30th b-day

I have an '86 with the 4+3. Anyway, I spent the winter rebuilding the engine, and have it all back together. The problem I am Having is that my overdrive will not engage now.(worked fine last summer)
I have searched in previous posts, but no luck on my specific problem. Alot of talk about relays and fuses, so I checked the relay, but I cant tell which fuse is for OD. Checked all fuses and were ok. Is there a fuse somewhere else or what?
Also which of the two sets of wires on the OD is the switch and where do I hook up a 12V jumper to test it?
Thanks in advance. :confused:

frank moran

does the dash light come on? check the gage fuse, if it worked before you more then likly have a wire off, is the speedo working, engine coming up to temp??? are all the wires intact under the car, trans?? etc.
at the first gear switch with the ing switch on you should have 12 volts...

scorp508

The most important thing is what Frank said, does the overdrive come on on the dashboard? If it does I would put a multimeter on the overdrive solenoid connector and monitor for 12v when that little light on the dash comes on.

30th b-day

OD light does not come on, But everything else on the dash and info center does. Dont Know where the first gear switch is or the solenoid. I have a haynes manual, but it doesn't say much.

scorp508

Correct me if I am wrong, but I think 87/88 are the only years to use a 1st gear switch. I have a 4-speed unit from an '86 in my car now and I had to swap the sideplate from my '87 unit onto it.

I'll have to look at the wiring diagrams, but I can't remember at the moment if the overdrive light is driven directly from the ECM or if it goes active when the solenoid is powered and sends a feed back to the display.

If you could get a scan tool onto the car you should be able to see if the ECM is getting the correct coolant temperature information. Believe it or not there are 2 coolant sensors. One strictly for display on the dashboard and one that sends the degrees to the ECM which is the one it'll monitor for overdrive engagement. 1986 has to hit 122* (F) before the overdrive will engage.

30th b-day

I ran the car up to 170F and the Od still wont engage, and no dash light. Don't know which is the solenoid to test with a multimeter. :confused:

scorp508

They are next to each other under your wiper motor. One is the fuel pump and one is the overdrive. Unplug one. If the car won't run then you know its the other one. :)

30th b-day

Thanks scorp. I jumped the wires on the two ends of the relay, and the OD came on. Took it for a test run and it was engaged. I am assuming this means that the relay is shot. Thanks for your help!!