The fans will come on with the a/c and with high temp
so with that if you have a freon leak and it is low enough it will not close
the pressure switch so the fans will only come on but only with a high temp.
this switch is on the high pressure line by the receiver dryer pull the leads
off and touch them together with key on and a/c on the fans should come on
if not your problem's are elseware
post back and let us know if this helped or what you find

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Keeping 'em Running
1990 300SEL
1994 E320
1989 E300