Aftermarket AC on an LS with stock harness & ECM

If you are running AC on your LS swap with the stock LS harness and GM ECU and are NOT using the stock GM compressor, there is a good chance the engine will idle poorly. This applies to almost any aftermarket or pre 95ish AC compressor.

Stock LS GM AC compressors used variable displacement pumps and used both a high and low side pressure switch. The high side switch likely does not exist on your swap chassis so you don't want the ECM to control the compressor. You do want the ECU to know the AC is running though.

You should connect AC desired signal from the chassis TO the ECU to command the idle to increase and fans to turn on. Pretty simple, it's just 2 wires and a little bit of tune adjustments.