07 Forester X about 80K miles. A/C works fine initially on startup, cools off nicely,  but after driving for 15-40 minutes, it starts cycling on and off and fails to keep the car cool. (Temp from vents gets warm.) Eventually it drops out altogether, the compressor doesn't run at all and there's nothing but warm air from the vents. The compressor clutch is not engaging when this happens.  Took it to a local shop, they said it was fractionally low on Freon, topped it up and said they couldn't duplicate the fault.  I found that clicking the A/C button to off, waiting a minute or two, then clicking on again can get the AC going again, but just temporarily.  Most recently, after a 45 minute drive, I restarted the car, turned the AC on and the compressor started short cycling, it would click on for a little under two seconds, click off for about two seconds, click on for about two seconds over and over again. This is on a 90+ deg day, with the blower fan set on max and recirc on.  Airflow through the vents is good.

Does anyone have any ideas? I'm thinking the high/ low pressure switch ( does anyone know the pinout of it? It's three-wire deal.) the evaporator temp sensor or.... ? 

There's also a single (black?) wire, with a small (black?) connector on top of the AC compressor, which sends 12V to the clutch.

You could try disconnecting the connector, and seeing if there is 12V on that incoming wire, when the system calls for the clutch to be engaged.

I checked that, there's no +12 at the compressor clutch when it's not kicking in, so it's an electronics/control issue. (Unless something is causing the system pressure to go so high or low that it's causing the high/low pressure switch to kick off.) 

Bad solder joints in the HVAC control panel. Or yeah could be the EVAP temp sensor. That's a dash pull. No Bueno. 

GD

Edited July 2, 20187 yr by GeneralDisorder

GD, is there anywhere I can probe the Evap temp sensor resistance without too much disassembly? I'm guessing the wires from that must run to the HVAC panel... I'm assuming the EVAP temp sensor is a thermistor? Do you happen to know the normal resistance at room temperature or have a resistance-temperature chart? I'd greatly appreciate any info that you can share. 

 

 

maybe swap the a/c relay with another?

I can tell you from my experience that this is probably related to the clutch or electromagnetic coil that engages it.  I had this issue with my last 04 forester and I was able to adjust the clutch gap and worked. not this time on my 2008 outback. It is a heat issue. I'm going to start a new thread. 

You need a set of gauges. Need to know your high and low side readings.

I own gauges I recharged last season. I haven't checked because when the temp is below around 72-74  the ac runs great very cold no shutdown. But I will check 

OK, this is embarrassing, but I should tell you folks to help others.  After chasing down wiring diagram, I figured out that the AC was shutting off due to the evap temperature sensor. I concluded that either the sensor was faulty or something was making the evaporator freeze up. I thought "Hmm, what would make the evaporator too cold...insufficient airflow." The volume of air coming out of the vents seemed fine, but I decided to check the cabin air filter. The "pleated" sections were filled with crud to the top! A fresh cabin air filter fixed it. In my defense, I brought this vehicle to a shop for A/C repairs and they failed to figure this out too. 

Just had to change the clutch and coil.  2008 outback this fixed the problem.