A/C clutch relay does not close

I have a 99 jeep cherokee classic. I have a problem where the A/C clutch does not engage at all. I can manually engage it by manually engaging the A/C clutch relay in the relay box under the hood. When the clutch is manually engaged by manually closing the A/C clutch relay, the A/C blows cold air. I checked the 20A fuse for the A/C clutch, also located in the relay box, and it is OK. I checked the relay itself to insure that applying 12V to it would cause it to close, and it appears to be OK (it does not appear that the relay itself is bad). I looked at the control unit in the interior (containing the knobs to control heating and A/C), and it appears that vacuum is used to control heat and A/C function when the knob is rotated to that A/C position, but I can't determine yet how the vacuum system ultimately manages to close the A/C clutch relay. It seems that 12V is not being applied to the A/C clutch relay for some reason, and I have read that there is a pressure switch on the system somewhere. Could it be that the refrigerant is low, even though the system blows cold air when I manually engage the relay? Also, is there a vacuum switch somewhere (under the dash, etc) that is actuated by vacuum when the control knob is rotated into the A/C position, that should be ultimately actuating the A/C clutch relay? Any help would be appreciated. Help! Thanks.

Reply to
carybriel
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Reply to
L.W.(Bill) Hughes III

Yep. You can safely jumper that switch for testing (and for filling) but it's not a fix. The purpose of the low limit is to prevent the AC compressor from burning out due to lack of oil. The low limit switch can kick in when the AC is still capable of delivering cool air.

The vac has nothing to do with the clutch or relay, it's purely for moving the doors that direct airflow.

The clutch circuit is

battery >-->fuse>-->lowlimit>-->highlimit>-->AC relay-->clutch solinoid-->ground

On the relay side it's a bit more complex and involves the switch inside and the powertrain control module (to control engine RPM).

The switch should be > vacuum system ultimately manages to close the A/C clutch relay. It

Reply to
DougW

I think I understand your reply, and will check the low pressure switch as you suggest. There is one thing that I don't understand though. You mention that on the relay side, a switch is involved inside. The only knob inside the cab that activates the AC is a rotating knob that allows selection of AC functions, as well as selection of heat related functions (floor, defrost, etc). Does this rotating knob have an integrated electrical switch as well as a vacuum switch? I do not believe that this internal switch is bad, but I'm just wondering what electically actuates the relay from the inside of the cab. Also, how does the powertrain control module figure into the logic of the relay side circuit? Thanks for the help! I'll check the low pressure switch and let you know what I find. Cary Briel

DougW wrote:

solinoid-->ground

Reply to
carybriel

In Defrost modes the AC is on. In all other modes it's on if the AC button is illuminated. The knob that controls position from ducts to bi to defrost has a switch that works in tandam with the AC button. (manual climate control) In defrost or any combination of defrost, the AC will run. Otherwise the AC only runs if the AC button is pressed in. The light doesn't illuminate in any of the defost settings even if the compressor is running.

If the light is lit in regular (center/center+floor) mode then the relay should have power.

Reply to
DougW

Reply to
philthy

Yes, the A/C switch is vacuum/electric.

The PCM controls the A/C clutch relay.

solinoid-->ground

Reply to
bllsht

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