| I had to troubleshoot a nonfunctional compressor clutch on my '83 300SD when I first got it. First, check the fuses and A/C relay by substituting a good known relay for it (there are several identical relays in the box so this should be no problem). There are two electrical switches in series in the a/C compressor clutch control circuit: one is in the climate control unit, it turns A/C on and off. The other one is a thermostat switch with a probe going into the evaporator. This was the culprit in my case. I simply jumpered the switch leads and the compressor clutch turned on. This switch is located under the dash on the driver's side, closer to the center console. You need to remove the lower panel under the dash to get to it.
The system must have enough refrigerant for the compressor clutch to turn on. Also, you need to check the clutch itself. Supply a 12 Volt battery power directly to the clutch terminals and see if it will engage. Also, measure the resistance of the clutch windings. It should be about 5 ohms. Infinite resistance means a burned out clutch. |