As I was scrolling thru the 2000 FSM (gas engine 8W section), I was wondering what the PCM uses as inputs to control the cycling of the clutch relay, Or is it that the compressor runs continuously? Continuously does not seem to be correct. I did not see any expansion coil thermocouple inputs for freeze protection or ice build up.

 

One thing would be the pressure switch on the drier.

 

Pcm gets a signal from the ac/defrost control in the dash, The pcm will give the relay a ground for it to engage the compressor. Not sure if any or the newer ones are any different but other that the pressure switches the compressor will stay on. The pcm will shut it off during heavy load on the engine.

 

After driving the truck for between 15-20min, the compressor drops offline and never comes back. When this happens, it never occurs at an opportune time to pull over and troubleshoot.

I'm curious about your comment and was wondering if you could elaborate
While at idle with an ambient air temp of 88-90degF, the refrigerant LoP hovers at 50psi and the HiP 160psi. The control switches, pressure switches, and the compressor clutch appear to all be working correctly. Removing the PCM connectors, continuity of the circuits has been verified to be good. What is the 'DIY method' to diagnose if the PCM is faulty?

From the 2000 FSM:
8W-42-5
The compressor clutch is energized by the relay following the PCM providing or closing a GND connection on C3 pin-1.

8W-42-4
The climate controls contain (2) switches and (1) dial. The (2) switches provide a GND for the blower motor but also provide a GND to the pressure switches on sht 8W-42-3.

8W-42-3
The GND is provided thru the pressure switches to the PCM. The PCM compares the GND signals between the pressure switches (C3 pin-22 switch sense) and the bypass (C3 pin-23 select input). If the PCM sees no difference between these two GND, then the PCM provides the GND on C3 pin-1 for actuation of the relay on 8W-42-5.

 

It would be interesting to see what pressures look like when the compressor shuts down.

The fact that it doesn't recover after a bit kinda indicates either high pressure, or, something is getting hot and failing (relay), or, if it has a temp sensor on the evaporator core, it's freezing up......

 

Shutdown only seems to occur when the truck is moving, which is why I'm curious about the previous comment relating engine power and deenergizing the relay. I had compressor engaged and truck idling for nearly 2hrs, every 20min running the rpms upto 2k for 5min to increase some airflow to bring the engine coolant temp down slightly. The onetime I was able to pull over to check pressures, by the time I had gotten the gauges on they had equalized, approx 100psi both sides.

 

Wonder if its over charged.....