Guys, I read the A/C sticky and couldn't seem to find an answer in there.

I am asking this on behalf of a buddy who has a '99 Durango with the 5.9. Manual temp control (did they even have ATC?). He has an FSM for a 2000 and a '98 but not a '99. I also have a '98 Jeep Grand Cherokee 5.9 Limited and I have an FSM for that. A/C systems seem to be the same. He and I are both very mechanically inclined but he is so frustrated with this problem that he is ready to sell the dang car, lol! He also has A/C tools such as a manifold and we know our way around a multimeter. We even have the capability to evacuate/recharge the system and he has a tank of refrigerant at his house.

Anyways, I have a vested interest in this because we are members of a 4-person car pool and when it is his turn to drive this affects me, too. LOL.

Okay, on to the problem. When we are driving home on the interstate (80 MPH) about 50 miles one way, the A/C is intermittent. You turn it on and it works fine (plenty cold), then after a little while, it just stops blowing cold air. Blower motor continues to run, the air just gets hot. At this point we usually turn the A/C off for a little bit. Like 5 mins later we can turn it back on and it will again blow cold air for a little while then it stops again. Wash, rinse, repeat ad nauseum.

I suspected a cutout switch and he finally did some diagnostics last night. He hooked up his manifold to monitor high/low side pressures and ran the system.

What is happening is the compressor clutch is actually kicking out when the air gets warm (big duh!). However, we can't seem to figure out why it is kicking out. The high side pressure is only about 200 psig when it occurs and low side is still up around 40 psig. I read the high side switch cuts the compressor at 450-490 (back in below 400) and low side cuts out at 20.5 (back in at 35-38). When it kicks out, the pressures begin moving towards each other until it equalizes around 150 psig. At this point I believe you can run the A/C again. (I will try and get him to register for this forum to participate in this discussion)

So this isn't making sense because the operating pressures seem to be within range when the compressor cuts out i.e. a switch shouldn't be cutting it out. At this point he wisely suspected a faulty switch so he jumpered each of the pressure switches, one at a time. This had no effect. He never jumpered them both at the same time, but I don't think that is saying much.

Any ideas?

Basically what I am surmising is that SOMETHING is telling the PCM that conditions are not right to run the compressor after it runs for awhile. What is telling it that, I have no clue.

The other option is that the PCM is malfunctioning but I find that highly unlikely since the A/C works sometimes. I would think that would be more of an all or nothing proposition.

 

I think there's also a temperature probe in the expansion valve that tells the computer if the evaporator is freezing up. It's mounted on the side of the expansion valve which is bolted to the firewall.

 

Well, it appears the problem has been identified. As a complete shot in the dark, we figured we might as well rule out the clutch. Well, we couldn't rule it out because it was the problem. After a while, the clutch just lets go and won't stay in. Apparently, giving it a few mins to catch its breath is enough to get it going again.

This was diagnosed by observing the state of the relay (through touching it while it was plugged and unplugged) and watching the clutch. After a while, the compressor would stop as before. At this point, he would plug and unplug the relay and he could feel it click closed as he did so. The final confirmation came when you could visibly see the clutch moving a little, but not enough.

A new clutch has been ordered and we expect that this will cure the problem.

 

That certainly could be it. Sounds like the electromagnet in it is overheating.

 

Yeah. Didn't even enter into my mind that something like that could happen. I guess I figured that if the clutch engaged at all, then it must be good.

My buddy was actually resistant to doing the test because we were convinced that it was so unlikely. I convinced him that it was a free test to do and if nothing else, we could eliminate one more thing from diagnosis. I myself pretty much figured there was about a 0.0001% chance that it was the clutch. I, too, figured it would be a waste of time but lo and behold, it was the problem.

The other benefit is that insofar as A/C system repairs go, it doesn't get a whole lot cheaper than replacing a clutch....