Stop poking the BMW Viscous Fan! A better way to test

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This is the second in the series on the BMW V8 and V12 Cooling System - and this is on the Viscous-Coupled Fan. The viscous-coupled fan provides airflow through the radiators of the big BMW's.

Instead of just telling you how to test it, I will show you exactly how it works, what its limitations are and the failure modes that are sometimes seen with this usually very reliable component.

However, I will show you how to test it without the need for rolled-up newspapers or losing your fingers!

I have used a generic two-state viscous coupling for the purposes of explanation - there are many variations on the design, especially concerning the fins that are used to transmit the force between the rotor and fan housing.

Viscous couplings that are used to transmit a lot of power (for instance, on a fan used on a John Deere tractor) use many more fins for the sheer-force coupling, even having fins on both sides of the rotor. The BMW viscous couplings only have fins on one side of the coupling - and no fins on the mating face of the housings as relatively little power is required.

As explained in the video, the viscous coupling is a two-state device, it is either engaged or dis-engaged and it is easy to tell when it is engaged - it makes quite a racket and moves a lot of air - and it is easy to see when it is dis-engaged as it just idles around - this makes it simple to test!

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