sk87

Ok, so I finally got my 4+3 OD unit replaced and it is working correctly. However, since it is winter, I frequently am driving on the freeway and my coolant temp will drop below 140 and the OD will kick off, then heat back up and kick back on, cool back down and so on. It's really annoying, so I would like to bypass the ECM and have manual control.

I have a 1985 early with the Overdrive switch on the console. I took apart the passenger kick panel to get to the ECM but there was no way in hell I was going to be able to get it out past all of the wires.

So my next option that I've heard is to ground a wire from the OD relay. First question, haha, Where is the OD relay!? What I've gathered, is you typically cut the wire going to the B terminal, run the cut wire to a switch in the cab, then from the switch to ground.

Second question, since mine has the constant on/off switch already on the console, do I just need to ground the B Terminal wire with no switch, and use mine like normal?

Is this the best way to accomplish what I need (manual control of OD not dictated by engine temp)? Thank you for the advice!

sk87

Happy Thanksgiving by the way... I'm back and forth between fiddling with my Vette and stuffing my face.

c4cruiser

I think the real thing to do is to fix is the coolant temp problem. 140 degrees is far too cool for the engine.

What thermostat do you have? I have a 180 'stat in my '87 and the car will always maintain coolant temps between 178 and 210 depending on the weather and how I'm driving the car. I was out in the car a couple days ago with the air temp around 40 and the coolant temp stayed between 178 and 180 in both 25 MPH city driving and out on the freeway at 70 MPH.

The OD relay is on the firewall in front of the fuel pump relay. Both are next to the windshield wiper motor.

DanZ51

Even with a cooler thermostat, your temp shouldn't drop that low. Replace it.

black88z51

I just switch it off from the switch on the shifter no big deal

sk87

Its a 195 degree stat as far as i can tell. If i let the car sit in the driveway running, I'll watch the temp rise to about 195 then drop ten or so degrees, then rise to 220 (i think) and the fan kicks on. But, like today, ambient temp is 53F and cruising on the highway at 70 mph engine temp never gets above 150.

Maybe the stat isn't closing all the way?

And thanks for the location, i figured it was one of the two.

sk87

I'm trying to keep the OD on, not off.

frank j. moran

Your 85 overdrive is designed to kick on at a minimum coolant temp of 140F, (84 mdl is 176) your first step should be to correct the low engine temp.
ps; if you have a fail safe stat it may be have failed in the open position. I have seen a number of them fail, get a standard thermostat.

sk87

Understood, thank you. Thermostat is now on my to do list, along with a good TB cleaning when i take it all apart.

I think i still want to bypass the ecm too, just for more control. Anyone have any insight on what to do with the relay?

vetteoz

If your setup is working correctly ,the O/d is always available when the programmed conditions ( inc engine temp ) are met unless you manually disengage it before driving off.

Cut the relay control wire ( tan /blk ?) to the ECM and run it to ground through a switch

sk87

What happens if i just ground the wire with no switch? What will my existing switch do then?

383vett

One of the first mods I made on my 84 when I bought it new 3 decades ago was to bypass the ECM and allow direct control of the overdrive using the overdrive switch on the console. It was relatively easy back then either grounding the switch wire to the of unit or supplying 12 volts. It's been a while. The do light on the dash still worked when the is was activated.

sk87

But am i correct in saying you didn't have to add another switch? I think that would only apply to the later years that had the momentary switch on the shifter, not for ours that have the constant switch on the console. I'm gonna give it a go tomorrow when i wake up from my food coma.

vetteoz

In that case , delete the relay and connect the relay power wire to the existing switch

AGENT 86

I don't have a schematic for an 85, but if my 86 switch was constant, I would cut the the wire from switch at ECM and cut the wire from ECM to relay and join them together beside ECM.
That would allow full control with no input from ECM.

Do you have a schematic for 85 OD ?

AGENT 86

Looked at some online 85 ECM pinouts.

A7 OD relay control- tan/black
C7 OD request black-/blue

May want to confirm if you have a an OD schematic.

383vett

I used the original toggle switch on the console. It's been a while so I don't remember how the wiring went. It was pretty simple though. Just bypass the ecm and go directly to the od solenoid wire.

sk87

According to this, that sounds about right to me.

https://wiki.bentleypublishers.com/d...g-1985%204.jpg

Can anyone confirm this? Last thing I want to do is damage the ECM by wiring something incorrectly.

powerpigz-51

When you bypass the ECM by running a ground switch to the relay, the switch (and you) essentially become the ECM. You just need to make sure that you cut or unhook the ECM wire from the relay. No risk of damage to anything. I do believe it is the "B" post on the relay. As far as the thermo, the fail safe will get stuck open. Mine did it several times until I cut the clips off that hold it open. I have an 85, and the ECM bypass is one of the best mods I have done to my car.

sk87

Did your 85 have the switch on the console or on the shifter?