Automotive Air Conditioning Information Forum

For anti-slug GM briefly pulses the compressor clutch during cranking.

Are you aware of a vacuum operated bypass valves that would be suitable?

Four Seasons 74781, 5/8 and 3/4 hoses, apply vacuum to close. Stock app is 96-2000 GM trucks.


Obviously you already know you can't use the PCM to run the compressor, because it needs the feedback from the 3 wire ...

Good plan, PF condenser and retain the TXV and POA that everyone wants to toss in the trash for cycling crap. Would be better to let the stock PCM handle compressor control. With the stock pressure sensor installed It'll handle the fans, under/over pressure, idle compensation, RPM cutout, etc ...

working right now on the 2nd injection system and can not get this thing above 13 mpg on 4 buck gas. I am vacillating between an LS and a Hemi. In either case if I use the compressor that comes with the engine it should solve the compressor at idle issue.


Injection as in EFI? Fuel-wise the only ...

The round pressure switch on the back of the compressor. Put a test light to that and battery positive compressor clutch engages

Ah, ok, so exactly what a shop told you a few posts ago? Then you need a wiring diagram and figure out what's wrong,

Can buy short term access from Mitchell or ...

When I hook up test light to battery positive and touch compressor wire harness from back of compressor - the AC clutch will engage and I get cold air.

Need to specify which wire(s) you're talking about. Picture would help.

Just not sure what the smartest approach would be money wise ...

It's a switch, no polarity, so doesn't matter A vs B.

It's a lot harder without a real scan tool but here's a few basic steps

1. Verify the two 10A fuses in the underhood fuse panel for the compressor clutch and clutch relay supply are good. ("Ign E" and "A/C Comp")
2. With a/c requested from the HVAC control, verify +12 volts at high pressure cutout ...

The engine computer controls the a/c clutch on that truck, there's many things non-a/c related that will cause it to not engage the compressor. Assuming gauge on death can is ballpark correct you have enough pressure to satisfy the cycling and low pressure switches - if it's not engaging it's an ...

Fan should come on with the a/c request not pressure alone. If using two speed fan(s) can use high pressure switch to activate the high speed.

Ideally the fans should disable over ~30mph. The stock ECM doesn't control fans but if you're into tuning there are code patches and upgrades to allow for ...