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AC Compressor overheating

8C 361 on Fri September 21, 2012 2:02 PM User is offline

Year: 2000
Make: Subaru
Model: Forester
Engine Size: 2.5
Refrigerant Type: R14-A
Ambient Temp: 90
Pressure Low: 40
Pressure High: 145
Country of Origin: United States

The compressor has a high temp cut out switch. At first when it was hot the ac would quit working. A friend with ac experience suggested changing the dryer. I ordered a new dryer and expansion valve. The valve would not fit right so we put the old one back in and changed the dryer. When it is working it cools pretty well but may not be working to full capacity. All the reading that I have done indicates that returning refrigerant is what cools the compressor.

It still overheats and cuts out, particularly around town, low speed low rpm. We have learned that if we are moving and keep the rpm over 2000 it will keep working for long periods of time. Once we stop or slow down it will cut out and not cool down for a long time.

I am wondering if the compressor is marginal and needs to be replaced. I have never observed the high pressure over 150.

mk378 on Fri September 21, 2012 2:44 PM User is offline

First, are you sure that it is stopping because of the high temperature switch? Typically that switch will only act when running with a very low charge-- cases where there is not enough to make any cool air but the driver keeps the button pressed anyway.

A compressor may fail to engage when hot because the clutch gap is too large. The magnetic force from the coil is weaker when the coil is hot because the resistance of copper wire increases with temperature, so the coil current drops. If the gap is large, the force won't be enough to pull the clutch plate onto the pulley. Once it does engage though, it will keep going.

I suggest measuring the voltage at the coil when it is not working to see if it is trying to engage.

8C 361 on Fri September 21, 2012 3:45 PM User is offline

The clutch is fine. The compressor has a high temp cut out switch. The compressor is hot when it cuts out.

iceman2555 on Fri September 21, 2012 4:44 PM User is offlineView users profile

The compressor utilized on this vehicle is a 'scroll' type compressor. It does utilize a 'case temperature' switch. This is a normally closed switch that opens when the case temperature exceeds a given value. A typical cause of this would be low refrigerant charge or a restriction within the system, thus reducing the amount of lubricant flow to the compressor. It is not the refrigerant alone that maintains case temperature. The reduction of lubricant flow to the compressor will result in serious compressor damage. It is quite possible that this has already occurred.
A suggestion would be to have the system serviced insuring that the system is charged completely...no cans....a service machine is required. Once this is accomplished measure the condenser inlet (discharge) and condenser outlet (liquid) temperatures. Pressures alone may not indicate a restriction within the condenser. Temp drop should be min of 13-15 degrees. An elevated temperature drop would indicate a restriction of the condenser. Do this test at idle, max cool, high blower, doors open. It would be possible to elevate the engine rpm, if the clutch disengages or the temperature of the discharge line escalates, this would also indicate a possible condenser restriction.
If the condenser is restricted...there is only one source of this and that would be the compressor. If so, the system must be serviced completely. This basically the same system utilized by Honda in the CRV and Honda recommends a TOTAL system replacement. It would be advisable to follow this procedure to insure proper operation and longevity of the replacement compressor. The system could be serviced without changing all parts, but the system must be disassembled to properly flush and clean, so why not spend the extra cash and insure a completely operational system. Failure to properly service this system will result in the failure of the replacement compressor....no matter from whom it is purchased.
If you decide to complete the repairs yourself, it is not a difficult repair. A strong suggestion would be to have the system charged by someone who has the proper equipment. The use of cans and pressures is not recommended due to the possibility of not completely charging the system....this will result in a reduction of lubricant migration and back to square one.
Good luck!

-------------------------
The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

GM Tech on Fri September 21, 2012 6:12 PM User is offline

Sounds like inop cooling fan(s) to me....from here...

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

8C 361 on Sat September 22, 2012 9:55 PM User is offline

The refrigerant charge was easy. There is a sticker that says 18-20 oz. That is what we put in and it cools fine until the compressor ovrheats and cuts out. Iget low pressure of 35 and high of 145. Could this indicate weak compressor?

The oil charge is a bit in question. When this first happened I put some in. When we evacuated the system we lost some and put more in. There seems to be a lot of oil in any mist that escapes. I lean towards too much rather than too little.

Both fans come on when the AC comes on, plenty of air flow. It does stay working at highway speed though. I will try to check the temp differential of the condenser. It is on my suspect list. I see no sign of contamination in the system yet. I think that overheat switch has saved the compressor from catistrophic failure. That is why I hessitate to bypass it.

NickD on Sun September 23, 2012 8:12 AM User is offline

Pressures should be posted running your engine at 2,000 rpm, blower at max, doors open, AC on. For a 90*F ambient your high side looks very low and your low side, kind of high.

