On a previous thread about higher-than-normal coolant temps, Benzmac pointed out that this could be due to a broken aux fan preresistor. He also cautioned that the wires leading to the preresistor often corrode and break. That was the situation with mine. Had to have a new wire spliced in. Now the aux fans operate on low speed as they were meant to. Now I'll see if it will actually drop my coolant temps, which run about 90 degrees in 70 degree ambient temps.

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Steve
'93 400E

Glad you now have the low speed fan, but I do believe that the low only comes on with a/c operation. [ high side pressure a/c sw.]
The high speed fan is controlled by the engine coolant temp, so you
may want to also check that out.
90C is not considered too hot and the temp sensor will not call for high speed at those temps.

Thanks, Arthur, for the suggestions. My high-speed fans do work. I probably obsess a bit about coolant temperature (but then, judging by the number of posts on this, I'm not the only one). Donnie had posted a reply a couple weeks ago to someone who felt his engine was running hotter than it should (though still below 100) and Donnie recommended checking out the preresistor and its wires since he'd replaced many of them. You're absolutely right, the low-speed fans come on when AC pressure is high, so I think Donnie's point was that there are circumstances when ambient temps are not high enough that coolant temp will trigger the high-speed fans but AC pressure would be high enough to switch on the low speed fans, which in turn would lead to cooler engine coolant temps in stop-and-go traffic.

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Steve
'93 400E

True.
Just thought you should know that the low speed will never come on if the a/c is not on . [ which I now see you already knew]
Jim F 's idea of lowering the coolant temp sensor cut-in by parallelling a resistor with the ECT sensors resistance for the high speed fan is something that has worked out well and you may want to do some checking into that.
That is an independent circuit and needs no A/C on to operate.
I have spoken to him about [and am also playing with ]the idea of doing some mods
with the resistor circuit to give high speed to the a/c high side pressure sw circuit.
That seems to be when both the radiator and condensor really
get taxed at high ambient and could use more air flow.
I never did like the idea of a condensor in front of a radiator, but I guess that's where it has to go.....
Anyway, possible resistor change/jumper or cabin switch...
something along those lines.....just thinkin about those real hot/humid areas of the country where the a/c just doesn't cut it.

Good thoughts. Stu Ritter has a mod. for the mechanical fan where he basically makes it run full time instead of coming on with high underhood temps. Says the only drawbacks are more noise and a little lower MPG.

These otherwise superb vehicles just aren't quite designed for prolonged heat and humidity, are they? Probably because Germany doesn't see really hot summers, I suppose. I know for a long time Europeons didn't bother to get AC since they didn't need it, but then again Mercedes has advertised how it tests its vehicles in all sorts of extreme environments, including a test in Death Valley.

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Steve
'93 400E

um -Yes, the "Comfort Zone" expected in the US is "Icicles hangin off the vents" A/C .
Much too cold for Europe.

It is also my thoughts that if the aux.fan cut-in temps were not so damn high, [105-107C], we would prob see a lot less head-gasket problems. I have been Benzin" for 35 years and have
done some sort of set-up to control the temps from getting up there. And never a blown gasket/over-heat....

But who knows.....