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  #1  
Old 07-07-2008, 05:09 PM
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Join Date: Apr 2005
Posts: 72
300SD/W140 A/C Compressor won't run under 2000 RPM

I've got a '92 300SD with 350K on it. It runs just fine, but last Fall the A/C seemed to run only when the RPMs were over 2000. I have done some more investigation and found some other hints on the problem.

1. The compressor seems to run OK when the Auxiliary fans are on, even at idle.
2. Refrigerant level is OK.
3. The dashboard control module shows error 74 which is "coolant circulation pump." What is a "coolant circulation pump?" And where is it? The compressor circulates the refrigerant and the water pump circulates the coolant—or at least that's what I thought.
4. System runs fine at RPMs over 2000.

What do I check now to find out why it won't run at idle, for instance.

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Old 07-07-2008, 07:57 PM
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Join Date: Oct 2004
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There are two likely scenarios, 1. Your compressor clutch is very loose and doesn't grab enough to turn the compressor until the RPMs are up. Typically not a problem but with 350K it is feasible. 2. Your RPM signal read by the climate control is faulty, or is being read as faulty. Possibly being read as a valid signal once the RPMs are up. You need to see if the compressor clutch has power going to it at idle. If it does then the clutch is too loose, if not you have an electronic problem.
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Old 07-16-2008, 11:11 AM
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Join Date: Apr 2005
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Where is the A/C Clutch wire? W140

I have been dealing with an alda problem and got that fixed, now am focused on the A/C issue.
Where is the clutch activation wire? On my 126 it's easy to find, but the only thing that I can find on the w140 is a pair of wires that are screwed into the bottom of the compressor half way back, so I don't think they drive the clutch. I think they are the temperature sensor as they appear to a unit. I am reluctant to take them out as they may be part of the pressure containment system and that would not be good.
Apart from these, I can see no other wires going into the compressor, though, in fact, it's hard to see much of anything down there! It is so packed in!

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