Fans and AC don't wanna mix
 

This is driving me crazy. I have a Vintage Air system and I'm trying to hook up the trinary switch to my fan relays. My PCM sends a negative signal to the fan relays to turn them on at the set temperature. I've spliced into those wires to send a negative signal when the AC gets to the set head pressure but the truck doesn't like that at all. It's actually killed the engine. As it turns out, I've got 12 volts on that leg (PCM to fan relay) until the PCM signals the fans to come on and then the leg becomes a ground. The PCM isn't putting out 12v on that leg, I'm sure it's coming from the relay, but I can't figure why it will take the ground signal from the PCM but not from the trinary switch. Any ideas? Here's the fan hook up diagram, with the PCM taking the place of temp sensor:

Re: Fans and AC don't wanna mix
 

Are you running the PCM control to 85 or 86 of the relays? if it's 86 and the relays are diode protected you may be getting the 12 volts from the relays through the diodes.

Just a thought.

Re: Fans and AC don't wanna mix
 

Well.. The PCM is on 86. I reversed 85 & 86 and had the same result. Tried a couple of other relays and the same thing. I'm getting 12v on that leg which doesn't make sense to me, but I hooked it up and it's working now. Before I soldiered it together, I was just touching the wires together to ground it and it sparked and killed the engine, which really confused me, but it seems to be working like it should now 🤯

Re: Fans and AC don't wanna mix
 

The inner workings of the vehicle computer is a little above my pay grade so I would have to say that the polarity change you mentioned is hard to comprehend unless they use gated diodes or logic circuits. The bottom line is that it is now working for you and congratulations on that.