A/C Compressor Clutch not engaging?

[Brent R. Duey]

My air conditioner doesn't work. It turns on and works as it should but no cold air, just vented from outside. Yesterday I was under the hood and noticed that on the A/C Compressor (above the PS pump), the black "disk" near the belt wasn't spinning. I thought if that wasn't working, my defroster wouldn't work either. I am perplexed. Thanks for any help in advance. Also I saw a vacumn line that is cut, but it didn't seem to be sucking when defrost or air was on.

 

[Brian1]

If it does not come on at all it is the a/c switch that is under the hood. It will be between $60-90 to have a shop replace the stupid switch. I believe the system has to be evacuated and then refilled when the switch is replaced but I am not sure. My dad's 96 F250 had this problem recently.

 

[mdrut]

Brent,it being a 91' it probally just needs charged..$6 a can, plus hose..Probally 3 cans to get it going..Add a can of oil to it while you're at it..8oz.

 

[Blaine Cooper]

I agree with mdrut. The compressor won't come on if the freon is low,I believe it's a safety thing so the compressor won't burn up. Hopefully all you need is a recharge. My recharge ended up running about $750 last summer...evaporator core, accumulator, line, o-rings, 134A conversion.

 

[sink]

Brian1 must be talking about the pressure switch that the others referenced. If you had to change that, you'd have to open the system. But I would believe it is just low on juice like the other said. I agree with mdrut if you can find a state or a store where you can still buy the cans of R12. Defrost only gets a boost from the AC being on. It still defrosts OK without it. I'm used to seeing the compressor clutch click in and back out rapidly with a low Freon problem. If yours is just staying out, you might want to try jumpering the pressure switch out for a few seconds to make sure that is your problem and not some simpler wiring thing.

 

[EvilleDave]

If I'm reading all this correctly I have bigger issues.
I checked all the fuses and relays.
I checked the plug at the compressor(no power)
I jumped the low pressure switch(nothing)
The cheapo gauge with the kit shows in the red but when I hit the needle valve there doesn't seem to be much pressure in it, I've recharged many times before.

I can't remember if I checked continuity at the plug but at the compressor side of the plug show no continuity showing either as ground. I'm on a limited income and would like to have this fixed, the wife is on oxygen as it is.

 

[Kevlar7R]

the clutch itself may have failed. The clutch is the black disk you dont see spinning. Under it is an electromagnet, that when enegerized, pulls in the disk and engages the compressor.

Usually there is a single wire going in right behind the belt pulley. Unplug this and put 12v on it with a clip lead (if you have 2 wires you'll need to figure out which is ground). Do this with the engine off. You should see and hear the clutch engage, and that disk press against the pulley. If you don't, your clutch is bad. Usually this happens because the glue seperating the windings of the electromagnet gets old and brittle and cracks.

The clutch can be replaced with the compressor in the vehicle so you don't have to crack open the ac system. This will be your cheapest option, but a real pain in the ass.

 

[bass]

OK so my husband and I both just went outside and did what kelvar7r said and it worked so therefore our clutch is NOT bad.. So now what?

 

[derfderf]

If you're not getting 12V at the clutch to engage it via the "proper" system settings, then I suspect it is either

1) an A/C relay issue (relay is not closing, and therefore the required current at 12V to engage the A/C clutch is not getting through)
2) a wiring issue between the relay and the pressure switch. Jumpering the pressure switch will engage the AC clutch only if the current is there to flow through the bypass. This is true regardless of charge level AFAIK. The fact that the clutch doesn't engage indicates likely there is no current flowing to the clutch, and since jumpering the switch didn't work, that implies that the switch is not seeing 12V either. So the issue is upstream.
3) There is also the chance that the "AC on" command from the BCM is either not being sent to the input of the relay -- this would be a BCM issue--- OR the wiring to/at the relay on the underside of the fuse block may be damaged/contacts melted, etc,
4) wiring issue between BCM and AC relay.
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Since you indicated that the system does not seem to release refrigerant when you depress the valve, you are likely low on refrigerant or completely out.

When this happens, I jumper the pressure switch long enough for the suction line to pull in refrigerant to exceed the low pressure cut-out.

Since you can't do this, you will need to manually apply the 12 V to the clutch as you did when you tested it. I would leave the pressure switch UNJUMPERED while doing this. Every so often, CLOSE and detach the refrigerant can and check the low side port with a gauge--with the 12V applied so the clutch is still being manually engaged. Get the system about half charged. You will have then eliminated low system pressure from the list of possible causes.

Remove the manually applied 12V at the clutch.

I expect the clutch will disengage.
Jumper the pressure switch.
.If the AC clutch kicks in, the pressure switch is bad.
If the A/C clutch does not kick in at this point, I would say AC relay or wiring which can be traced to where the 12V current feed to the circuit disappears as discussed above