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Old 07-23-2009, 12:59 PM   #1 (permalink)
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Default AC is Fine - till it's Hot!

This is the 2nd summer that my 04 Forester (145,000 mi.) has had this AC problem, and the third mechanic look-see, this time at the dealership. Works fine most of the year (use it to dehumidify), until it gets hot - 83 degrees or so. Then it cuts out. The fan still works, but no cool air. Sometimes stopping the car (like at a stop sign) and then starting up again will kick it back on. Occasionally will start up again with no apparent ambient changes. Always suddenly off and on. Every time I've taken it in, they've all noted that it's "slightly low on refrigerant," but either reproduce the problem, when it happens, they can't tell where the problem is. Two have recharged the system but couldn't find leaks. (Also said the amount it was down would affect the temp of the cooled air, but not this problem.) The dealership is speculating on an evaporator temp thermistor, but says they can't test it - that it would be a leap of faith, and $650. They said they checked the electrical system. We're driving through South Dakota in this thing in a couple of weeks - it was miserable last year. Help!

Last edited by dragonflyeye; 07-23-2009 at 01:00 PM. Reason: Repeaters
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Old 07-23-2009, 01:06 PM   #2 (permalink)
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See My recent problem and find:

Intermittent A/C



PS WELCOME to the Forum and our little corner of the internet ....... I just noticed it was your FIRST POST !!!!!
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Old 07-23-2009, 01:16 PM   #3 (permalink)
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Thanks, ferret. My problem is totally related to the ambient air temp. It doesn't cycle off and on frequently (at least doesn't sound or feel like it) when it's running - just doesn't want to work much at all when it's hot. Does this behavior suggest the same problem?
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Old 07-23-2009, 01:38 PM   #4 (permalink)
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You can always try swapping the A/C relay in the under hood power box with one of the others there ( like one of the fan relays ).

This would prove if it might be the relay, and if it is, then replace the culprit.
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Old 07-23-2009, 01:46 PM   #5 (permalink)
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Since it's at the dealership now, I may ask them to do this one last, little thing. I printed out the whole intermittent thread (w/pix!), so we'll see!
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Old 07-25-2009, 07:48 AM   #6 (permalink)
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A common problem on all compressors w/ on-demand clutch is failure of compressor to engage due to wear of clutch plate, causing clutch plate gap to become too large. This gap results in coil magnetic field being too weak to pull the plate in against the spring. You can verify if this is the problem by rapping the clutch plate sharply w/ a stick when AC should be running. If the clutch plate closes and AC works, you've verifed that clutch gap is too large.

Fix is easy also. Remove clutch plate (usually a 10mm head size bolt) and remove a single thin shim from beneath the clutch plate. Measure clutch gap w/ a feeler gauge to verify it is w/i spec, usually 15-25 mils.

PS, your auto shop should know about this issue, so perhaps something else but your symptoms are classic for too large clutch gap. The reason it is heat sensitive is 1) gap may be slightly larger under higher temps and 2) clutch magnetic field will be reduced w/ increased temps.

good luck
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Old 09-03-2009, 06:01 AM   #7 (permalink)
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Were you able to confirm a solution to this problem? I have the same issue.

My mechanic has checked that the AC is engaging and replaced the relay, but that did not fix the problem. He suspects at this point it is a temperature sensor that's behind the dash. The problem is, it's a significant amount of labor to open up the dash to get at the part and he can't even order a new one ahead of time because the Subaru dealer can't locate the part number for him. So he's reluctant to get into it at this point in case we find out that's not the problem.

Any suggestions?

Thanks
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Old 09-03-2009, 06:36 AM   #8 (permalink)
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Unfortunately Vors, we've been traveling a lot lately, and my regular mechanic is busy as well (added a school bus maintenance contract to his already packed schedule), so I haven't been able to get to it.

