JohnC485

Bought my 85 with a full service record and all papers just recently. It has 91,500 miles on it (I put 1500 miles on it) and the overdrive didn't want to work on the way home. Randomly, one day, it all worked fine and continued to work. Just recently I'm having an issue with it. The light won't go on and OD won't engage, but if it does it'll be after driving for a while and it'll magically come on after shifting. Downshifting into first gear makes the light go off, but it takes longer than it used to.

1. New fluid and filter.
2. Reverse works.
3. When shifting back to first then into second it's a crap shoot as to whether or not I'll get overdrive.
4. Checked and cleaned all electrical connections.
5. Overdrive light does not appear on dash.
6. New Relay
7. OD switch on the stalk used to work flawlessly.
8. I only really used OD for fourth, 1, 2, 3, 4, clutch, OD
9. It had no slipping issues before.

I'm thinking gear switch. Where is it? Though, what role does the fourth gear switch play with the first/second gear switch? Is there a part number for this thing?

frank j. moran

It sounds like a first gear switch which is on the side of the trans near the front shifter arm. But, before you do anything get the factory service manual, it has 2 very specific flow charts on trouble shooting the electrics on this unit. I dont have them but have used them in the past. I had a similar problem and it was the 1st gear switch on an 84, which has different electrics than teh 85. Follow the charts and it will take you to the problem.

JohnC485

Would anyone have access to said chart? Better yet, what wires can I jump in either the relay or in the system so I can put it in od anytime I want and say f*** the ecm?

vetteoz

On 1/2 selector shaft on side cover



85 doesn't have a 4th or 2nd gear switch so if you have them it is a trans or side cover from a 84
2nd and 4th switches are only inputs to the ECM which in 84 was different to yours
so it probably is not looking for those inputs
Are those switches wired up ?

Indicates no power out of relay to O/d; relay is not being operated by the ECM
diagram for later model ( different O/d switch but wiring to O/d the same )
.1st gear switch not shown

vetteoz

As per diagram above,
disconnect the tan / blk relay control wire at the relay and run it to ground through your O/d switch.
Full manual control ( assuming relay / wiring / 1st gear switch working correctly )

Or supply fused 12V supply to pin F on the ALDL connector and bypass the relay altogether by joining the brown and green wires @ relay together

JohnC485

Both of these options wil retain normal operation through o/d switch? What's the easiest way to get 12v to F?

Before that: how do you diagnose the first gear switch, or bypass it? Are you sure 1985 didn't have 2 switches, thought I saw it but could be wrong.

Ill recheck wiring and everything Monday, do the fixes etc.



Also: it came on about 10-15 minutes into a small trip and to my knowledge OD should disengage as soon as you go into first but it wouldn't if I was rolling in the slightest.

jhammons01

Well, before you go crazy wiring things up, you state that you have new fluid and filter.

You had the pan off, did you check the clip on the pressure switch? Did you check the pressure switch itself?? Have you gone under there to make sure the fluid is not leaking out?

Also, you claim the that OD Switch works, mine is old and crusty and when it acts up, a few on/off cycles cleans off the terminals.....pretty soon that won't work with age. Maybe yours (different than my '84 but same principle ) is getting old.....I mean, you state the light goes out etc. Could be just the switch on the shifter....

87FIREREDRAGTOP

Do you have the double relay setup next to the firewall? My 87 had the same problem and it turned out to be the wiring harness had bare wires that were shorting together from the relay down about 3 inches. Repaired that and it worked great. (not a great setup).

vetteoz

Answers here
http://www.5speeds.com/43table.htm

glava2876

I don't know if the 85 is the same as my 84, but I had the same problem a couple of years ago and it ended up being that the thermostat was stuck open and the coolant has to reach like 185f before it will allow the OD to come on. So when it was a cold day it would take forever to come on or would only engage after I had been driving slow and sometimes on a cold day and I got up to speed the temp would drop the OD would kick out.
Just another easy one to check.

jhammons01

^^Yup.....I don't know why or how....but mine was at 160° F.

At least that is what my gauge reads......So, I put a stock temp Thermostat in there and it cured that issue all together.

Before, when my coolant was thick and brown from years of the PO neglect, I am sure it was having cooling issues as they reared their ugly head with me.

A new Water Pump, and "New" radiator and a system that was filled with Distilled water and glycol and the thing no longer over heated......imagine....

Anyway, once I addressed the cooling system, going down the highway in the winter was hairy....it would just kick out of O/D....CentralCoaster helped me diagnose the issue and sure enough....someone had a too cool of a thermostat.....$15 later and the issue was gone.