Air conditioning controller wiring

[neiltee]

I have a '75 Chevy K10 and had a new wiring harness installed by a guy that's no longer around (I know, my bad).
The fan switch wiring must be wrong because all speeds blow very minimally. I installed a new fan motor and it seemed to help a little. Before I start disassembling the dash, I would like to have a wiring guide. Anybody have some help with that?

 

[neiltee]

Oh yeah, the highest switch setting used to go on maximum when the A/C was turned on but no longer works at all.

 

[gmbellew]

Oh yeah, the highest switch setting used to go on maximum when the A/C was turned on but no longer works at all.

I think there is a relay involved. and maybe a resistor. assuming the system is similar to my 1990 burb. I'd check those out before i dug too much further.

 

[75gmck25]

The high speed is controlled by the blower motor relay, so it does not go through the resistors used for the lower speeds. It goes on high speed using the blower switch, or if the temp slider is moved all the way to far cold. You can feel the slight detent as you move the temp slider to the extreme left.

Original design - The high speed blower power is provided by a separate fused wire that runs across the top of the firewall from the junction near the brake booster to the blower. That wire has constant power, and a separate 30 amp inline fuse. In my ‘75 the fuse holder was flaky and it kept melting. I replaced it with a new fuse holder and it’s worked fine ever since. Maybe they left that constant power line out when they installed the new harness.

The original GM wiring diagram is posted on this site. Not always fun to read, but the information is there.