viscous fan problem!! - PeachParts Mercedes-Benz Forum

#1 09-07-2002, 04:50 PM

Took my fan off today and found out if you take remove the bi-mettalic plate and take the pin out or push it in thefan doesn't lock nor does the tension of spinning the fan change. Is that normal?

Ali Al-Chalabi · #2 09-07-2002, 09:15 PM

The bi-metallic strip is what bends according to temperature and locks the fan or allows it to freewheel. Also keep in mind that it takes high temp along with centrifical force in order to lock the fan. You will still be able to turn the fan when the engine is off with your hand even when it is over the temp at which it should lock up.

psfred · #3 09-08-2002, 09:56 PM

It isn't possible to test a visco clutch by spinning it by hand. The "clutch" is actually a two speed fluid coupling, sort of like a torque converter.

The bimetalic spring rotates the pin, and the pin then operates the stator in the fluid coupling, changing from high slip at low temp to low slip at high temp by changing the angle of the blades. GM had auto transmissions many years ago that worked this way.

The easy way to test a visco clutch is to allow the car to idle and get warm. At the temp where the clutch should engage (220F or so), it should start to roar and blow large volumes of air. Sounds like a jet engine on the highway.

If you them shut off the engine, the fan should stop rotating within one revolution after the engine stops. If it spins more or less freely for many revolutions, the visco clutch isn't locking up.

The two things that go wrong are for the fluid to leak out (spins freely all the time) and for the guts to freeze (never locks up). You can sometimes fix the latter by rotating the pin back and forth by hand. Never pull on it, you will break something or disconnect the stator, in which case it won't lock up anymore. Occasionally, the clutch will freeze, and the fan will run at engine speed. Very annoying, very noisy.

The cure for a visco clutch that won't lock up is to replace it. Can be expensive, but much cheaper than a cracked heat from overheating.

Peter

JimF · #4 09-09-2002, 07:52 PM

go to my page and do the Ritter mod. Pics are there to guide you. Really works well.
http://pages.prodigy.net/jforgione/MB_S500.html See MENU #19.

Some corrections:
psfred: The bimetalic spring rotates the pin. . . No it doesn't ROTATE the pin. It PRESSes on the pin clutch with about 5lbs of pressure.

psfred · #5 09-09-2002, 10:32 PM

Jim:

Most of the bimetallic springs on visco clutches are coiled -- no way they will generate inward pressure. MB may be different, I've never really looked!

Peter

JimF · #6 09-09-2002, 11:09 PM

of the Mercedes VFC. It requires about 5 lbs of pressure to keep the pin fully "pressed" in; this causes the fan to freewheel.

The bimetallic strip (bms) is made from steel and brass. So it's amazingly stiff! There's pics of some tests of the Sachs VFC in menu #19 and #20.

Not sure of the mixture but it appears to be alot of steel. Also takes very high heat (above 110C) to ge sufficient "bend" to fully release the pin. That's why they don't work very well!

Page can be accessed by clicking on the 'www house' icon below.

psfred · #7 09-09-2002, 11:20 PM

Jim:

I don't think they are supposed to lock up below 110 C. Should lock up at the mark between 80 C and the red zone on the temp guage. Aux fan kicks in at 100 C.

I looked at the replacement clutch I have for the 300 TE I've never gotten around to replacing (overheating problem last summer that turned out the be a bad radiator cap), and there is a strap type spring. The Volvo has a coiled spring that rotates a pin in the center, so do most American visco clutches I've seen. I'd expect the MB type to be more reliable -- less chance of crud making the pin stick!

Peter