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.

 

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.

 

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!!

 

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.


--
Gunnar

240 Turbo Wagon '84 200 K Miles
940 Wagon '92 150 K Miles
on Swedish roads