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Old 07-25-2010, 04:55 PM
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Join Date: Oct 2008
Posts: 88
W126 AC/Heat Sensor?

1990 300SEL

AC has been converted to R134. Worked fine last year after a system flush and charge.
This year is hottest summer ever and it was blowing warm, had it charged up, worked fair for an hour, then compressor stopped working. Let it sit 2 days and it worked again (marginal) but not great. Had it checked and topped off, no problems found. Today in high 90's and city traffic compressor turned off again.

Seems when outside temperature is high 90's, it doesn't blow cold, the engine temperature goes up to between 100 and 110 (fans work), and then the compressor cuts off. After it sits for a while and the engine cools down, the compressor works again. Blows almost cold when the outside temperature goes down to low 80's or below.

Is there some type of temperature sensor that keeps turning off the compressor when the engine gets hot?

Thank you in advance for any help.

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Old 07-25-2010, 05:00 PM
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Join Date: May 2009
Posts: 398
Sounds like the relay is telling the compressor to cut out. Could be the rpm portion of the compressor. Its part of the comprssor but you have to pull it and rebuild to get to it. They get dirty and cause intermittent faults. You could try brake cleaner and spray it down to get all the crud off, not sure if the old seals would like that tho.

Also freon charging entails you reving the engine to 2000 rpms to draw in refrigerant enough to hold a system charge. Compressor must be engaged full time. AMbiant pressure with car off should peg in the red. When you start car and compressor engages should be like 50 psi low side, when you hit 2k rpms it should be about 30-35.
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Old 07-25-2010, 05:16 PM
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Join Date: Oct 2008
Posts: 88
Thank you for the help.
When it cut out from a charge by my friendly local mechanic, I thought he didn't know what he was doing so I took it to the dealer. They whacked me on the price (naturally) just to tell me it was fine and only needed to be topped off.

If I understand what you wrote, if I take it back to the dealer and complain I should expect them to try and sell me a new compressor or otherwise eat me alive?

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