01 Yukon A/C electrical mystery

[01SLE]

I've been struggling with this A/C for a year now. I put it on the back burner after last summer because I had a work around and had already put many hours and dollars into it. Summer is coming and it's hot in Phoenix.

The A/C didn't work when I received this truck in trade for rebuilding a transmission. The compressor was seized and destroyed.

I replaced the compressor, condenser, accumulator, rear expansion valve, and orifice tube. Flushed the lines, refilled.

A/C wouldn't turn on with cabin controls so I tried jumping the low pressure sensor and the other sensor on the drier (forgot the name of it) and no luck. Jumped the compressor at the relay and it turns on and it's ice cold.

So if I drove the truck last summer, I just jumped the relay. With a suspension/steering rebuild, oil pan, oil pump, etc last fall, I was tired of working on this damn thing and left it at that.

This year, I decided to replace the AC Heater controls in the cab because they don't seem to work right. Sometimes the controls would turn the blower on, sometimes it wouldn't. Sometimes it would switch hot to cold, sometimes it wouldn't. Plus it was an import unit in Celsius that annoyed me. So I bought a new one and installed it today and still no luck.

So I'm stuck. I understand the cabin controls send a signal request to the PCM to turn on the A/C compressor when you push the button. I understand the PCM only forwards that "ON" signal to the relay if certain conditions are met such as a positive signal from the low pressure switch and the other underhood safety switch on the drier (why can't I think of it's name?). But that's where I'm stuck. I don't know how or where to test the signals from and to the PCM.

Mechanically, the system is new and sound. It doesn't leak and it functions flawlessly with a jumped relay.

 

[01SLE]

I figured this out so I'll follow up on my own thread.

The sensor on the accumulator is called the A/C clutch cycle switch. I couldn't remember the name the other day.

My problem was the compressor mounted high pressure switch. It was bad. It's a closed switch so when it senses high pressure, it opens the circuit and the PCM shuts off the compressor ground signal. Or when it's bad apparently it's also open and your compressor won't turn on. It has a round two wire connector and it's mounted directly to the back of the compressor by a snap ring.

The differences I've found with the 3 sensors going bad (low pressure [line mounted], clutch cycle switch [accumulator mounted], and high pressure [compressor mounted]) are:

When your low pressure switch is bad or you have low line pressure, the system will usually try to turn on a couple times and shut itself off. Your A/C button light on the cabin controls will then flash and go out. The flashing A/C light on your controls is usually the tell sign of a low pressure problem.

When your clutch cycle switch is bad the system will compressor will turn on and off over and over.

When the high pressure switch is bad or open due to high pressure, the compressor won't turn on at all unless you jump it at the relay. The A/C button light on the cabin control will also stay constant on.

 

[01SLE]

Just to add: The switch I couldn't name that is mounted on the accumulator/drier is called the A/C clutch cycle switch.

 

[01SLE]

And to follow up on my own problem:

I found out it was the high pressure switch that is mounted directly to the back of the compressor using a snap ring. It has a two wire round harness connector.

If this switch is bad, you have a bad connector, or a break in the wiring, the compressor will not turn on at all and your A/C light on the cabin controls will stay on and not flash. This is a closed switch when pressure is below threshold and the circuit is complete.

To test it, unplug the connector and jump the harness side connector. It is open/closed so it won't hurt anything.

FYI: If you have a 3 wire connector on a different style compressor or vehicle, do not jump the harness. You could damage the PCM.