AC Compressor Does Not Engage?

[CedarGrove]

I checked the fuses just inside the driver's door and the relay under the hood. Both seemed fine. The light on the ac button in the cab lights up when I push it, but the compressor clutch does not seem to engage. Checked 2 plug-in connections to the compressor and they both seemed fine. What else may cause the clutch to not engage and what other diagnostic things can I do to narrow it down

[DmaxMaverick]

First, eliminate the pressure switch. Unplug it and short the contacts. Temporary only! Don't run the A/C if it works. If it engages, the switches can go bad, and are relatively cheap. Just replace it. If that doesn't work, you need to check the freon pressure. If you have a leak, it will bleed of until the pressure is too low to trip the switch. A really slow (undetectable) leak can put the pressure just under the envelope. The leak may not be large enough to need addressing. If it takes 5 years to leak down, I wouldn't mess with it. Just get it charged and go on. If it's an R134A system, you can do it yourself with a Walmart fix-it kit on the cheap. R-12 isn't so easy, and may require a full service to get it up again (unless you know an A/C guy that can throw in a few ounces).

[CedarGrove]

Jumping the pressure switch did not engage the compressor. Check the charge pressure now?

Thanks.

Lewis

[ronniejoe]

You also need to check the wiring from the underhood fuse relay center over to the low pressure switch. I just had to repair an open in that circuit this spring on my truck.

[Hubert]

Will it engage at all? During defrost, fan off and A/C button pushed in or anything wierd? How are the other functions Mode (vent selection) and Fan? I just fixed my A/C and it was the control board unit. But thats just my case. Found it diagnosing the mode actuation motor.....

First check is it getting the signal to engage ie 12 V on the green wire at the compressor with A/C on? Then you know if its control circuit (somewhere could be lots of things) or clutch issue. Then there are other flowchart checks to see if A/C clutch relay is good and pressure switches to check.

Do you have the helm manual or alldatadiy subscription?

[rustyk]

Jumping the pressure switch did not engage the compressor. Check the charge pressure now?

Thanks.

Lewis

You won't be able to if the compressor won't engage. Static pressure won't tell you much (unless it's totally empty), as the Reed vapor pressure is governed by temperature, not volume.

Shorting the pressure switch eliminates it as the culprit.

If it were I, the next test would be to check voltage at the plug on the clutch. If that's ~12V, then jump the clutch electrical connector (+12VDC and Ground) to test it.

[CedarGrove]

It won't engage at all. The other controls seem to be fine.

I remember last summer when my water pump failed...I believe the computer shut off the compressor when the coolant temp spiked. I was just wondering if it is possible that there is a sensor somewhere that has failed and the computer has shut down the compressor. I know this is way out there but it's on my list of possibles.

I have a Haynes manual that has some wiring diagrams so I guess I'll start there and see what happens.

I'll check for 12 V on the green wire at the compressor with A/C on and give you an update.

Probably unrelated...Truck has been randomly stalling lately. Also experiencing random "fishbite" type stumbles. I'm pretty sure it's a FSD issue. The new module will be here tomorrow.

Thanks for the hand!

Lewis

[CedarGrove]

There does not seem to be 12 v on the green. I have 3 pairs of wires going to the compressor and they all have a green in them. First pair is up top by the clutch. This is the one that I checked and there was no voltage. Next pair is up top at the rear of the compressor. Last pair is down low at the rear of the compressor.

Looking at my wiring diagram, I think I can trace it. I'll take my book and meter out their sometime later this week and probe around andlet y'all know what I find.

Thanks again for the input.

Lewis

[CedarGrove]

"If it were I, the next test would be to check voltage at the plug on the clutch. If that's ~12V, then jump the clutch electrical connector (+12VDC and Ground) to test it."

Good call. I'm putting that on my to do list.

Thanks,

Lewis

[rustyk]

BTW, a lot of us oldsters use this technique for tracking down issues (electrical or mechanical): Check first what's easiest to get to!

