My 300SDL started to run hot at times, so I searched the forums (as always)for suggestions. I refilled the viscous clutch with silicone and then checked the electric fan. I found that the electric fan was not operating on low with the AC and would not go high when the temp rose above 105 C. Checked the fan motor and circuit breakers which checked good. I next bypassed the fan resister and swapped out fan relays, and still no go. So I reversed the wires from the temp switch to the pressure switch so that the fan runs on high when the compressor is engaged. That seems to work well. Temperature stable at about 87 C. However, I then noticed that my AC compressor is on all the time, even when I press (EC) or the off button (O) on the control unit. How is that possible? Other than running all the time, the heating/air system works well. Should I worry about it?

The first thing would be a frozen A/C Clutch. Disconnect the two wires going to the clutch and see if it still turns with the engine on. If it does, you do have a frozen clutch. If it were a V-belt drive, I'd then remove the belt (But you have a serpentine belt so that is NOT an option.) ASAP, take it to an indie shop that is MB only or A/C only.

Alway's on, you can generate some extreme pressures in the A/C system, depending on air temps. If you have a low/no charge of R-134a or R-12, you can damage/kill the compressor.

Good luck, don't wait to fix this.

Thanks for the reply. I didn't think of a frozen clutch. Went immediately out and checked. The clutch spins freely with the ignition off, however, when I turn the ignition on, the clutch engages without even starting the car. I'm looking through the electrical schemantics but am getting somewhat lost. Is there a typical relay, switch, or connection that might cause this?

I think you need to put those wires back into original position while you are trouble shooting....

Hal, my SDL also has a habit of engaging the compressor when in the economy mode. However, the compressor shuts off when the system is turned off using the far right button.

You can certainly pull the Klima relay, which I have done, and this will definitely shut the compressor down if it is not frozen. The relay is about 1" x 3" x 3" and is sitting right on the firewall directly in front of the driver. It unplugs straight up and has the word "Klima" somewhere on the top of the black box.

Mine is sitting in the coin tray right now until I change the CCU.

Thanks to all. I replaced the wires back as they were, and pulled the KILMA. That stopped it. I'll trouble shoot when it gets a little warmer. I suspect a new KILMA might be in order. Appreciate the help.