After fixing the A/C in my van a couple of times, I thought I knew all the problems. But this one has me stumped because of the limits of the information available to me.

While driving down the freeway last week, the A/C decides to stop cooling. It would switch randomly get cold and not cold for a few minutes at a time, until it finally stopped cooling completely. So I start checking it today and find:

As the engine is idling, I hear the A/C clutch relay cycling on and off every few seconds. However, the compressor never turns on during any of the times that the relay was on. Normally, if the refrigerant was low, the pressure switch would disengage as the compressor sucks from the low side and pressure drops below the switch's trip point.

I hook up my gauges and see the refrigerant pressure at about 75psi whether the engine was running or not, since the compressor never engages. That should be enough to engage the pressure switch.

The coolant pressure switch is working; I pull its plug off, and the relay stops cycling.

The clutch is working; I jumped it with wires from the battery.

I pull the relay and test it, and it's working. The relay is actually controlled by the EEC to disable the compressor when the throttle is pushed wide open. So I disconnect the TPS, but that didn't do anything. The on/off cycling indicates that the EEC is sensing some condition and deciding that it needs to shut off the compressor. Each time it engages the relay, it wait a couple seconds before disengaging. Then it tries again a few seconds later.

A couple things I still need to check:

Is there voltage coming to the plug for the A/C clutch. My feeling is "no", as when I applied the voltage directly, the clutch engages. Of course, this test requires that the engine is running.

An EEC self-test to see if it detects anything strange.

If anyone else has experienced similar symptoms, please let me know. Like, if 75 psi static is too low. I'm still using a R-12 equivalent refrigerant, and I think the stock pressure switch switch point is around 30 psi.

And if you have wiring diagrams for the circuit that powers the relay, I'd appreciate if you would share it.

 

It sounds like your compressor is toast. From what you're saying, it has a mechanical problem preventing it from turning. The easiest fix for that would be to replace the compressor. Be sure the system is clean so contaminants from old compressor don't ruin the new compressor. Also be sure to pull a GOOD vacuum on the system before refilling it with refrigerant. If there's sludge and blackness in the old compressor, you really should flush the system to clean it up. You'll need to remove the filter-drier to do that. System flush is available from the auto parts store. Try to determine why your old compressor failed before swapping in the new one.

 

The actual symptom is that as the relay cycles on and off, no voltage is getting to the clutch. I traced the problem to the pressure switch. When engaged, it's supposed to be a dead short, but mine was showing 80+ ohms resistance. As I activate and deactivate it, I see its resistance jump around, so it's contacts must be dying. I installed the pressure switch from another vehicle, it measured 0 ohms when pressurized, and the compressor runs normally with it.

So I need to get a new pressure switch. I'm glad it's something simple and cheap.