Doug Nash 4+3 Overdrive help

I have a 1986 coupe with the 4 speed. The previous owner stated that the OD has been “bypassed”. The shifter button does not engage or dis engage OD and now OD is generally always on in 2nd/3rd/4th at all times UNLESS the gas pedal is depressed more than about 35%. If you give enough throttle to dis engage overdrive, you lose the ability to re activate overdrive for the rest of the drive. If you park the car for about an hour and re start it, overdrive will be back engaged during normal driving.

If you get on it in 1st gear, OD will not engage in any gears and the shift button does not engage anything when pushed.

If you are in overdrive in 2nd gear and put the pedal to the floor OD will dis-engage but once you are off the throttle it will no longer engage. You lose the ability to have the car in OD if you smash in the gas. Even if you are at highway speeds the car will not go back into OD if you’ve lost it.

I’m running into the problem of either barely pressing the gas in 1st gear or at anytime in order to keep the OD engaged for highway driving (most of what I drive this car is around 65-70mph) or actually being able to drive the car how I want but then being stuck without OD gear ratios because of that. If I want to keep OD engaged I also need to really wind out first gear before dropping it into 2nd gear with OD engaged to not bog the engine down.

Ultimately I want to add in a toggle switch that truly bypasses all factory logic and I have total control of when OD is engaged, getting rid of the momentary switch on the shifter all together.

I looked at the OD relay on the firewall, under the passenger side dash where the ECM is housed, under the shift boot and under on the transmission and I don’t see any splices or clear as day hacks for how this is being “bypassed” by the previous owner.

Any insight would be very helpful.

 

Is the indication of 'overdrive' being engaged an amber "OD" light in the center Driver Information Center, or in the LCD cluster? (The icon is a square circle outlining a "D" in the middle.)

If it it is in the DIC, does the light follow the overdrive / direct state of the unit itself? Ie, the light = ON is Overdrive, light = OFF is Direct?
If it is in the LCD, does the indication follow as above, or is it different?

Another way to tell which/where the "light" is: Are there 1 or 2 (in the same cavity) dk green wires on the relay? 2=DIC, 1=LCD.

The distinction is that the light in the DIC is wired to the output of the relay and thus is directly indicative of what state the OD should be in.
If the indication is in the LCD, the indication is derived from data from the ECM and is indicative of what the ECM is commanding.

I believe that the 86 still has the light in the DIC. If the light does not follow the state of the unit (direct = OFF, overdrive = ON) there is something else 'wrong'.

 

yes the light still follows how the car drives.

the amber indicator light will come on when it is engaged and will stop shining when overdrive is not engaged

 

Ok. There's nothing wrong with the unit itself. Cool!

For complete manual control: At the relay: Disconnect the tan/blk wire in Cavity B. (This is the control wire from the ECM.) Connect a new wire to Cavity B and run this to your switch. Grounding this wire will command Overdrive ON. (There are 2 tan/blk wires on the relay. If in doubt disconnect them both. With the key in RUN, grounding Cavity B (the correct cavity) will make the light come on.)

Do not use overdrive in 1st gear. The torque multiplication of 1st gear exceeds the torque capacity of the overdrive clutch. This is why the OEM strategy does not allow 1st-Over under acceleration.

 

This smells like (poorly) modified programming. I can think of a few possible variations that might explain the behavior.

1. A hacked 86 4+3 bin, with strange unlock to lock tuning, or strange 4th gear detection settings along with a disabled on/off switch (primitive)
2. A hacked 87-88 4+3 bin with strange unlock to lock tuning, or strange 4th gear detection setting (bonus points).
3. A hacked 89 six speed bin with strange CAGS settings (ugh).
4. A hacked auto trans bin with strange settings (ugh).

Is the speedometer correct?
Tire size?
3.07 gear?

 

This may help, I did this with my car where the button didn't do anything and OD was on all the time making the car a total dog.

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

If in fact this is a BIN on your ECM this won't help I dont think.

 

I cannot thank you enough for this insight, got around to throwing in a simple toggle switch from the mentioned location and overdive is now at the control of my finger tips and works flawlessly.
My OD relay was above the master cylinder, mounted near the firewall. It was the front of the two relays mounted there. One wire from a chassis ground on the front cross member near the steering rack and another wire from the “B terminal” on the relay were ran through a rubber chase grommet and through the dash to a switch mounted on the center console.

thanks again

 

Watch out for mechanical over-rev (money shift) if you ever accidentally switch it off at high rpm. I'm no fan of hardware based overrides and much prefer to fix it in the tuning, but it is your car.