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Dodgerench, ASE Certified Technician
Category: Dodge
Satisfied Customers: 3406
Experience:  30+ years Dodge/Chrysler exp., ASE Master with L1 certification. Driveability/ combustion specialist
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AC stops working after driving 15 minutes. I pulled over and

Customer Question

AC stops working after driving for about 15 minutes. I pulled over and the clutch on the compressor isn't engaged. I brought it to a shop and did a vacuum test on the system, all good there. They refilled the system and added die, no leaks. Any ideas?
97 ram 1500
5.2 v8 auto
165xxx miles
Submitted: 2 years ago.
Category: Dodge
Expert:  Dodgerench replied 2 years ago.

Hi, welcome to JustAnswer!. This is Ed.

I'd suspect an excessive air gap in your compressor clutch with that sort of problem. Clutches really don't lock, when they engage... they slip, but slowly. Wear and tear from years of use can cause the air gap between the outer and inner portions of the clutch to increase, making it more difficult for the electromagnetic effects of the clutch coil to draw the two sides together. Compressors with excessive air gaps often work OK until hot, at which point electrical conductance of the circuit drops and magnetism does, too. You can sort these problems out by checking the compressor front end (clutch) for magnetic draw of something like a screwdriver when you have A/C turned on, or by lightly tapping the clutch with a hammer to bring the friction plates closer together. If it inspires immediate engagement, it's a positive diagnosis.

You can also check your gap with a common feeler gauge. The acceptable range is 0.016 to 0.031 inch. If much more than that, there might be a spacer shim that can be removed from the mounting base of the unit. Spacers are used during initial assembly to achieve the gap needed, but removing spacers later after the friction surfaces of the clutch have worn may make the clutch usable again. You won't know if there were any spacers used without a teardown and if it comes up empty, your only choice will be to replace the clutch or entire compressor if the clutch isn't serviced separately (it happens).


Customer: replied 2 years ago.
Thanks Ed. Even when the engine is cold, hasn't been on for hours, the clutch won't engage. I did try your suggestion anyways but with no success. Sounds weird but it seems like it only works when I start it for the first time after a day or more of sitting. Anything else I can try? Could it be something electrical?
Expert:  Dodgerench replied 2 years ago.

Electrical? Certainly!

The clutch does use a relay that will be located in the underhood fuse and relay panel (called the PDC). Look at the under-cover road map of the panel to find the location, then swap that relay for another one just like it (there will be several). If the A/C now works... but the thing you swapped it with does not... it pretty much condemns the relay, which is pretty cheap compared to a compressor clutch.

If you're not feeling any magnetism on your clutch when you request A/C operation and relay swapping doesn't help, you could try jumpering the circuit manually inside the PDC. Terminals 30 and 87 will be power in and power out to the clutch, with the terminal numbering cast into the base of the relay. The two terminals will be across from each other and they'll be the two that point different directions.

If it still won't engage, check your circuit resistance between terminal 87 and ground. Terminal 87 is the output circuit to the clutch and the clutch is normally grounded, so expect to see resistance of around 10 ohms or a bit less if OK. The specs say that the clutch coil shouldn't be less than 4 ohms, in which case it's shorted internally, but you have half-a-mile of wiring you're also testing, so resistance will be a bit higher.

And then there's the whole controls thing. The clutch relay is commanded on by the engine controller (PCM), which in turn gets its urge to fire from the A/C controls inside the truck. To be sure that the compressor relay is being requested to fire, you'd need to look inside the PCM's head with a scan tool ... to see what it sees.

In addition (you knew there had to be more), the daisy chain of switches used to protect the refrigeration system have to be aligned and OK before the PCM will allow the compressor to turn on. You have a low-pressure and a high-pressure cutout switch that are wired in series in a sense circuit to the PCM. The switches are normally closed and switch electrically open if they detect the fault they're designed to. So, testing for each switch is to just put a jumper wire/ paper clip into the socket to complete the circuit. If the low pressure switch is preventing compressor operation (and the system is filled properly), jumpering the low pressure switch will complete the signal circuit and allow compressor operation. Same for the high pressure switch in case the switch believes the system is overcharged. So, yeah... it does get a little involved.

As for the pressure switches, they'll be mounted in the high and low pressure lines. Look for a 2-wire switch, then jumper it and test your compressor for operation. Replace the one that was preventing the system from running and that should do it.

Expert:  Dodgerench replied 2 years ago.

I'll be gone for a day or two, but will address any questions you may have upon returning. Good luck!