Fans and AC don't wanna mix

[GM man]

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: