BCM climate control electrical help

[rodnock]

I just picked up a 1990 SHO that has been sitting for a while, and the blower motor would not come on. I pulled the blower motor out and retrieved a dead rat and the remains of his 5 star hotel he had built. Anyway, I checked the motor by straight wiring it, and it runs fine. I went to the junkyard today and pulled every BCM I could get my hands on, which ended up being 3. Two are out of 1989 cars, and one out of a 1991.
After trying all 3, the fan will still not come on. The manuals I have dont have an accurate wiring diagram of this set-up, so Im hoping someone on here might be able to help.

First I want to make sure I have this right. From the way the BCM acts, it seems that it only sends low to midrange (0-9) voltage to the fan, but when I manually put the fan setting on high, the BCM contact switches (clicks) and then the voltage source wire becomes a ground source, and the main wire coming from the dash unit would send 12v to the motor.
Im guessing this, due to when the BCM contact clicks, the wire that was sending 8 volts, drops to zero. It then shows continuity to the chassis ground.

When the BCM is sending say 8 volts to the motor, the motor doesnt have a ground source. The wire that should offer a ground shows no continuity to the chassis.

I guess Im wondering if I need to replace climate control unit in the dash due to its not providing a ground for the blower motor. Any info would be greatly appreciated.

 

[rodnock]

In manual mode, and blower motor unplugged, and the climate control fan speed set at low, the orange wire from the BCM to the blower motor shows 8.9 volts.
As I increased the fan speed dial, the voltage going to the blower motor decreases (medium speed about 4 volts etc..).
When I get to high speed, the voltage hits zero and the BCM clicks. The orange wire that was showing voltage will now show continuity to the chassis.
The black wire that comes from the blower motor and heads toward the climate control unit will not show any voltage ever, and it has no continuity with chassis.
Maybe this will be more understanding than what I wrote above.
If I could see a wiring diagram of this actual system, and not the generic haynes crap, I could probably figure it out.

 

[hawkeye18]

 

[rodnock]

Thank you very much.
Mike

 

[rodnock]

Hawkeye18, thanks again. The diagram made it clear, and I have it working now. 2 of the 3 BCM's I picked up at the junk yard were bad, but for $10 each, I cant complain.
Anyway, it helped me trace down a chewed wire thanks to the rat that rented out the blower motor housing for the last 5 years. My original BCM is also still good, so now I have a spare.
One problem fixed, and countless more to dig into.

 

[hawkeye18]

So the rat was chewing through the ground wire then? I have to admit, I got about as far as "need wiring diagrams for BCM" before I got a headache lol...

But I'm glad it helped! :wave: