On 28/02/2020 18:28, Tim+ wrote:
T i m wrote:
On 27 Feb 2020 14:51:23 GMT, Tim+ wrote:

snip

The last car that I had that definitely had a viscous coupled fan was a
Volvo 240 which always gave a distinctive “roar” on start up from cold
until the coupling started slipping.

Whilst that makes sense on a PC fan to blow the dust out g, I don't
remember it happening on the viscous coupled fan on my 2L Sierra
Estate?

On cold start it may have idled round from the drag in the system but
you could very easily hold it when starting and you could only just
feel it trying to turn?

Once the heat starts blowing though from the rad however, it really
firms up and does it thing.

It sounds like the Volvo one worked differently?


It just seemed to be €œdraggy€� when first spun up from cold. Within seconds
I guess the oil or whatever inside it would warm and the drag would
dramatically drop, at least until the engine was hot enough to actually
need cooling down.

I dont think it was different in principle from any other, it just had a
high start up drag.

During normal running, the fluid is effectively "pumped" to the hub
reservoir and only released when heat opens the valve and lets the fluid
out. Once the valve opens, fluid continuously circulates, hub to
periphery to hub - and the viscous drag when it is between those
positions drives the fan.

It is possible that when stopping the engine, the heat means that valve
is open, but the lack of rotation prevents it pumping back to the hub.
If so, the fluid will remain between the sections and provide drag on
startup, which will only reduce as the fluid gradually ends up back in
the reservoir?

SteveW