My wife's 98 E320 A/C recently stopped. I looked under the hood and the compressor clutch isn't moving. Is this more likely a switch problem in the car, or a compressor problem? I haven't had time yet to check the wires at the clutch with a voltmeter.
You are on the right track, there is voltage supplied to the compressors clutch which then pulls it in. There are also a series if switches and safeties, the A/C conrol sends the command but befire it can do so it must go thru a pressure sensor so it does not run if it it low on charge, there probaly is a high pressure sensor in the line.
This will give you some help MERCEDES-BENZ: A/C Diagnostics & Evaporator Replacement
The current for the clutch does not go through a pressure sensor on the 210 with automatic climate control (which I assume you have). This was the case on older cars.
There is a pressure transducer in the high-side line which feeds a signal to the pushbutton control unit, N22. If there is insufficient pressure, the clutch will not be activated, and the EC LED will be illuminated and won't go out when you press the button.
Follow the instructions in the above link and find the value for the refridgerant pressure. It's #7 in the current-value readout.
Also make sure that the temperature values are reasonable. For example, a very low or high evaporator temperature reading will inhibit the compressor operation, without illuminating the EC LED.
Finally, there is a thermo-fuse on the clutch itself. This breaks if the belt slips, disengaging the compressor forever (it can't be changed). This replaces the compressor speed sensor which was used in prior models.
Finally, there is a thermo-fuse on the clutch itself. This breaks if the belt slips, disengaging the compressor forever (it can't be changed). This replaces the compressor speed sensor which was used in prior models.
Ohhhh...it can be changed (it's actually epoxied within the electromagnetic coil which comes as part of the replacement clutch kit)...but it is a bit of a job.
I should have been more specific (isn't that often the case). M-B says that if that fuse blows, it is because the compressor locked up and the whole assembly should be replaced.
That Wiki article is for replacing the clutch itself, which may be necessary; but other than that article, I've not read of someone blowing the clutch fuse. That's why I listed that last. I consider it the least common problem in this system. Low refrigerant or a bad evaporator temperature reading seems to be the two most common problems which prevent operation (in that order).
Linear Mode
