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Diagnosing fan clutch

My 740GLE overheats badly when idling in hot weather but OK when moving moderately fast. Except for the idle speed that is rather erratic (some day exactly where it should be around 750 rmp, other days at 500 or 900 or switching to different speeds in between) there is no other problem. Of the few items I have to check to fix this overheating the thermal fan clutch seems to be the hardest. When the car is cold there is a resistance when I try to turn it by hand (it won't spin.) This morning when I got to work (after some 30 km drive) it seems to offer the same resistance when I tried to spin it. Is this indicative of a faulty fan clutch? Not knowing how stiff a hot fan should be I have no clue as to whether the fan clutch is good or bad. Can somebody tell me if it should offer subtantially more resistance once the car has reached normal temperature or does this "more resistance" only occurs when the car is over the normal temperature range and only when hot air is blowing past it. I will do this "cover it with a blanket test" later if this is considered inconclusive.

Reply to
Duong Nguyen
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Now that I have re-read the message is rather misleading. What I mean is "it won't spin freely." It does move with a slight resistance.

Reply to
Duong Nguyen

What I do to test these is use a balled up rag to stop the fan with the engine at idle. When it's cold, the rag stops the fan quite easily, use this as a baseline. When it should be locked up (engine temp above normal) this method won't stop the fan, don't try too hard. Watch your fingers!!

Reply to
Mike F

Before investigating the fan, did you check the radiator?

If the exterior of the radiator is full of insects, dust, leaves and whatever may get stuck in there, it tends to block the air flow, typically causing overheating when idling.

If the radiator is old try cleaning it with a hose, spraying water from the engine compartment through the radiator towards the front of the car. Use a moderate pressure since the aluminum radiators are not too sturdy.

Reply to
Gunnar Eikman

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