The aux fan was coming on and turning off randomly. I pulled the plug on the sensor mounted on the front side (closest to the radiator) of the themostat housing (not the acc coolant temp sensor) and the fan stopped running. I believe that in fact if you have this sensor it's function is to turn on the aux fan at high coolant temps.

I need to replace this sensor/switch or probably just pull it and clean it to see if I can again get the proper control again.

In the meantime it is disconnected for now since the car wouldn't hardly warm up with the fan running all the time. Of course this is probably a sign that the t-stat is going too and is stuck partially open becasue it sometimes will not even get up to 80C while driving down the highway.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique