87-88 car. But may simply be a bad switch or related wiring issue. Try grounding pin C8 to see if it will upshift/downshift.

86 diagram: http://crowz.narmir.com/corvette/trans/Odcontrol.jpg

If using an 87-88 bin in an 86 car, the shifter switch won't work as expected since the ecm pinout is different between 86 and 87-88. The 87-88 4+3 shifter switch is pin C7 and 86 is pin C8. The 86 shifter switch will appear to an 87-88 bin as first gear, since the 87-88 first gear switch is pin C8.

If this is the case, you can repin the shifter switch at the ecm, so that it will be read properly with an 87-88 bin. More on this later...

Need to determine what you really have.

First check if the memcal is plugged into a moates G1 style adaptor inside the ecm.

If no adaptor present, check the label on the eprom (under the blue cover on the memcal) to determine which bin file or broadcast code (bcc) you have in the car.

If there is no label, or if there is a hand written label, the bin file can be read from the eprom with a programmer and appropriate header.

This is interesting info. I'll get the info and report back. Anything is possible as I don't know any history on the car.

My 88 had a problem with changing into and out of overdrive when I first bought it. I tested the little micro switch at the base of the shifter and found it faulty. Replaced it for 14 bucks and it's worked perfectly ever since.

Also something I need to inspect for proper operation. Do you remember where you picked the micro switch up at?

I bought it locally online here in Oz. I just Googled the part number written on it and it led me to a supplier. The one I bought was identical.

That great info to know thank you. I'm gonna pull mine apart this weekend and see whatbi come up with. I'm thinking that the microswitch in my car is faulty as well. Just want to get it working as it should.

I haven't had much time to look into the issue. My mom has had some serious health issues and we nearly lost her. I'm starting to have more time so I will have to jump back into this as I would like to get the issue resolved.

Wanted to update this thread. I found the switch had broken wiring. I was able to solder the wires back onto the switch assembly and the overdrive unit is working perfectly now. Thank you for the suggestions and help!