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When I call for AC on the dash the clutch on the compressor doesn't engage. I disconnected the clip that connects the wire to the clutch and checked for voltage and there was none. People say that means I "need a new computer". But that seems drastic. And expensive. Is there some fuse that can be blown? Some broken relay?

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Engine Size? Ram 1500? –  Larry Sep 4 '13 at 16:12
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Yeah, it's a Ram 1500. No idea what the engine size is or what the correct measurement for that is. I assume it's in what I remember as "combined cylinder volume" and I don't know what that is but I do know it has a V8. Or is it a straight 8? It's probably a V. What car has a straight 8? –  tooshel Sep 5 '13 at 16:57

2 Answers 2

There are several things that can keep the compressor from engaging

  • the heater-A/C mode control switch
  • the low pressure cycling clutch switch
  • high pressure cut-off switch
  • compressor clutch relay
  • Powertrain Control Module (PCM).

One of the most common problems is a low refrigerant charge, the low pressure switch prevents the AC clutch from engaging when the system is low on refrigerant.

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Nice! Thanks. I'm digging into each of those now. I got a new compressor clutch relay (which I had to order and just showed up) and checked fuses and so far it's none of those easy things. Someone else mentioned it could be a stuck "high RPM shutoff switch" or something like that. The "low pressure cycling clutch switch" sounds like a good contender because the previous owner mentioned it would come on for a bit but then turn off (which is why he thought the compressor was fine since it would blow cold for a bit). I haven't experience it ever come on. And now I need to find that switch! –  tooshel Sep 5 '13 at 16:51

Definitely check the fuses before replacing anything else. Your interior fuses should be on the drivers side as pictured below in a video from carcarekiosk.com.

1998 Dodge Ram Interior Fuse Location

The video shows a page in the owner's manual that indicates that a fuse connects to the "A/C Controller" titled "MIRR HTD". It is a 10 Amp fuse. Here is an image from the same video. (The panel is being held sideways).

fuse panel

Another fuse is under the hood on the drivers side titled "A/C Clutch". The picture below is from justanswer.com. Look at the lower left corner of the image. It's labeled "J".

hood fuses

Those are the only two fuses that I could find. You should check them both and any others that might be related, such as the blower fuse, just in case before you move on to more expensive repairs.

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Why 2 fuses to the same thing? I checked the one under the hood and it was fine and still need to find the one in the cab. I ordered a new relay and I'm trying that too. And that schematic for under the hood box is RIGHT ON! Too bad there is no central repo for that information on every car. –  tooshel Sep 5 '13 at 16:59

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