I think its the faulty auxillary fan thermo-sensor switch. when this switch is faulty it can do 2 things. Keep the fan on all the time or keep it off all the time. Also check the auxillary fan relay. Also there is a connection between receiver dryer temperature switch and auxillary fan.

It seems like someone had purposely jumped the switch to keep the fan running all the time as soon as you turn the key in. Some people do this to compensate their faulty cooling system. Not a good idea though.

MVK

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1985 300D Turbo 179,000miles