cftvols

And I've read a dozen on this topic and I don't understand what I'm looking for so apologies if the answer is in 40 different threads.

98 Cherokee Sport 4.0

I love this old truck but I just moved back South from CT and I had forgotten how hot it is. AC Compressor seized and broke belt in CT a year ago. Not a big deal - I ran a bypass pulley and survived the few hot days.

New compressor and evap/dryer ordered and installed. Vacuumed system and added 20 oz refridge. Checked fuses and relays. Everything looked great.

Fired engine. Fans work. Climate control unit lit up and responding to fan and temp changes - however, the clutch isn't engaging. The compressor itself won't fire. I checked voltage (and again, I am terrible with this so I may be saying it wrong) and the clutch leading from the dash is all over the place. No way it's a steady voltage to fire the clutch.

So what on earth do I do? I took the center dash off and everything is plugged in to climate switches and no obvious frays or melts. The fan changes speeds appropriately. The temp dial gets very hot on red and tries to get cooler as I get into blue. The different settings move the air from floor to vents as needed. It just won't fire the clutch.

I would hate to have a shop charge me $200 for an easy fix when I've made it this far on my own.

Any help would be greatly appreciated.

Jimbo Slice

The voltage jumping around sounds like a bad ground. You also have an AC clutch relay that you might check for function. Swap it out with a known good relay of equal value and see if it works.

cftvols

Relay is good. Neighbor had one of them relay-checkers.

So I know what a ground is (kinda) but not where to check that/how to fix it specific to the clutch.

Jimbo Slice

Have you got a manual?

cftvols

I do not, shamefully.

Jimbo Slice

You need to get a schematic and trace the wiring for the AC compressor. I'd do it for ya, but my book is in my buddy's garage.

I want to say the ground is dark blue/black, but I can't be sure.

This is just a start though. When I troubleshoot a circuit I like to start at the failed component and work my way back in the circuit.

Make sure you have a good power and ground to the compressor/clutch and if it still doesn't work move your way along the circuit.

petejeep95

CCKen

I know you said you checked fuses, but check the AC Clutch fuse in the PDC.


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BlueRidgeMark

I agreed with that when I read it, but on second thought, no. If the meter has a good ground, it's measuring true voltage. He should also check the ground, certainly, but he's got a supply problem.

Possibly a flaky conductor (corroded wire or bad connection) carrying the voltage.



Circuit splitting is a faster method than going from one end to the other.

In circuit splitting, you check both ends of a circuit (doesn't make sense to check the end of the circuit if there's no power at the start), and then find a place that is electrically in the middle. When I say "electrically in the middle", I mean something that is functionally in between the source voltage and the load (the compressor clutch). Distance is irrelevant here. Maybve that point is the relay, or the fuse. It's not critical. Find out if everything checks okay at that middle-ish point.

If YES, move farther out in the circuit. If NO, find another middle point between where you are and the source voltage.

Get the idea? Split the circuit, and find out if the fault is BEFORE or AFTER your split point, then split the half that has the fault.

Of course, you aren't going to cut into the middle of a wire loom. You are going to measure at junctions.


This is kind of like using bracketing fire with artillery. It gets you to the target with the fewest attempts.

Jimbo Slice

Thanks BRM for putting it in more clear terms.
In this case I would have taken a reading at the compressor, if it's a no go I would move to the fuse, go=after fuse/before compressor... no go=check at switch.

As far as the ground issue. I was thinking that he was taking his reading at the compressor with the leads touching the power and ground terms. This wouldn't show a reading until it kicked on though (relay closed) though would it?

BlueRidgeMark

Yeah. Hope he wasn't measuring it that way!

If I'm working in the engine compartment, I usually ground right to the battery terminal. That eliminates some variables.

93XJeeper

I had this same problem. Turned out to be a blown fuse but the location of the fuse isn't labeled for the compressor. In ur fuse box under the hood, look at the mini fuses, it's the bottom right blue 15 amp fuse. Check that one. Mine is labeled ignition but actually controls the AC clutch and electric fan.

CCKen

Your system pressure may be too low. Verify it is serviced properly.


The compressor clutch is controlled (ON/OFF) by two switches, wired in series. A high pressure cut-off switch which is normally closed, and will open if the system pressure gets too high, and a low pressure compressor cycling switch which is normally open, and will close when the system pressure rises to a preset value. The low pressure switch is mounted on top of the AC Accumulator.


The low pressure cycling switch will shut down the compressor if ice forms on the evaporator and the system pressure drops, then starts it again when the ice melts and pressure is regained.


You can check the operation of the low pressure switch by removing its electrical connector and jumping the two pin cavities in the connector using a small paper clip, then try operating the AC system (see pic). If the compressor clutch engages the low pressure switch may be defective (rare). You can check it by running the system (with the connector jumped) and, using an Ohmmeter, check for continuity between the two pins in the low pressure switch. With the system properly serviced, you should see continuity. If not, the switch is defective.


Go from here...

CCKen

The AC Compressor Clutch is grounded through the compressor mount, thru the engine(G101), to the battery NEG post.


Here's how to test it. Follow the numbered steps.



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Compressor clutch wiring walk around:



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CCKen

Any progress?