Last year my 1993 5.0 V8's AC would momentarily start then shut off. I had the remaining Freon drained, put a vacuum of 30 inches on it and then did an R-12 to R-134a retrofit on the system. A/C ran great afterwards. Turned on the A/C this spring and the compressor no longer turns on. I'm wondering if the the compressor died because of the R-134a lubricant 'mixing' with any left over R-12 lubricant in the compressor?

 

There is a low pressure switch that will prevent the compressor from coming on if it's low on refigerant.

 

I tried refilling system with R-134a to just under 45 psi on the low side with the A/C set to Max and the compressor failed to kick in.

 

Are you getting electric power up to the compressor?

 

Did you check the gap on the clutch? If too much the electromagnet won't have the power to pull the clutch to the compressor pulley to engage the compressor

 

when you did the conversion did you flush the system, or just put the "ac killer" kit on?

you said yo you refilled the system to just under 45 psi. are you using the cheapo one side gauge, or a manifold set? to properly charge an AC system, you really need a manifold set to see what the pressures are on both the high and low side.

 

I disconnected the power plug to the compressor, started the truck, then the A/C. There was no DC voltage at the power plug.


I don't know how to check the gap in the compressor clutch.

 

Check your wiring for breaks, and also fuses

 

I replaced the A/C compressor fuse with a new one. I again checked for power at the compressor and still no power. Following the wiring looks like an impossible task. Wondering what else might prevent power from being send to compressor.

 

Have you also checked the relay? You can temporarily jumper the clutch wire to +12v to see if it engages.

 

If you charged it to 45 lbs on the low side and the compressor was off, that does not tell you anything about the state of charge. That should be enough to close the low pressure switch, however.

 

Disconnect the low pressure switch and put a jumper across the wiring harness terminals, then check for voltage at the compressor again. If it works then, then you have low refrigerant or a defective low pressure switch. If it still doesn't work, you have a wiring problem.

 

what is the high side pressure?
you may have a blockage in the system that turned the high pressure switch off.
just reading low pressure is useless when trying to diagnose a problem with an AC circuit.

 

After finding no voltage at the compressor, I verified the low side was above 30 psi and disconnected the plug to the sensor on the accumulator. Using an ohm meter I checked the resistance across the terminals on the sensor (not the plug) and found it 'open'. This would appear to mean the sensor is no longer working as I would have expected to read zero resistance if the sensor detected the 30 psi pressure. I apparently need to replace the sensor on the accumulator but am wondering if I have to drain the entire system of coolant before removing and replacing the sensor?

 

You know I've never replaced that switch when I had a system charge. I would assume its not a schrader valve so it will vent refrigerant if you take it out.