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1992 Subaru Outback AC Problem

Recurring intermittent problem with the AC. It has been in and out of the shop three times in the past year. First time, a basic checkup with service to specs. Worked okay for six weeks or so. Second time, revolution sensor replaced. Worked okay for a little over six months. Next time, revolution sensor replaced under warranty. No change - problem still there. Dealer shop wants to replace compressor/clutch assembly. I’m not convinced. The problem only occurs in the morning after the engine has been at rest overnight . No AC until the engine warms up and the engine is switched off and then on again. You can check it after a few minutes and no AC. Shut down the engine after 20 minutes or so, turn it back on, AC is good and will be good for the rest of the day, even if the engine is at rest for a few hours. Next morning, same drill. There is a three wire connector at the clutch with one black wire running to the clutch. I am unsure what voltage I can check there to see if the system is calling for clutch engagement. Any ideas out there on how I can narrow it down to clutch/compressor malfunction or some other sensor problem?

Sorry, should have been 2002 Subaru Outback AC Problem!

After reading your post I’m not convinced either, but I haven’t worked on it like the shop has. Going by your description of the problem my guess is the trouble is just due to a faulty wire connection to the sensor rather than the sensor itself or perhaps a relay if one is involved with this. Since the shop wants to replace the expensive part rather than clean or repair a simple connection connection point I would hope they have already proved the trouble is with the part and not just a simple wire connection or relay problem. Since the trouble is intermittent, to me this points more to a wire connection problem than a problem with the sensor itself, but I could be wrong.

I assume the clutch is at least working ok when the trouble happens. Since the problem always appears in the morning it should be pretty easy for the shop to determine what is going on if they are troubleshooting the problem correctly rather than just guessing at it.

Without looking at some data I would assume the three wires are power and ground for the clutch and the third wire is for the sensor. I am guessing there is a magnetic sensor inside that provides a pulse for each revolution of the compressor. If you have a digital meter you might be able to see that pulse action by checking the AC voltage on the wire connection. If the meter can check frequency also then using that function might show the pulsing action also. An O-scope would be the best thing to use but most folks don’t have access to one. If I was working on this I would also want to check to see if the trouble may be due to a problem at the other end of the line, in the control unit. Checking for any DC voltage on the sensor line and comparing the results while the trouble is happening and while it is working ok may indicate there is a input problem with the controller.

Do both radiator fans come on, when the AC is malfunctioning?

Worked perfectly yesterday, but this morning, same drill - not working. I have parked it in the Texas sun. This afternoon I will check it after baking in the sun. If not, warm up the engine and try to get a better handle on when it kicks in. I have downloaded a picture showing the three wires. As you can see, there is a three wire connector with one wire going to the clutch. Seems to me I should be able to check voltage potential at that point when the compressor will not work to see if it getting a signal to engage. That is if I knew what to check, which I don’t. Thanks for any input.

I am guessing it is no longer under warranty. If so I would start looking for a local A/C radiator shop. Generally if you live in the south you will find lots of A/C shops and in the north you find radiator shops. They generally do the same work, Check with friends, neighbors etc for a good one.

Answer to RemcoW’s question about the fans: Yes, the fans came on this morning when the AC didn’t work. The car sat in the sun yesterday and when I cranked it up in the afternoon, the AC come on, no problem. Still checking to get more history. Thanks.

I really dont think its the compressor if it works fine in the afternoon, compressors eather work or they dont… they dont care what the temp is of the motor or out side.

I am wondering if its a low side pressure switch issue, where is it not reading pressure correctly…I think you need to take it to a REAL AC shop, not just a shop that also does AC…