How do you know you can't do something if you haven't tried


I find myself still stumped with the AC wiring to the ECU.

i already have a fan override relay that will kick the fans on when i ground it out. What i need now is the PCM to disengage the compressors clutch above a certain RPM. I believe the PCM will also slightly increase idle while the AC is on. (not sure if that is true, i read it on a corvette forum)

I found the PCM pinouts for my factory C5 Corvette ECU.

As i mentioned (almost a year ago) in this thread these are the pins that i think i need.
C2 Pin 14 A/C Refrigerant Pressure Sensor Signal 
C2 Pin 17 A/C Request Signal 
C2 Pin 18 A/C Compressor clutch supply voltage

But it looks like these pins may also be involved.
C2 Pin 43 A/C Clutch relay control 
C1 Pin 45 A/C 5 volt reference for Refrigerant Pressure Sensor 

So here is what i am currently thinking.

  • There has to be a way to program the PCM to ignore the pressure Sensor, I shouldn't need it as i believe that the Trinary switch does this already. This means i can ignore C1 45 and C2 14
  • I believe the request to turn on the AC compressor should go to pin C2 17 (my guess is pulling this to 12 volts tells the PCM that we are requesting the compressor to turn on)
  • I believe the C2 43 grounds out when all of the conditions are positive and the AC clutch should be engaged. I think this should be fed into a relay that drives the clutch on the compressor, or maybe its goes high to 12 volts when conditions are met?
  • c2 Pin 18 could help drive that relay above but is not required, i could just pull IGN +12 instead.

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