Good afternoon. I am a new corvette owner and new to this forum. I have spent hours on this forum looking for answers to the problem and trying everything I have read. The car is a 1985 with the 4+3 Doug Nash. I start the car and let it get to proper operating temp before moving. When I initially take off it is fine going through the gears and then the overdrive kicks in as it should. However the overdrive never disengages and the light is always on, wont disengage with button on shifter eiter. Now for what I have checked and changed; Overdrive relay on the firewall is new, also checked wiring below relay plug which looks good except possibly where wires go into plug (will post picture) overdrive switch on shifter is good along with wires to the plug, fluid is at the bottom of the filler hole as forums state, and it has had regular service as I have the records from the previous owner dating back to 1996. With everything ive read iv almost come to the conclusion that the unit is good otherwise it wouldn't go into overdrive at all? Shifts fine through all the gears forward and reverse works as it should. Would a bad solenoid possibly cause this? Also read that possibly a bad TPS could cause overdrive yo kick on all the time? (I have one on order i think its bad fue to eratic idle up and down at running temp) Im open to hear suggestions and input on where to go next.

 

I would not suspect the TPS. At full throttle the ECM should command a kick-down & deactivate the O/D, but other than that, the TPS has no say in when the O/D is activated...wouldn't ever fail in a manner which forces full time engagement.

A stuck or shorted solenoid could potentially be at fault, but the best approach would be following the FSM diagnostic tree & do some diagram-aided wire tracing. I don't have one for '85, but I'm sure '84 is very similar; if nobody is able to post pics of that section from your year soon, I will put some from my manual tomorrow.

 

I have a shop manual and repair manual for the car. Wiring and myself don't get along to well. Any leads to which section this diagram is in?

 

The light is in parallel with the wire to the solenoid. If the light is on, so is power to the OD.
If it does not shift into OD unless the light comes on at the same time, the OD unit and solenoid are functioning correctly.

Does the OD light go out when you come to a stop? (It should).
Does the light stay out until you shift out of First? (It should).
Drive the car from cold. Does the light come on when coolant temp reaches ~135F? (Or thereabouts. It should).

If all of the above = YES, the ECM is commanding the OD relay correctly.

I would go back to the shifter switch. Think of the switch as "overdrive request" to the ECM. If the request = YES, the ECM will command the OD ON when the other paramaters are also met. (Not in First gear, temp over 135).
If the request = NO, the overdrive remains off. (Exception: At 110MPH, OD is automatically engaged without the button being pushed).

When the two wires are shorted togeather by the switch being CLOSED, OD Request = NO.
When the two wires are open-circuit, by the switch being OPEN, OD Request = YES.

Noobies to this system usually don't understand the request circuit. Again, open circuit = OD ON, closed circuit = OD OFF. If one of the wires is broken, or the switch is bad and the contacts are not closing, the OD will be on at all times that the other parameters are also met.

 

I received your Visitor Message regarding this.

The shifter switch is NOT a "momentary" switch. It is an OFF or ON. Each push of the button changes its state. Off = Overdrive is Active. On = Overdrive is off, or in Direct.

You indicate your switch is a momentary action. This is incorrect, and is the problem with your system. You can double-check me by connecting the wires to each other and going for a drive. With them jumpered together, OD and the light will be OFF.

 

I would like to thank everyone for the help and input on this issue. It turned out it was the switch, once I replaced it the vehicles overdrive works as it should.

 

So mine will go into OD on it's own sometimes but most of the time it will not at all. So of course I can't drive it on the highway due to the rpms are way too high because it won't go into OD. Does anyone have this issue and if so how can I remedy it. Because I'm just about had it with this car. Already replaced the clutch and slave cylinder so what's next it only has 63k on it. I know it's an old car but it's in mint shape otherwise.