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,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)