I didn't run into anything about a temp sensor behind the dash. When I do get into see my guy, I'm going to have him check into that clutch plate gap issue, but may just go ahead and have him replace the evaporator, which includes a thermistor that that someone mentioned to me on the Car Talk forum. (I'm sure you can find my post there.) Although not cheap, it's fairly straightforward. Like most things, you have to get to it to test it (if testing would reveal anything), so it's a gamble.

This problem is ugly, and obviously a weak link with some Subes. It's starting to get cool here in WI, and my Sube uses the AC to dry the interior air, but it's not working very well (yes, it's starting to malfunction at lower temps too), and my windows are often all fogged up on the inside. Who knows - maybe there are 2 things going on.

Let's stay in touch.
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Old 09-04-2009, 09:29 AM   #9 (permalink)
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Sounds good. I'll let you know what I find out.
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Old 09-05-2009, 05:13 AM   #10 (permalink)
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You don't have to be a mechanic to rap the clutch plate sharply w/ a stick. If this causes clutch plate to engage (starts spinning), the clutch gap is too large.

good luck
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Old 09-13-2009, 09:34 AM   #11 (permalink)
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Well, it's a little too cool now to verify the fix, but my mechanic did find a problem.

I spoke to him about the clutch plate, and he said they had ruled that out by manually triggering the relay, which engaged the A/C just fine. So they knew it was being caused by the computer shutting off the A/C for some reason. Before spending a bunch of time digging into anything too complicated, he wanted to check the freon level, as that was easy. Turned out to be down to 50%, though he couldn't find a leak (seems plausible it lost that much over 5.5 years).

Won't find out for sure if that fixed it unless we have some more hot days so I can see if it happens again, but for now I'm going with that and hoping.
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Old 09-13-2009, 09:46 AM   #12 (permalink)
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Anyone thinking of a bad highside safety switch? This happened in my Honda. Compressor would just turn itself off. It was building too much pressure do to a clogged expansion valve. (stupid name, but it's a filter inside the line near the dryer) Check for voltage to compressor clutch while unit is not working.
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Old 09-21-2009, 09:35 AM   #13 (permalink)
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Freon recharge did not, in fact, fix the problem. Guess this is going to get more expensive...
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Old 09-25-2009, 02:00 PM   #14 (permalink)
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To put my particular problem to bed, I went back to my old (BP station) mechanics, who took everything I told them into consideration, but also thought the compressor looked totally shot - wrapped in rust. They located a used one in like-new condition with a 90 day guarantee for $300, I agreed, they popped it in and it's been working great for more than 2 weeks now. They said that the clutch plate gap was likely a major contributor to the on-off behavior, but the unit needed to be replaced. How the other two entities (including the dealership) that looked at it didn't think the compressor was the problem, I'll never know - it looked like giant, rusty bird's talons were wrapped around it. Although the car's not new, and cause I live in the country, it gets beat up, I think I'm going to blame this on acid rain damage. The car was classified that way when I bought it new (had been on a lot down south, next to chemical spewing smokestacks), and there have been some things that I had to have replaced early on that the same BP guys said looked "older" (drier, warped, corroded), than they should have been. Thanks for all your wisdom! Vors, good luck with yours. Might want to consider the obvious.
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Old 09-25-2009, 02:23 PM   #15 (permalink)
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Glad to hear it's finally fixed !!!! But I have to agree, Texasforester nailed this back in post #6.

These Zexel compressors use a clutch that is a wear item. As the clutch wears, you see more and more rusty looking dust under the hood. You usually only have 1 shim to remove to adjust this. It also requires a special tool to remove the clutch plate. Nice thing IF you have the tool, you do NOT have to remove the compressor, or open the A/C lines and loose your charge.

I know I will have to do this soon, as when I had my relay problem, I measured my gap at .0022". The spec is .0010"-.0014" when adjusting it. The running check is .0010"-.0020", so I am just beyond that at .0022". But mine would NOT engage when I tried to tap the clutch plate, so I knew it was missing voltage.

Replacement compressor, but more important, finding someone who KNOWS WHAT THEY'RE DOING is better yet.

Thanks for posting back your fix for others to find.
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