1999 Dodge Ram 2500 Diesel.
Clutch stays engaged when switching fan off, air conditioner off, or in vent only mode. However,when the ignition is turned off and back on with fan off, air conditioner off, or vent only mode, the clutch stays disengaged as expected.
I have done the following:
Vacuumed and recharged system.
Pressures check okay
No leaks found
Swapped relays

 

Is the clutch just sticking?

If you turn the HVAC system completely off, or set it to heat, does the compressor disengage?

 

It does not disengage if I shut HVAC completely off or set to heat. The only way to disengage it is to turn the HVAC off then turn the engine off and then to restart the vehicle.

 

Sticking relay?

 

I swapped the relay with another and the problem persisted.

 

Perhaps there is another relay besides the compressor relay that controls the fan/ac switch??

 

Not that I am aware of. Setting your HVAC controls in certain spots "requests" A/C from the computer. The computer energizes the control circuit, and if pressures are right, triggers the relay.

According to theory, when you turn Off the A/C, the PCM should notice that, and kill the control circuit. Think I would test that particular theory first...... make sure the control circuit IS getting turned off, when you turn off the A/C.

 

If I understand correctly, I can test the the feed to the relay at the relay socket. I'm not sure what contacts to test and what voltages to expect, I do know that it would be the three smaller contacts.

 

Terminals are labeled on the bottom of the relay. (generally)

Pin 86 is power feed from the fuse block on the side of the dash.

Pin 85 is the control side of the circuit going to the PCM. PCM grounds that wire to activate the relay.