Thread: Viscous fan diagnosis

The way I was told to test them . and it worked well . was to warm the motor then force a rolled up news paper into the blades while someone revs the motor up for you .
If you can stop the fan it is cactus . Do it on the side that goes with the lay of the blades . Usual disclaimer , do not do anything that might be dangerous .

IIRC it doesn't free spin when cold as such, but slips relative to the speed of the engine rotating.
When the unit is hot, the clutch engages, and you get your 1:1 engine to fan rotation ratio.
I too have heard of the newspaper trick. There may be a more scientific way to evaluate (turning torque when cold vs hot), but for the layman, use a newspaper.

The way I tested them in my workshop was to have the engine hot, ie when it arrived at the workshop, have someone watch the fan and turn off the engine. If the fan rotates more than 2 turns it is not within specs. This way you are not putting anything in there that can cause damage.
LuckyLes

If it turns your water pump, its 1. seized and 2. your fan belt is too loose.

Originally Posted by bee utey

.... and 2. your fan belt is too loose.

Yeah, that's the other question to ask.

If it turns the waterpump, does it also turn the fan belt, and other ancillaries along with it, or does the pump pulley slip on the belt as it turns?

On a V8, the working of the viscous unit is audibly obvious .. one of the loudest fans I reckon going in any passenger vehicle.

From cold, it'll make a cyclonic sounding whooshing, and then go more quiet after about 30sec - 1min.
As engine heats up, and reaches operating temp, with some revs on the engine as the viscous unit started to lock up, you hear that loud cyclonic air rushing noise again.