Kevin, First a little clarification, then on to your problems. In the 124.133/.193, the high speed only runs at temps over 105C. The switch is on top of the engine at the water outlet. The low speed only runs when AC refrigerant pressure exceeds 16 or 20 bar, depending if you have the old green or new red switch. That switch is located at the receiver-drier. Nothing related to fan operation is located at the thermostat housing (the switch down there is for the tranny cold-shift-softening vacuum circuit).
1) If you can short the 2 pins and make the fan come on high speed, replace the switch. These switches have a high failure rate. Every one of the 4 cars in my family came from the previous owners with bad switches. Replace it!! I use the 603.971 switch which is cheaper and has a lower temp rating. Part number and photo is here: <http://www.w124performance.com/images/W124_stuff/Temp_switch_105-12x.jpg> 2) The low speed switch requires AC evacuation to replace. If you have the green one, replace it with a red one. The resistor is right next to the receiver drier. If bad, or bypassed, replace it or un-bypass it. Hopefully this switch is working ok. 3) Your engine should almost never, EVER exceed 105C and need the aux electric fan. If that's happening, you have other issues. If the fan clutch is truly bad, either try refilling with silicone, or replace it. Yes it does something at freeway speeds too, don't expect ram air to cool the engine. 4) If the radiator is more than 5 years old it is also highly suspect. They can corrode internally and although flow is fine, it won't cool worth squat. My car had the radiator replaced at the dealership in 1993, and when I bought the car in 1997 and had hot running problems like you, I assumed it couldn't be the radiator because it was only 4 years old. Guess what? It was the radiator. It's awfully hard to diagnose, though. I replaced everything else and that was the only thing left, so I took the gamble and it worked. :-) -Dave M. > ---------------------------------------------------------------------- > Date: Tue, 11 Apr 2006 08:53:42 -0400 (EDT) > From: "Kevin J. Slater" <[EMAIL PROTECTED]> > Subject: [MBZ] Cooling problem with my 87 300TD > > > Folks, > > I have a few problems with my 1987 300TD and I could really use some list > wisdom tracking them down. > > The foremost, and the one I've forgotten about since the last warm days of > last fall, is that my auxillary fan never comes on. On the way home from > work last evening (outside temperature around 68 degrees F) after I got > off the highway and was making my way through some local lights, the > temperature gauge went about 110 (probably over 115) and no aux fan. I > made it home without incident and decided to check out what I could. > > Last night, with the key in the run position, I shorted the pins on the > temperature sensor on thermostat housing and the fan came on as it should. > (Didn't sound great, but it hasn't run in months either.) I did the same > with the pins on the high pressure switch and again the fan ran as > expected. > > A couple of other datapoints: 1) the car has had the #14 head replaced (by > me) with a #17. 2) I don't have any coolant in the oil or oil in the > coolant problems. 3) previous owner was a hack and I think the aux fan > resistor is missing, or maybe I don't know where to find it. 4) my viscous > clutch for the rad fan is probably mostly dead. > > What diagnosis do I need to do to find why the aux fan isn't running? > Eventhough this might not be the only problem, I think if it was working > correctly, I'd get some relief from those high coolant temps in stop and > go traffic. I know the aux fan used to come on at times when the a/c was > on and I was pulling into the garage b/c I would hear it as I pulled in. > > ...Kevin (Slater, Mars, PA - Marshall I'm extra-signing for you and I'll > come and cut your grass if you help me out!) > > 87 300TD 282k miles
