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.

Don't condemn the Klima immediately when the CCU is the more likely culprit.
This is especially true if the CCU keeps the compressor engaged when you press the off button all the way to the right.

Thanks Brian. I decided to open the Kilma just to see if anything looked abnormal. To my surprise there were two pieces of small wood (match sticks I think) shoved in a switch type thing and a burnt spot in the middle of the board. Obviously, some previous PO did some interesting mods to this unit. Also, I managed to remove the aux fan resister (under brake boost unit) and it fell apart in my hand. It obviously has been toast for some time now. I intend to replace both before any more trouble shooting. I'm not sure if the AC system was ever working correctly since I bought it 8 months ago. I bought this car from a mercedes shop, which I will stay well away from. I keep getting surprised at some of the shotty maintenance that has been performed. Thanks.

Anytime.

Well, it appears that the Klima now is the more suspect component.

I went out today in the balmy 26 degree weather to fix my heat. Sure enough, the monovalve has a small tear in the diaphragm.

I cannot, for the life of me, figure out why this results in no heat. The valve needs to actuate downward to close off the flow. You would think that a torn diaphragm would not allow this.

I took the rubber stopper off the end of the valve and put it back together. I might not freeze if I need to use it tonight.

The switch that you are referring to on the circuit board of the klima relay is the actual relay that carries the current to the compressor clutch. forcing this relay closed activates the compressor. Sounds like a little used car lot engineering to get "ice cold air".

Well, my new Kilma and aux fan resistor arrived and are now installed. The system seems to work correctly now. I just have to wait until summer to see if the air conditioning can keep up. I am inclined to believe what jhal suggested about the previous PO using a little used car lot engineering to get "ice cold air". If I ever buy another 300SDL, I'll need a week to check out everything I have learned in the past 8 months. However, I love the car.

1987 300SDL (174,000)
1984 190D (305,000)

Glad to see that the Klima fixed the problem, Hal. Hopefully, the a/c will work properly when the heat returns.

If its any consolation, I believe these vehicles have a never ending "owner do" list. I just got my first oil analysis back. The 603 has some sodium and potassium in the oil, indicating a "slight" head gasket leak.

Never ending...........