For some reason, the compressor on the 81 380sl I own started running constantly whenever I push DEFROST button, even though the compressor button is OFF, and hot air is coming out the correct vents.

The compressor is also running constantly when I push the BILEVEL button, and the temperature is changing when I move the temperature wheel. So I know the wheel is working correctly.

I also know that the compressor button is working because the compressor is coming on when I push it and the AUTO HI or AUTO LOW buttons, and the center flaps are opening as they should. But in this case, pushing the compressor OFF button turns the compressor off, as it obviously should.

I've replaced the servo and the amp (twice) with units I know are good. But I've still got the problem, and I don't know where to look next. So any help that anyone could give me would be greatly appreciated.

Thanks very much in advance.

Compressor always running on bi-level is normal. Running on defrost is something that I am experiencing myself. Don't know the cause. Let me know if you find out.

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1978 300CD
1984 300D
1985 300SD

I didn't realize that. Thanks for the info. I thought the compressor button had to be on for the compressor to be running, even in BILEVEL?

Wow. I thought for sure I was the only guy that ever had this problem. Is your defrost air as hot as it was before your compressor started coming on? Or is it cooler because it's mixing with cold air?

I will. But I've posted this on several forums, and nobody seems to know the answer. I've had servos stuck on hot or cold before, and changing the servo usually cured that. But nothing like this. I was guessing it's electrical, but now I'm not so sure.

Running A/C Compressor during defrost is by design. Outside air is passed over the evaporator, which is cold, causing the moisture in the air to condense. This produces dehumidified air. That air is then passed over the heater core before being ducted up to the windshield.

Result is a more effective defroster.

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W. Brian Fogarty

'02 S55 AMG (W220)
'92 300SE (W140) - sold
'76 450SEL 6.9 Euro #1164
'76 450SEL 6.9 Euro #521

"Bond reflected that good Americans were fine people, and most of them seemed to come from Texas..." Casino Royale, Chapter VII

Thanks for that info, W.. I didn't realize that either.

Now I'm wondering if the compressor went on when I turned the defroster on a few days ago because the ambient temperature was high, whereas the compressor doesn't go on it the winter because the ambient temperature is low?