Change the switch...its most likely the fan switch. Inexpensive. Give your vin# and you should get the right one but I believe there is only one 3 prong one. The secondary fan will kick in when the AC is on. Does the override switch engage the big fan? I have never seen this...is it an aftermkt add on? If so is it fused?

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yal
88 230E (W124)
euroheadlights,
clearcorners,
smoke/red tailights
98 e420 sport replicas