[Hubert]

I believe the PCM does supply a ground to the compressor relay switch on the diesel. Not sure if any codes will prevent it from working??? Does your cruise control work that might be an indicator?

The flow chart to diagnose the wiring, clutch, pcm ground, relay, and/or control unit says to check several pins with either a self powered test light (continuity check) and some with a regular test light.

Several switches have to signal ok for compressor to engage. If your haynes manual dosen't have a flowchart to diagnose I'd suggest alldatadiy subscription. Its involved circuitry.

If you like I can scan a page or 2 from my manual so you can see which wires to test at the A/C relay.

[CedarGrove]

I tinkered with it today. Took me a while to figure out that the low pressure switch isn't on the compressor but is on the condensor at passenger side firewall.

Anyway...Had 12 v on one wire going to the low pressure switch connector and no voltage on the wire coming out of the low pressure switch connector with it plugged in. Unplugged it, jumped the connector, it sent the voltage on down the line and the clutch engaged. So it's either low on pressure or the switch is bad.

Thanks for the tips!

Lewis

[DmaxMaverick]

That aluminum cylinder near the firewall is the accumulator (or receiver/dryer, depending on the system). The condenser is in front of the radiator.

Yes. You either have a bad switch, or the freon is low. It's probably low freon. On your '98, it is R134A, so you can get a "repair/recharge" kit at Walmart for not too much $$. Get one with a gage, and system leak sealer. Unless you have a noticeable leak, you won't need the kit with oil.

[CedarGrove]

Ahh...Very good. I'll do that.

Thanks,

Lewis


That aluminum cylinder near the firewall is the accumulator (or receiver/dryer, depending on the system). The condenser is in front of the radiator.

Yes. You either have a bad switch, or the freon is low. It's probably low freon. On your '98, it is R134A, so you can get a "repair/recharge" kit at Walmart for not too much $$. Get one with a gage, and system leak sealer. Unless you have a noticeable leak, you won't need the kit with oil.

[rustyk]

And get the kit with the dye - if it gets beyond your ability to track down (and if it's a leak), the dye shows up under UV light, and localizes the problem. An oil film can be a clue, but in some locations, it's hard to see.

BTW, on most systems, the Hi/Lo pressure switch is screwed into a Schraeder valve, so the switch can be replaced without evacuating the system.

Also, if you plan much A/C work (of course, one doesn't "plan" much A/C work - it just arises), a set of gages is worthwhile to have.

[Low_Bridge]

4.3 in a 94 Safari Van AC Clutch doesn't engage - Rusty on my AC R&R this looked like a good place to start-

[rustyk]

A set of jumper wires (just wires with alligator or crocodile clips on each end) are handy - one to ground, one to hot, and hot wire the compressor clutch. Remove the chassis' connector, and clip them on. If the clutch engages, then check the hi-lo pressure switch; reconnect the chassis connector to the clutch, and retract the connectors on the switch enough to jump; if the clutch engages, the issue is either low refrigerant or a bad switch.

If you have an aftermarket compressor by Modine, the Modine adaptor for the compressor clutch electrical connector is a POS - I had to rebuild mine to get reliable connections...

[Low_Bridge]

OK - If I can't get that clutch to kick in by direct wire - replace the clutch - Right? ;)

[rhsub]

ac clutch is the plate on the very front of compressor that contacts pulley when magnetic force is applied by coil
coil is behind pulley, if you ground 1 terminal and apply bat v to other terminal of plug at rear of pulley you should get a strong magnetic field that pulls the clutch plate in to contact pulley
if no magnetic field you need a new coil
there are special pullers and installers required to remove and install clutch plate. no hammers or pry bars allowed, you will damage the compressor
hope this clears up your questions
Ron

[rustyk]

Caveat! - later GM compressors feature a press-fit clutch on the compressor shaft - they AREN'T user-replaceable!