I have an '85 300SD -- live in the mid-Atlantic (some warm temps). The running temp is around 95-100C, sometimes goes over 100 but drops back at highway speed. Two questions: (1) What is the proper thermostat? I'm thinking of putting in an 80 degree T-stat, and would expect the running temp to be, say, 85-90? Does this seem right? (2) Where is the relay for the auxiliary fan? The fan is not coming on automatically (tested the fan itself, and it operates). I can't replace the temp switch on the A/C drier myself, but I think I can replace the relay -- if I can just find it. Thanks in advance.

Curt

The 1985 car has 2 speed fan, low speed with a/c pressure & high speed with engine temp over 100c. There are 2 relays for the 2 different speeds. T/stat temp is the fully open rating. If the rating is 80 but engine runs at 90 then cooling system is doing all that it can do. Engine temps around 100c this time of year are NORMAL! If temp goes up at highway speed then you have problem. Air flow at 60mph is greater than fan can pull, also volume of waterpump increases with speed.

response is appreciated -- thanks for reassurance on the temp. Where are the relays located? The auxiliary fan isn't coming on at all. I don't know what t-stat is currently in the car -- was thinking that it might be rated at higher than 80 degrees. All things being equal, wouldn't the car run at, say, 85 degrees or so with a properly working 80 degree t-stat?

The relays are in the rear section of fuse box. MB wiring diagrams list position of each relay & there is a "letter"(alpha) under each relay. Find a diagram, check fuses by removing them & good luck. The 1985 300SD was the first W126 diesel w/two speed fan, make sure to find correct dia.

There is a thermo switch on the receiver/dryer that will turn the aux. fan on when the refrigerant reaches a high enough temp. don't confuse this sw. with the low refrigerant cutoff sw. for the AC comp. The thermo sw. can be replaced without losing any refrigerant.Not so with the low pressure sw. Check the thermo sw by un pluging the two wires and connecting them together. Aux.fan should come on.(ign.sw.must be on.) Wm. Lewallen

Thanks, William. I'll try jumping the wires on the thermal switch -- tried that once, but don't recall whether I had the ignition on (think I did). When I jumped the wires I got a clicking sound (chatter?) in the vicinity of the fuse box -- which made me think the problem was probably a relay. Can you tell me which relays control the aux. fan? (don't have a wiring diagram -- purchased a manual for the engine, but no wiring diagram.) These are very helpful responses. I didn't know this was a two-speed fan, for example, and I wasn't sure whether any of the relays in the fuse box belonged to the fan. getting closer, hopefully.

I'm not sure which relay controls the aux. fan. You can take a relay that you know is good and exchange it one by one with the other relays. Hopefully you will find the one that controls the aux.fan. Wm. Lewallen.....