Do you really have a high pressure cutoff switch or a thermistor, later is becoming far more popular as it is a heck of a lot cheaper, but using a P-T chart, its not temperature, but the equivalent pressure that will kick off the compressor, typically above 400 psi. For what I work on, definitely require a circuit diagram so I know what I am dealing with, or just working in the blind. I don't like to guess.

Can directly activate the clutch solenoid with a starter switch to first verify the mechanical condition of the system, that has to be okay first with proper pressures and great cooling. If that is okay, then dig into the electrical. Latter can be a problem because just is a maze of wires and components without any identification, need a manual for that as well. Or spend hours tracing it out.

High pressure kickout normally occurs with a restriction, pressures tell that instantly with a proper charge. But if the charge is not correct, and refrigerant is low, you have a leak that must be found and repaired first. Speculation is never a means to find the source of the problem, it all has to be tested. Thermistors or high pressure switches can be tested, but require the proper equipment to do that. Proper engagement of the clutch is one of the first and easiest steps. Could have shorted turns in your clutch coil that only short out when the compressor gets hot, and they do get hot, even in normal operation. This reduces the magnetic field strength, again can be tested. I just slap on a DC metered power supply and leave it for about 15 minutes, if the current shoots up, coil is bad.

Lots of stuff to know when repairing an AC system, first step is always to clean the system up, is air conditioning, and without proper air flow, nothing will work correctly and pressures will be way off. Even with a proper charge.

alexcuma on Wed October 17, 2012 11:16 AM User is offlineView users profile

Hello,

According to your observation compressor high temp and cut out the switch its obsessively compressor get warm, if you want run again then you all need to do is just change the switch hope it will definitely lift up for more information read:

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ezautoac
Junior Member

Posts: 25
Joined: Oct 2012

Thu October 18, 2012 8:26 PM User is offline

If you have an infrared thermometer, can you post the temperature of the compressor when it cuts off? also if your there, check the temperature of the pulley/clutch.

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ice-n-tropics
Senior Member

Posts: 913
Joined: Dec 2003

Thu October 18, 2012 10:26 PM User is offline

FYI
1) GM decided that the Thermal Protection Switch (TPS) was unnecessary. The scroll TRSA12 did not become a warranty problem after the TPS was eliminated.
2) TPS will trip prematurely if the bimetal switch outer housing bottom is deformed. Freezing Canada weather has a negative effect if there is any gap in the RTV silicon surrounding the TPS. Consecutive freeze thaw cycles can dent the TPS base and raise the trip temp.
Just tapping on the top of a TPS can deform the fragile brass/Cu base and lower the cut out temp setting.
3) TPS setting was raised for R-134a vs. R-12.
4) A leaking discharge valve will allow high temp discharge gas to reverse direction through the scroll's single discharge valve (wire drawing) and raise discharge temp.
5) Higher scroll rpm causes a rapid temp rise and at 110 F make the scroll very sensitive to refrigerant charge quantity.
[email protected]

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

Edited: Fri October 19, 2012 at 2:27 PM by ice-n-tropics

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8C 361
Junior Member

Posts: 7
Joined: Jul 2011

Sun October 21, 2012 12:17 PM User is offline

Thank you everyone for the replys. We have made it through another hot season with marginal AC operation. I am leaning towards verifying correct R134 charge before going any further. I see evidence of an O ring seeping so I am going to do an O ring change and put a precice charge in the system and go from there. I may be back next summer.

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ezautoac on Thu October 18, 2012 8:26 PM User is offline

If you have an infrared thermometer, can you post the temperature of the compressor when it cuts off? also if your there, check the temperature of the pulley/clutch.

ice-n-tropics on Thu October 18, 2012 10:26 PM User is offline

FYI
1) GM decided that the Thermal Protection Switch (TPS) was unnecessary. The scroll TRSA12 did not become a warranty problem after the TPS was eliminated.
2) TPS will trip prematurely if the bimetal switch outer housing bottom is deformed. Freezing Canada weather has a negative effect if there is any gap in the RTV silicon surrounding the TPS. Consecutive freeze thaw cycles can dent the TPS base and raise the trip temp.
Just tapping on the top of a TPS can deform the fragile brass/Cu base and lower the cut out temp setting.
3) TPS setting was raised for R-134a vs. R-12.
4) A leaking discharge valve will allow high temp discharge gas to reverse direction through the scroll's single discharge valve (wire drawing) and raise discharge temp.
5) Higher scroll rpm causes a rapid temp rise and at 110 F make the scroll very sensitive to refrigerant charge quantity.
[email protected]

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

Edited: Fri October 19, 2012 at 2:27 PM by ice-n-tropics

8C 361 on Sun October 21, 2012 12:17 PM User is offline

Thank you everyone for the replys. We have made it through another hot season with marginal AC operation. I am leaning towards verifying correct R134 charge before going any further. I see evidence of an O ring seeping so I am going to do an O ring change and put a precice charge in the system and go from there. I may be back next summer.

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