[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