There is no sensor for that operation. The climate control pushbutton unit gets the engine temperature from the instrument panel and works with that.

Without the AC running, the fan should not start until the engine temperature reaches about 100C.

How are you going to check the main fan without idling the motor?

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2005 Pathfinder LE
2001 E55
1998 M3
1982 320i

deltajet, matt is talking about aux fan. matt on my 1999 e300d the high used to come on way earlier then that and stay on after car was stopped for about 15 seconds now niether is happening. so i am assuming either the module for aux fan is defective, and im out $250 ! what im reading is the 98/99 are different from 97,96,95 yrs, thats why i was hoping for a wiring diagram or someone who did this already

If you do have a mechanical fan, your system uses different parts from the earlier years, but the mode of operation is the same.

Assuming that you do have a mechanical fan, if the fan starts in test mode, it will also start for any other mode of operation. If you're not getting fan demand but the temperature gauge works and the fan test works, the problem could only lie in the climate control pushbutton unit N22.

However, the fan is not supposed to run with the engine temperature below about 100C, so if it was starting before the demand would have to have been from the air conditioner system, either the refrigerant pressure or temperature.

matt , what would keep the power on high from the air condition side of things, I feel if i find that sensor it would remedy atleast one thing, it used to go on high in summer time then stay on for a bit after that to cool things down! and temp wouldnt go over 85

The sensors that run it are the two sending units at the receiver/dryer (refrigerant high-side pressure and temperature) and the ECT (engine coolant temperature) which comes from the instrument panel. There is not a separate ECT sensor for the fan, as used on older models.

Matt - Would a B1231 fault cause problems with the aux fan operation? In post #1 he talks about high coolant temps.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)