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Old 03-12-2001, 09:38 AM
Al M.
Posts: n/a
87 420 sel
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.
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Old 03-12-2001, 11:52 AM
Ali Al-Chalabi's Avatar
Senior Member
Join Date: Nov 2000
Location: Knoxville, TN
Posts: 1,837
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.
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Old 03-12-2001, 02:20 PM
MBenzNL's Avatar
Join Date: Jan 2001
Location: the Netherlands
Posts: 1,417
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)

1990 300SL-24
1993 C250D
with a minor 600+k kilometer
(the Netherlands)
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Old 03-12-2001, 03:11 PM
Al M.
Posts: n/a
is this a very serious problem that need immediate attention? can this possibly hurt the engine, cooling system, etc...
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Old 03-12-2001, 04:08 PM
Ali Al-Chalabi's Avatar
Senior Member
Join Date: Nov 2000
Location: Knoxville, TN
Posts: 1,837
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.
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Old 03-12-2001, 06:12 PM
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Join Date: Jan 2001
Location: Santa Clara, CA
Posts: 414
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.
'76 240D-Sold
'78 240D-Sold
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
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'86 560SL 124K Miles
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