1984 corvette "overdrive"

[muskiemike]

hello folks,


have a 1984 Corvette with the doug nash 3+4 overdrive.

car has 34K on it, i've owned it since 1984, had the overdrive unit rebuilt 8K miles ago, from a rebuilder in FL who has done thousands of these.

i put less than 500 miles per year on this car.

overdrive ran fine last year, just took out today, and car ran great, but no overdrive.

i waited until the coolant temp was 190*, as that's when the overdrive unit will kick in - i flipped the overdrive switch "on" from the console, and the "overdrive" light indicator did "not" display on the console, nor did the overdrive unit go on.

i turned off and on the overdrive switch several times of the course of 30 miles, but still not no overdrive display light or no overdrive.

thought i would ask if you have any suggestions?

in advance, "thanks" for your time and replies. . .

Mike

 

[WhalePirot]

electrical

From memory here, but no dash light and no O/D means no electrical signal is getting through. That's probably good news.

Easiest to test is the console switch, which has two screws under the trim panel that holds it in place. They can get dirty contacts from spillage or dirt.


BTW, that 190 temp is not in concrete. The ECM must be in closed loop for O/D to be available. Long ago, my car had rich-running issues, due to some performance modifications and I 'fixed' the O/D by changing to new spark plug wires. The Doug Nash was switching in/out at steady, flat freeway speeds. The mods had the ECM on the edge of open/closed loop and the old wires had degraded the spark to tip it into intermittent open loop sensing.

 

[muskiemike]

WhalePirot

Thanks for your suggestion . . .

BTW - NICE CAR you have.

From memory here, but no dash light and no O/D means no electrical signal is getting through. That's probably good news.

Easiest to test is the console switch, which has two screws under the trim panel that holds it in place. They can get dirty contacts from spillage or dirt.


BTW, that 190 temp is not in concrete. The ECM must be in closed loop for O/D to be available. Long ago, my car had rich-running issues, due to some performance modifications and I 'fixed' the O/D by changing to new spark plug wires. The Doug Nash was switching in/out at steady, flat freeway speeds. The mods had the ECM on the edge of open/closed loop and the old wires had degraded the spark to tip it into intermittent open loop sensing.

 

[muskiemike]

Heres' an update:


I looked at the Overdrive (defeat) switch, inside the car on the console, it looked ultra clean, was unable to disconnect it, but did attempt to clean it using electrical deoxite cleaner (very good stuff), and worked the switch back and forth, repeatedly - still no overdrive light or overdrive.

I was able to locate the "overdrive fuse" it's called the GAGE (I found this info on this or another forum), the GAGE fuse looked good.

Then, I replaced the overdrive "relay" located in the engine compartment, the original one (34K miles) had gobs of heavy duty grease on the (4) contacts within the old relay as well as gobs of grease packed into the backing plate of the (4) wires that connect into the overdrive "relay" - thought for sure this was the problem - but nope, still no overdrive light or overdrive.

All comments are appreciated . . .

hello folks,


have a 1984 Corvette with the doug nash 3+4 overdrive.

car has 34K on it, i've owned it since 1984, had the overdrive unit rebuilt 8K miles ago, from a rebuilder in FL who has done thousands of these.

i put less than 500 miles per year on this car.

overdrive ran fine last year, just took out today, and car ran great, but no overdrive.

i waited until the coolant temp was 190*, as that's when the overdrive unit will kick in - i flipped the overdrive switch "on" from the console, and the "overdrive" light indicator did "not" display on the console, nor did the overdrive unit go on.

i turned off and on the overdrive switch several times of the course of 30 miles, but still not no overdrive display light or no overdrive.

thought i would ask if you have any suggestions?

in advance, "thanks" for your time and replies. . .

Mike

 

[WhalePirot]

Thanks for your suggestion . . . BTW - NICE CAR you have.

