when my w124 88 reaches operating temp between 80 - 90C it runs wonderfully but will sometimes climb over 90, i don't know what the temp must be before the aux fan starts but I have'nt seen it operate in years! it however does work becuase when the aircon is on it starts, does this fan come on only for the aircon or also incase of overheating? if so what temp triggers it and how can I check its working?

Assuming you have a six cylinder motor in your 124, there is a coolant sensor in the top of the cylinder head that switches the electric fans on at specific temperatures. It will be the sensor with three or four pins. The other sensors feed the dash gauge and the fuel injection. Just climbing a little over 90C is no big deal. IIRC, the fans switches on at 105, but I'm not certain of the number.

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Norm in NJ
Next oil change at 230,000miles

It is 105 degrees. That triggers high speed aux fan. There is a simple mod you can do that will makes it come on at lower temp (around 90) which I'm about to do. The air cond. pressure switch triggers low spd aux fan.

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car

[QUOTE=dylant]when my w124 88 reaches operating temp between 80 - 90C it runs wonderfully but will sometimes climb over 90, i don't know what the temp must be before the aux fan starts but I have'nt seen it operate in years! it however does work becuase when the aircon is on it starts, does this fan come on only for the aircon or also incase of overheating? if so what temp triggers it and how can I check its working?[/QUOTE

The engine driven fan clutch tightens at about 100C and when it does you can usually hear fan roar at 2000 and above. If the temp continues to climb the electric fan will engage at high speed about 105C. You can test this by letting the engine idle in warm weather until the temp gets up to about 105C.

With the A/C on the electric fan will engage intermittently at low speed based on a pressure switch signal in the A/C system.

Duke