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Old 07-26-2004, 03:58 AM
Johnson Chan's Avatar
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Join Date: Aug 2003
Posts: 229
How to test viscous fan clutch

This question is on my 1992 300D 2.5 turbo.

I suspect the viscous fan clutch is bad or going bad. WHen at idle, the temperature would rise, when the car is in motion or when i rev it, the temperature falls. Is there any way to test it without removing it?

When i turn on the heat, the temperature falls, when i turn on the A/C the temperature rises.

thermostat replaced, coolant flushed, nothing changed....
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Old 07-29-2004, 02:25 AM
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Join Date: May 1999
Location: Pensacola, FL USA
Posts: 48
Two years ago I experienced the identical problem, and the identical symptoms, and replacing the fan clutch solved it.

With the clutch still on the car, I'm only aware of two tests that the professionals use, by sight and feel, and even then you can be deceived.

The first, and most obvious, is to observe that the clutch engages. When it does engage, and with the engine compartment open, no one standing near it has to wonder about it because it comes on like "gang busters" and immediately moves an enormous amount of air, like a wind storm. Here, however, is the point where an inexperienced person can be deceived. If the clutch is slipping, it will still do everything I described, except the flow of air will not be delivered with the same force and volume. In fact, it will do just what has happened in your case. The air flow at idle speed will not be sufficient to keep the engine at nomal temperture and the temp. gradually climbs.

In my case, I had given the fan clutch a passing grade but an old timer at a "radiator shop" disagreed, and he was correct.

I hope this helps.

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Old 07-29-2004, 11:00 AM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,055
Behavior sounds normal to me Quantifying "heat up" with actual temperature gage readings would help.

The viscous fan clutch keys off the radiator exit air temperature. On my car it typically tightens at about 100C coolant temperature. At idle speed it does not produce much air flow, but once up to 2000 RPM audible "fan roar" is noticeable.

If the idle period is long the coolant will continue to heat up, and the electric fans engage high speed at about 105C. The electric fans rapidly bring the coolant temperature back down to about 100, and it will continue to cycle this way at idle.

Once I get the car moving at steady speed over about 35 MPH the temp will drop below 100C.

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