Thread: Viscous Fan Operation - Measuring its RPM

AFAIR I have read in the past that they are designed not to exceed 3000RPM and that seems to be confirmed by your test.

So don't pin them if they are failed as those fan blades make a hell of a mess of batteries and bonnets.

I think a good way to test is to just look at it and see if there is silicon fluid leaking from front or back ( dirt build up) , whether there is play in the bearings. IMHO another test is to listen on cold start up as they should engage for the first couple of minutes ( loudly) due to the silicon fluid pooling in the bottom.
I can certainly hear and feel mine engage at about 95C.
Regards Philip A

Originally Posted by PhilipA

AFAIR I have read in the past that they are designed not to exceed 3000RPM and that seems to be confirmed by your test.

Quite possible, Ive never seen it past about 2700.

Good stuff! How did you actually measure rpm? Proximity sensor counting each blade? Or something else?

Sent from my GT-I9305T using AULRO mobile app

Originally Posted by ROAMER_AUS

Good stuff! How did you actually measure rpm? Proximity sensor counting each blade? Or something else?

Sent from my GT-I9305T using AULRO mobile app

I used a Hall effect sensor measuring just one blade, I'm going to improve it with 2 blades for balance and increased response resolution. I have it wired to a microcontroller and output to an LCD screen. Quite interesting to see how it works, far more than I thought it did, I thought it was mostly an idle, slow moving fan, but no, it works quite a bit with normal driving.

Thanks finally some real world data to convince people to leave the viscous fan on their vehicles. A decent designed fan and shroud is far more effective and reliable than electric fans.

Good stuff!☺

JC

FWIW,on a Disco TD5 you can often here the viscous fan cycle when driving slowly,particularly doing hard off road stuff in hot weather.

Apparently the Puma has the same fan but I have never noticed it coming on and off.

The D4 has a viscous with electric override,bloody noisey when it is at full noise.

As others have said good to have some real world operating parameters.

Nice graphs man. I take it the x axis is a time measurement or is it vehicle speed?

Another interesting point is that it seems transitional, ie, not just on and off.

For instance this morning for maybe 10km open road at about 90km/hr it remained around 850rpm which I would think is completely OFF. As the air temp under the bonnet (inside shroud - inflow) increased from about 25C to 50C the rpm moved up to about 1000 rpm, then gradually to about 1200 rpm.

With AC on the coolant temp slowly moves up to around 83C and at some time in this higher range the viscous fan moves into total lock to around 2400rpm which then brings the coolant back down quite quickly to about 78C, the fan after a while slows back down to 1200rpm, the coolant temp again gradually increases to 82-83C and the same process occurs....sort of cycling.

Without the AC the fan pretty much stays at around 1200rpm as the coolant temp sits around 78C.

The interesting thing is how efficient the fan is at removing heat once its fully locked..