81 300SD.

Last year I was running a bit hot in traffic and I'm trying to be pro-active this year. The question concerns the aux fan control.

I can get the fan to come on if I jump the dryer plug, so I know the relay/fan is OK, and last year the fan would come on when the ac was on and the pressure was high enough. But I never heard the fan come on without the ac on even at 110C.

I removed the wire from the sensor on the t-stat housing and grounded it - I even grounded it to battery neg just to be sure. Fan never came on. I suspect the sensor, but I tried the same thing with my wife's 300SD (Practically Perfect in Every Way) and the fan would not come on either. I find it strange that both cars sould have a bad sensor, but maybe.

Is there more in the circuit for the t-stat sensor? Does it use the same circuit as the AC sensor?

Am I on the right track?

Thanks

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer