over drive not working
Thread Starter
6th Gear
Joined: Mar 2011
Posts: 6
Likes: 0
over drive not working
I have a 1985 corvette coupe. the problem I have is with the overdrive unit. I can push the shift **** button to activate the overdrive and it may engage or it may not. I have read in articles that if the overdrive unit itself is bad that reverse will not work. well reverse has always worked since I have had the car. the shop that has done work on the car said the overdrive unit needs replaced. But I think since it does work at times and then doesn't work that it could be a electrical problem. I have had filter and fluid changed Shift button and wire harness and relay it runs to replaced. Could it be a pressure switch is bad or bad ground. I really would appreciate any thoughts someone may have what could be wrong and any test or some thing I could tell the mechanic to check. thank you
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
There is an internal solenoid in the OD unit that may be failing. Also check the wire harness connector at the OD unit to make sure the wires are making good electrical contact. Also, there is an OD relay on the firewall near the wiper motor. It will be next to the fuel pump relay. It's possible that one or more wires in that harness connector are not making good contact.
Your mechanic should have some sort of access to the specific diagnostic procedures and repairs for OD issues. AllData and Identifix are two online automotive repair websites that will have the same information that is found in the '85 Corvette Factory Service Manual (FSM) These sites require a paid subscription so ask the mechanic if he has access to either or both.
I would recommend that you purchase the FSM book set for your car. They are year-specific and can be occasionally found in the C4 Parts For Sale section or online auction sites. These manuals are the same ones used by Chevrolet dealer techs. If you get the manuals, it could help to have them in the car when you need to take it in for service or repairs.
Your mechanic should have some sort of access to the specific diagnostic procedures and repairs for OD issues. AllData and Identifix are two online automotive repair websites that will have the same information that is found in the '85 Corvette Factory Service Manual (FSM) These sites require a paid subscription so ask the mechanic if he has access to either or both.
I would recommend that you purchase the FSM book set for your car. They are year-specific and can be occasionally found in the C4 Parts For Sale section or online auction sites. These manuals are the same ones used by Chevrolet dealer techs. If you get the manuals, it could help to have them in the car when you need to take it in for service or repairs.
Melting Slicks
Joined: Jan 2014
Posts: 2,192
Likes: 381
From: Lunenburg MA
If the OD light doesn't come on, it's electrical. It could be the shifter switch, OD relay, or the 1st gear switch on the trans (and a few less likely things).
If the light comes on but the OD doesn't, then it's time to make sure the OD is full of clean ATF and apply 12V directly to the white connector between the 1/2 shift lever and the speedo connector. This should engage the OD as long as the pump pressure is OK (rear wheels spinning).
If it does engage, it's electrical (see above), if it doesn't then you have an internal problem (could be electrical or mechanical).
If the light comes on but the OD doesn't, then it's time to make sure the OD is full of clean ATF and apply 12V directly to the white connector between the 1/2 shift lever and the speedo connector. This should engage the OD as long as the pump pressure is OK (rear wheels spinning).
If it does engage, it's electrical (see above), if it doesn't then you have an internal problem (could be electrical or mechanical).
Thread Starter
6th Gear
Joined: Mar 2011
Posts: 6
Likes: 0
a dash OD light. have never seen one come on even when over drive engaged. way it acts really makes me think it is electrical. a bad ground or something. will look for OD light on dash. have a shop manual for a 85 will look in it. tell the truth though I am not that good of a mechanic. really do appreciate the advice.
Melting Slicks
Joined: May 2014
Posts: 2,894
Likes: 592
From: New Port Richey FL
a dash OD light. have never seen one come on even when over drive engaged. way it acts really makes me think it is electrical. a bad ground or something. will look for OD light on dash. have a shop manual for a 85 will look in it. tell the truth though I am not that good of a mechanic. really do appreciate the advice.
Or if you make a O-D request and no Voltage/Light look in circuit
One thing that I have seen more than I like to remember is was that PITA O-D switch in shiftier having a broken connection of just being Temperamental, the Early ones having sticky goo or Corrosion in the ones in the console
Last edited by s carter; Aug 4, 2016 at 08:40 PM.
