I installed a new compressor in my '99 Blazer (4.3L) and now my clutch won't engage. I can bypass the relay and get the compressor to engage and I know I charged it with enough Freon (Low pressure = 35 psi, High pressure = 190 psi) outside temp is about 85* F. I bypassed the relay that's how I can get the pressures to read on my gauges.

My question is related to the Low Pressure Switch on the Accumulator. When I jump the connector with the relay installed, nothing happens. The clutch doesn't engage. I checked to make sure the connector was okay by peeling back some of the insulation on the two connector wires and then jumped the exposed wire and I still can't get the jumper to engage the clutch. Seems that I have something wrong with the wiring and not the connector. Any ideas? Where do those two wires on the connector go to so I can continuity check if they are broken somewhere?

 

More background info. Compressor failed and I had a leak prior to that which pointed to the evaporator. I couldn't get the old Red high pressure switch on back of old compressor off so I changed that too. In total, changed both fuses, relay, compressor, high pressure switch on back of compressor, evaporator, accumulator, condenser, 8 oz of oil, and 1 lb. -12 oz of R134a Freon.

New testing: I put the low pressure switch connector back on the low pressure switch and jumped the connector for the new high pressure switch located on back of compressor. The compressor clutch did turn ON.

I checked the part that I got from O'reillys and it looks like it might be the wrong part that came in the right 35974 box. My original high pressure switch was Red and the 35974 should have been a White Switch but the one in the box and now installed on my compressor is Blue. See the instructions that came in the 35974 box below.

According to the O'reilly webiste the Blue high pressure switch is a 35973 and has the pressure settings On Pressure (psi): 51 psi and Off Pressure (psi): 44 psi. The White 35974 has the pressure settings On Pressure (psi): 200 psi and Off Pressure (psi): 430. So my question is what pressures did my compressor with OEM Red switch originally have to engage the clutch at the proper pressure? I could just change the switch to the White one but that's a complete recharge of Freon so I want to check my thinking first.

Thanks in advance. This is driving me nuts.

 

The climate control module makes a request for compressor out of pin D8 on a lt blue wire through the compressor mounted high pressure switch which is normally closed. Then on a DK grn/wht wire to the vcm on pin c3/25. The accumulator cycling switch is grounded on one side on a blk/wht wire and then heads the the vcm on DK grn into pin c3/30. The call for compressor relay leaves the vcm from c3/1 on DK grn/wht and goes to pin 1 of the relay.

the high pressure switch opens at pressures that exceed the safe operating limit of the compressor. So closed at rest and normal operation but open Above the high limit which is usually 3xx-4xx psi. The compressor specs should tell you that limit.

george

 

I've had a similar issue with my 99. I replaced the whole system last year and I checked the pressures and when it did run they were close to spec (on the edge of the values) and when it ran it gave cold air. But it won't turn on. I'm annoyed that I don't have time to look at it at the moment.

 

Latest update, still puzzling me.

I checked and rechecked everything. And I changed everything so I still couldn't figure out why the compressor wasn't kicking on. So I thought that I'd check to see if the high pressure switch was bad, didn't want to do that because that meant losing all the Freon. Anyway, when I pulled off the high pressure switch, the hole that it came out of was completely dry. I expected to see a little oil/Freon seepage but it wasn't there. So I figured that maybe the brand new Delphi compressor might be bad. I took it back and installed another new compressor and now the clutch engages. Very surprising that a brand new compressor turned out bad. So much frustration on this one.

I finally just gave in and bought the GM OEM part for the high pressure switch and it's Blue (go figure). I didn't know if that would make a difference. I had it apart and didn't want to change it again.

Anyway, my pressures seem a little off. I have exactly 1 lb - 12 oz. (28 oz.) of Freon in the system and I have over 225 psi on the high side. My low side pressure is just below 40 psi. Temp outside is around 88 degrees and 60% humidity. AC is cooling to about 46 degrees. What should normal pressures be reading?

 

So, I found this attached presentation about how the AC system works that includes the pressures.

Here is also a nice manual that describes the pressures. https://www.ariazone.com/wp-content/...ing-manual.pdf

I have a 1999 service manual but I have not scanned the diagnostic pages for the AC system yet. If I have time, I'll try to post them soon.

 

Heres a guide:

AC Pressure Chart | R-134a Temperature Pressure Chart


George

 

Thanks. Looks like I'm within the range.

 

So does replacing the pressure switches on the AC system require removal of the refrigerant? I have a 1999 GM service manual and it makes no note of doing that for the High-Pressure Cut-Out Switch nor for the Pressure Cycling Switch. However for other AC service, such a hose replacement the manual will (not surprisingly) mention recovering all of the refrigerant.

 

The pressure cycling switch on the accumulator has a schrader valve so you can do a live swap but go slow just in case. I can’t guarantee that every version is made this way.

ive never done the high pressure switch just the whole compressor but I believe that swap requires recovery.

George