Automotive Air Conditioning Information Forum

I am trying understand why my AC Compressor clutch sometimes doesn't engage. The most reproducible behavior is as follows

Start the Jeep from cold, turn on AC - it works, compressor engages
Let Jeep idle for a long time, AC continues to work - Jeep at operating temperature for 10 mins or more.
Turn jeep off and start it up again
Compressor does NOT engage.

I put manifold gauge on, the pressure in the AC at cold start is essentially the same as when I restart. I have to wait a minute for the static pressure to return to balanced state after running the engine (occasionally it take a very long time, maybe it depends on where the compressor comes to rest?)

When I rev the engine and the high pressure side gets to 292 PSI, the electric fan comes on just like it should so pressure transducer seems to be working properly. The fan also goes back off at 227 PSI, again exactly like it should. (The high pressure cutout on the Jeep is a pressure sensor with analog voltage to the PCM so it can tell when to turn fan on/off for additional cooling)

The low pressure switch must be working since the AC comes on initially. I monitor the low side pressure and it never drops below the point where the low pressure switch should activate. I wiggled the connector and wire harness just to be sure there wasn't a problem there, the clutch stays engaged. (The low pressure cut out is the usual switch, not a pressure transducer)

There really isn't anything in the wiring diagram that could explain this. There are some more global power down things, but they would cause other symptoms.

Does anyone know if there is a thermal cutout switch as part of the compressor itself? I can't see one, but have limited view of the compressor. I am thinking this would partially explain the behavior, and it wouldn't be shown on the vehicle wiring diagram since part of the compressor. It seems like it is never going to fail when I idle the Jeep, but when I turn it off and back on, maybe the cutout gets activated from increased temperature above the engine just after shut down. It seems like a long shot, but how else can this be explained?

Thanks for any ideas.
JB

Check clutch gap. I did not read your entire post.

That is good idea, not sure why I didn't consider that. I think I will monitor signal to clutch as that is easier to do.
Thx

This is a real common scenario- seen it numerous times. Clutch air gap has worn too wide such that the magnetic pull of the coil cannot pull in clutch driver after the coil warms- and electrical resistance increases. To prove this, next time it "should be running" tap on front of clutch with a hammer handle- the clutch will snap into action because you helped it pull in. Heck, I 've even just slammed the hood shut and it will come on because the clutch was disturbed. This is the failure mode of a wide/worn air gap. Starts out running then never again- eventually never at all.....so take off clutch driver, remove the thickest shim--and throw it over your shoulder and replace clutch driver...all that is important, is that the clutch does not rub when not engaged....

good luck.

The other thing you can do is, when the compressor won't kick on, give a little tap on the front of the pulley with the tip of a pry bar or something. Usually the magnet just doesn't have enough strength to pull the clutch in but if you give it a little nudge it'll suck it in the rest of the way and the compressor will kick on. It's either a gap issue or a clutch going out or a combo of the two. Obviously be a little careful because if it does kick on, the front of the pulley is going to take off spinning. . .probably not an OSHA approved maneuver.

If you watch this video from about minute 15:00 to 17:30 you'll get a good example of what I'm talking about.

https://www.youtube.com/watch?v=DNx0X4INfiw&t=106s

Edit to say gmtech beat me to it.

Regarding wiring troubles, I did wiggle the wires a bit to check for intermittent connections and it didn't matter. It is also probably too repeatable to be intermittent.

I have cobbled together a clutch wiring pig tail now with and LED to know when power is present. I plug this into the compressor clutch connector, and the Jeep wiring harness connector into it so I can monitor power. I want to give it a try, but currently raining so I will have to wait until tomorrow to test it and see if it is indeed the clutch not working when hot.

My pressures are indicative of compressor going bad. The low side is higher than it should be, the high side is lower than it should be. Would I be a fool to hope that this is because the clutch was slipping and that I might fix that problem at the same time? I will update tomorrow when I get it tested out.

Don't overlook that the AC clutch could be slipping. If the AC clutch is getting positive voltage (can test with a light) and the clutch does not engage, high likelihood !!!

Last summer I removed an AC clutch shim on my own 1998 Frontier to fix it, and it's stayed working (Arizona heat). I didn't even need to unbolt the compressor or remove the belt on mine to do this.

All of that work to avoid using a feeler gauge. It took more time to unplug the clutch connector.

That was exactly the problem! It was interesting to monitor the signal to the clutch. When the engine was at ambient, the clutch engaged as expected. As the engine warmed up, I cycled the AC and you could see a time lag grow between power to the clutch and engagement. One time it took almost a minute. Finally it didn't engage after waiting 3 minutes and I gave it a whack with a broom stick. It engaged so I am very confident this is the problem. (This is why I wanted to put the test LED in place, to be 100% sure I nailed the problem. I know, I probably out-anal-ed just about everyone here)

Thanks so much for the help, I embarrassed I didn't think of it myself as it so obvious now. I was just stuck on trying to find a sensor that keeping it from running.

It is not the engine warming that does it so much as the coil warming--the resistance has to increase and the windings get warm- both by engine heat and more so by electrical resistance...