Automotive Air Conditioning Information Forum (Archives)

Provided by

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

Help, high pressure cutoff switch suspected

Biloux on Fri April 29, 2011 1:37 PM User is offline

Year: 1999
Make: Subaru
Model: Forester
Engine Size: 2.5L
Refrigerant Type: 134a
Ambient Temp: 75
Pressure Low: 29
Pressure High: 250

I had an AC problem since purchasing a 99 Forester last year. The compressor would lock up when ambient temps got hot, causing the clutch to screetch as the compressor cycled on and off. I has already adjusted the clutch gap last summer which helped for a while. I just had the AC compressor and drier replaced with aftermarket parts I bought and now the compressor runs all the time. The mechanic said the switch on top of the reciever/dryer is bad and the only way he could get the switch was to buy the dryer with the switch for $150 from the dealership. He could not get just the switch. I told him to leave it for now and I would figure out what to do next.

I confirmed that the compressor clutch disengages when the switch is unplugged. I have gauges and low is around 28-30 on idle and hi side is 200 ish on idle increasing to 250 or 260 while revving the throttle. I have not yet swapped the AC relay with some other relay.

The sticker on the switch reads "Calsonic 8F" and "Hi 3.04MPa" and "Lo 0.278MPa" .
3.04 megapascals equals 30 Bar or 441 PSI and 0.278 megapascals equals 2.8 Bar or 40.3 PSI.
The AC comes on/off with the interior controls, and the compressor clutch disengages when the switch on the drier is unplugged.
The compressor never cycles on and off, it just runs all the time.

Is this switch at fault and why is the high side pressure of 260psi so far away from the 441 psi rating on the switch?

Is this the correct switch?
Does someone have a used switch to sell? I cant find any new.
Can I use a different switch? There are lots of 30bar/3 bar generic switches on Amazon and Ebay.

Thanks, Walt in NC

mk378 on Fri April 29, 2011 9:51 PM User is offline

This seems to be a conventional TXV system with a "trinary" switch. The switch cuts off the compressor when high side pressure is abnormal:
40 - 440 psi : On (normal operation)
> 440 psi : off (overpressure).

So your compressor should run with the pressures you have. It would cycle off based on evaporator temperature. If it never cycles off that's because you have extremely hot weather or inadequate cooling performance.

Again, the switch is supposed to stay closed all the time during normal operation. Overpressure occurs mostly for the following reasons:

*Inadequate condenser cooling. I think that model has an engine-driven fan. The fan clutch could be worn out. The fan would still spin but not as fast as it is supposed to.
*Overcharged with refrigerant or oil (improper servicing)
*Air in the lines (again improper servicing)

It is possible you have inadequate condenser cooling, leading someone to undercharge the system to keep pressure down. This would give inadequate cooling and the compressor running all the time with the vents not very cold.

Edited: Fri April 29, 2011 at 9:52 PM by mk378

NickD on Sat April 30, 2011 9:55 AM User is offline

Saw that era of Forester in Colombia last year, that is one heck of a cute little SUV, how do you like yours outside of the fact that your AC doesn't work? Gather the reason I don't see those here, the nearest dealer is 120 miles away. Looked at the new ones, look like they came from outer space.

Mk378 gave a good response, these cars are suppose to have a TXV valve and the compressor should run practically continuously on hotter days, but would definitely should cycle at 75*F ambient. Cycling is not done by the trinary switch, that is there strictly for protection, like preventing the compressor from running at either low ambient temperatures or low on refrigerant with normal AC temperatures. High temperature cutoff prevents your system from blowing up.

What controls the cycling is an evaporator thermistor, think your vehicle has that, earlier ones used a capillary tube type. But at 75*F ambients, if you compressor is running continuously, would expect a freeze up. Vent air would greatly decrease.

AC works starts with a good housecleaning, can't have air conditioning without the air, flow in this case. Seen that space between the condenser and radiator 75% full with debris. View the blower wheel most like a leaf and debris chopper loading up the face of the evaporator. Then all the problems mostly with electric fan cooled vehicles. Without doing this work first, your pressure readings are next to worthless. And without good condenser airflow, that trinary switch should pop open and darn well better do so or your system will explode.

Your vehicle is over 12 years old and more than likely needs a good cleaning. Any, that is where you are suppose to start, see way too many guys, even shops that overlook this very important step.

Biloux on Tue May 17, 2011 10:34 AM User is offline

Thanks for the replies guys. It seems like system performance is a little less than optimal, but kept me cool last week at the beach. It does blow properly cold off idle. I will inspect and clean the condensor. As for the car itself, I absolutely love it. Just fitted new tires (oversize 215/70/15) KYB struts and a 1/5 inch lift kit. Looks almost like a Jeep Grand Cherokee, only half the size. Went out on the beach every day fishing, not even close to getting stuck or bottoming out in the ruts.

Back to Automotive Air Conditioning Forum

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.