I have a 91 turbo and I've replaced the ECU, but I want to keep A/C but bypass the standalone. It was working before, but afterwards its not. I've jumpered the L/W and L/Y wires as others have done, but still nothing. I noticed that in the wiring diagram there is a 3rd wire that goes between the logicon and ECU, which isn't present on the S4 cars, that is G/R. If I'm reading it right, that should see 12V as well. Is that correct, or is there something else going on?

 

Your tuner should have zero issue integrating this with the ECU. It's an Input Switch (from the center console) and an Output Switch (to the compressor).

Here's how it is on the FDs and it's very similar on the FCs:



Reach out if you need anything.

 

There is an input to the ECU to request AC and an output to control the compressor clutch. There is also a thermoswitch and a pressure switch. The thermoswitch will open and turn the AC input request to off if it senses the evap core is icing over. The pressure switch will open and turn off the AC request if there is low charge pressure. Bypass the pressure switch to test function of the system. If it works, you need the charge the system. If it doesn’t, bypass the thermoswitch as well and test. The thermoswitch usually doesn’t go bad though.