Compressor won't engage and cool. Mechanic checked system and said ac control head is bad and it cost like 800.00. But when I go and search the part they are like 150-200... Did I miss understand what part is called. He said he checked the compressor with a scanner and thecompressor will not communicate with the head...said he thinks compressor shrted out the head. He also jumped the compressor but it still would not work...any ideas? Any clue what part I should look for? Thanks

 

Seems wrong..... Not sure what he meant when he said he "jumped" it, but to me that says he either supplied power or ground (or both) to the clutch, and if it STILL won't engage, that sounds like the clutch is bad. Hard telling what someone else did though... Many A/C systems use the PCM to ground the clutch, could it be that the the PCM (powertrain control module) has gone bad, not the climate control system control head? What year is your truck, engine, manual or electronic climate control, etc.? And I'm lost as to the compressor not communicating with the control head; there is no "communication" between the control head & the compressor, just a power wire in most cases and either a body ground or a PCM ground.

 

xI have a 2004 f150 4x4 5.4L. I think I over charged the system. Mechanic said I did, and I may have, and he said "this could of shortes out the control head" He said that the compressor wheel turns, he saw no sign of metal savings and he ground it with a power cord tool and there was no signal, and he could not know if the compressor was bad without replacing what he termed the ac control head. The heater works and all the fans and ducts work...the compressor just wont engage. 150K miles on engine.

 

Something seems odd here but I'm tired and my computer at home doesn't like to access manuals and wiring diagrams. Tomorrow when I'm supposed to be working but feel lazy I'll have a look to refresh my memory on the wiring for the a/c system, I'm almost positive the compressor is powered through the cut-out switches and is grounded by the PCM.

 

OK Thanks!

 

OK, if you have a manual climate control system (three ***** on the control head in the dash) the compressor clutch uses a body ground and is powered through the a/c clutch relay, which in turn is powered on or off by the PCM. SO, in theory, all that should be missing is a power signal from the relay (IF, and this is a big if, the ground is in good condition) which might be related to overcharging (if you in fact DID overcharge it.) There are two cut-out switches that must be closed to send a signal to the PCM; one is a low pressure switch which opens when system pressure is too low, and the other opens when pressure is too high. If either one of these switches is open, the PCM will not ground the CONTROL wire for the clutch relay, which, finally will not close the CONTROLLED side of the relay to energize the clutch.

The system works ESSENTIALLY (but not exactly) the same way if you have Automatic climate control. In any event, if your mechanic powered up the clutch and it still wouldn't engage, sounds like a bad ground; if he supplied BOTH power and ground to it and didn't work, it might be a bad clutch. Or maybe it's just not getting power due to... many different things. Next step is to find out "What's missingower or ground" and work from there.