auxillary fan

87 420 sel
150k
Recently, My auxiliary fan comes on when I turn the ignition (the car isn't turned on yet!). I hadn't noticed this before but now it comes on and stays on through out the drive. The car hasn't been driven so its cold when I start it but the auxiliary fan still comes on. Is this something that I should be concerned with.

[Ali Al-Chalabi]

This doesn't sound normal to me.

The fan is supposed to come on high when the coolant temp gets to about 110C.

It will also cycle on and off low when the air condition is on and the refrigerant pressure reaches a certain limit.

Don't know how to fix it.

[MBenzNL]

The auxiliary fans are powered by either the airco (pressure in the airco system to high) or by the engine temperature being too high. I think that either a sensor of the airco or the motor temp is broken. The module for the auxiliary fans probably is out of one ground pulse (coming from airco or engine temp sensor), which results in the fans blowing at maximum speed...

...or something close to that...(I have no experience with the W126 S class)

greetingz,

is this a very serious problem that need immediate attention? can this possibly hurt the engine, cooling system, etc...

[Ali Al-Chalabi]

It shouldn't necessarily hurt anything, it just gets really annoying and wastes a energy.

It isn't an emergency, but I would get it taken care of.

If the air conditioner is on, the fans could cycle on and off low even before the car gets a chance to reach full temp.

[Ron Johnstone]

Usually, if you slap the thermo switch when the car is cold and the power is off, the switch will reset. However, it may again stick in the "on" position when it gets hot. At that point, simply buy a new one as it isn't salvageable.