A/C compressor

[MMeier]

Are the compressors controlled by switching the ground... in other words, always hot, can't find my darn test light to check myself. #@$%!

 

[B.G. Smith]

The AC clutch is operated by a relay in the PDC (+12V), the relay receives it's signal from the PCM when the AC switch is on position and the clutch cycling switch contacts are in closed position. bg

 

[MMeier]

The AC clutch is operated by a relay in the PDC (+12V), the relay receives it's signal from the PCM when the AC switch is on position and the clutch cycling switch contacts are in closed position. bg

So, the one lead is not hot all the time going into the compressor itself, correct?

 

[B.G. Smith]

So, the one lead is not hot all the time going into the compressor itself, correct?

I am relying on a 99 wiring diagram, it shows a ground to the clutch coil with 12V coming from the relay? bg

 

[MMeier]

What I am thinking of doing is wiring in a switch to open and close the ground side... . if indeed it is controlled by switching grounds. reason being is I am getting the P0647 code... AC clutch relay voltage hi... WTH that means? Don't want to but a $400 compressor if it is not at fault, apparently can't buy just the clutch itself anymore. Anyone else?

 

[Pawpaw]

You can download a service manual, complete with wiring diagrams, from here:



http://www.hemitruckclub.com/



Look half-way down the left frame.



Bob

 

[rbattelle]

So, the one lead is not hot all the time going into the compressor itself, correct?

That's how I read the manual for 2003. Compressor clutch relay provides +12v controlled by the ECM (connector C2, pin 41).



So it's NOT ground-switched.



Does that help?



Ryan

 

[MMeier]

That's how I read the manual for 2003. Compressor clutch relay provides +12v controlled by the ECM (connector C2, pin 41).



So it's NOT ground-switched.



Does that help?



Ryan

Just went down and bought a cheapie test light. No, it is not hot all the time, so does anyone see any possible issues of me just going and wiring in a toggle switch, or will that throw more codes by having that one wire open and not connected to anything?

 

[B.G. Smith]

What I am thinking of doing is wiring in a switch to open and close the ground side... . if indeed it is controlled by switching grounds. reason being is I am getting the P0647 code... AC clutch relay voltage hi... WTH that means? Don't want to but a $400 compressor if it is not at fault, apparently can't buy just the clutch itself anymore. Anyone else?

Relay voltage high????? The voltage from the relay would be whatever voltage the battery is at wouldn't it? I would think that if something were not right with the compressor clutch you would get a high resistance reading or maybe pop a fuse but "high voltage"? Maybe the relay is operated at less than battery voltage coming from the ECM??? Just wondering. Maybe we have a tech on the forum who could enlighten me/us?? bg

 

[rbattelle]

Just went down and bought a cheapie test light. No, it is not hot all the time, so does anyone see any possible issues of me just going and wiring in a toggle switch, or will that throw more codes by having that one wire open and not connected to anything?

I was thinking about this when I wrote my last reply. In theory, there's nothing wrong with wiring yourself a manual toggle switch to kick on the compressor manually. However, I feel like it might set codes, and of course you'd have to be careful not to overpressurize the system.



I assume (big assumption here) the ECM monitors A/C coolant pressure and actuates the compressor clutch to maintain some certain range of values. If you manually actuate it, it's unlikely you'd be able to maintain the proper range, which would cause the computer to complain.



The PDM says P0647 is set when:

... the signal from the A/C clutch output does not match the signal from the A/C clutch feedback.

Not much help there. But here are the possible causes:

Loose connection

Fused battery supply

A/C clutch relay

Intermittent condition

Bad ECM

Of those, I'd say go get yourself a new relay and see if that cures the problem! Looks like a fairly standard automotive 12v relay, so it should be very inexpensive.



Why do you think running a manual toggle will fix the problem?



B. G. - the clutch is definitely 12v.



Ryan

 

[MMeier]

Ryan, I've used toggle switchs before for this same scenario, but being the tempramental things these trucks are, I'm not sure I wouldn't be creating more problems then I'd be solving. Going to my buddies shop tomorrow and use a scan tool to troubleshoot more. Anyone know if the A/C clutch is available seperately, and if so, how much of a pain it is to change out... . needing special pullers and such? Your second qoute above is what has me thinking the clutch is at fault here.

 

[rbattelle]

Your second qoute above is what has me thinking the clutch is at fault here.

But notice the PDM never says that a defective A/C compressor clutch is a possible cause of this code!



Ryan

 

[MMeier]

But notice the PDM never says that a defective A/C compressor clutch is a possible cause of this code!



Ryan

True, but like someone told me the other day there is a hold up coil... or something to that effect in the clutch itself that may be affecting the voltage in that circuit. sag2 where are you????