Gee, thanks. I hope to have it on the road (again) soon. Lemme check the FSM. <pause>

Not a lot new here, but the (rare) FSM electrical supplement (Section 8A) says:
"Manual overdrive is controlled through the Overdrive Relay. The ECM provides a control signal through ECM terminal 19 to the coil of the Overdrive Relay . By controlling the ground path, the ECM energizes and de-energizes the Overdrive Relay as conditions change.

In the energized condition, the relay allows current to flow from the Gages Fuse, through the relay contacts to the 1st Gear Switch. This switch is closed in all gears except 1st gear.

The Overdrive Solenoid can be energized in any gear except first. The ECM receives a ground signal input from the 2nd Gear Switch or the 4th Gear Switch when the transmission is in either of these gears. In this way, the ECM can determine which gear the transmission is in. The Overdrive Relay control signal from the ECM is applied only when the conditions are proper for the applicable transmission gear."

As pristine as your car is and the climate where you are, there is slim chance of much corrosion on contacts in the system. I think the relays for the fuel pump and OD are identical, if you have a spare top swap in. Actually, I like having a spare of this double-duty part, especially given the import of it and its low cost.

Using the below diagram, I'd check for proper grounds and 12vdc, as normal, given the configuration as you test it. I'd look under, for any disconnected wires on the tranny switches and the solenoid connection itself. If I read the diagram correctly, Pin F on the ALDL will indicate a path to ground with 1st gear selected and the switch ON.

I think I changed the O/D solenoid long ago, accessed, I believe, with the pan removed.

FWIW, my car far from stock., I rewired the switch so the ECM is out of the logic path, but 1st gear logic remains in; hence, my car will engage O/D in other gears, regardless of closed loop or the odd logic dictating the oft-misunderstood "proper conditions". The final straw for me, was the surprise loss of O/D in third gear at, 60 mph, is it? I did not like the sudden, high revving engine.

I suspect you wish to keep the car all stock, however.

 

[jseremba]

Gee, thanks. I hope to have it on the road (again) soon. Lemme check the FSM. <pause>

Not a lot new here, but the (rare) FSM electrical supplement (Section 8A) says:
"Manual overdrive is controlled through the Overdrive Relay. The ECM provides a control signal through ECM terminal 19 to the coil of the Overdrive Relay . By controlling the ground path, the ECM energizes and de-energizes the Overdrive Relay as conditions change.

In the energized condition, the relay allows current to flow from the Gages Fuse, through the relay contacts to the 1st Gear Switch. This switch is closed in all gears except 1st gear.

The Overdrive Solenoid can be energized in any gear except first. The ECM receives a ground signal input from the 2nd Gear Switch or the 4th Gear Switch when the transmission is in either of these gears. In this way, the ECM can determine which gear the transmission is in. The Overdrive Relay control signal from the ECM is applied only when the conditions are proper for the applicable transmission gear."

As pristine as your car is and the climate where you are, there is slim chance of much corrosion on contacts in the system. I think the relays for the fuel pump and OD are identical, if you have a spare top swap in. Actually, I like having a spare of this double-duty part, especially given the import of it and its low cost.

Using the below diagram, I'd check for proper grounds and 12vdc, as normal, given the configuration as you test it. I'd look under, for any disconnected wires on the tranny switches and the solenoid connection itself. If I read the diagram correctly, Pin F on the ALDL will indicate a path to ground with 1st gear selected and the switch ON.

I think I changed the O/D solenoid long ago, accessed, I believe, with the pan removed.

FWIW, my car far from stock., I rewired the switch so the ECM is out of the logic path, but 1st gear logic remains in; hence, my car will engage O/D in other gears, regardless of closed loop or the odd logic dictating the oft-misunderstood "proper conditions". The final straw for me, was the surprise loss of O/D in third gear at, 60 mph, is it? I did not like the sudden, high revving engine.

I suspect you wish to keep the car all stock, however.

When my 84 did that. I went and crimped the blade that connects to the overdrive. It was just a bad connection. So go under and make sure that all the wires are on the connectors tight.