I have a 2000 Blazer with an AC pump clutch that won't engage unless hot wired. I have read many posts but have found no information on where to start troubleshooting. The gas charge is where it is supposed to be. I know the Hi/Low cutoff sensors control the pump, but don't know where they are or how to test to see if they are bad. Fuses are all good, and blower works correctly when the AC is engaged.

 

Automatic climate control or manual HVAC?
What is the low side pressure, (at the accumulator service port) with the engine OFF?
Does the dark green wire at the clutch cycling switch on the accumulator show battery voltage when AC is requested?

 

Thanks for the reply Captain, I value your advise:

I went to Autozone and used one of their pressure testers. It read about 32 PSI on both Hi and Low Side. No change when engine started.

Manual HVAC.

There is no battery voltage at the pump. The electric clutch does work correctly when hot wired.

The AC worked last year.

 

Your system is low on charge. The clutch cycling switch, (on the accumulator) will not activate the clutch if low side pressure is below ~42psi. Add some UV dye, locate & repair the leak, (very common for the compressor to leak where the case halves come together). Drain the system and evacuate it for a minimum of one hour, (mandatory). Recharge with the proper amount of R134a.

 

Thanks Captain: I was using incorrect information when I said it had a full charge based on an AC Pro gauge. When completed, what are the final reading supposed to be for a charged system? I have a second 2000 Blazer I would like to check.

 

Pressure test is not accurate for determining state of charge. On older systems using R12 refrigerant, the compressor was on all the time. A sight glass on the receiver drier was used for checking bubbles in the system. They needed to clear within 2 seconds after compressor engagement. On clutch cycling systems like these, there is no sight glass. The only way to know the state of charge is to drain, evacuate, and recharge with the correct amount of refrigerant.

 

The pressures for both the low side and high side are dependent on ambient air temps. You will need a set of AC gauges to get read your pressures. Those cans of R-134 at the parts store with a plastic gauge on them are not good enough. HF sells decent gauges for a decent price.