On 28 Feb 2020 18:28:27 GMT, Tim+ wrote:

snip

It sounds like the Volvo one worked differently?


It just seemed to be “draggy” when first spun up from cold.

As was mine but not anything like enough to keep up with the engine
RPM, even if given plenty of time (seconds). It did spin up in time
(10's of seconds) but still only a percentage of engine revs (because
of aerodynamic drag on the fan part of the blade) and if you slowed
and stopped it with something (not a finger), you could then still
hold it with a finger until you got enough hot water though the rad to
heat the bi-metallic actuator on the fan itself.

Within seconds
I guess the oil or whatever inside it would warm

If like mine it was an aluminium hub (bolted on the front of the water
pump) filled with an oil, that was only allowed into the engagement
area on the hub when the bi-metallic actuator was hot enough (from air
the rad as it wasn't touching anything, other than at the two ends).

and the drag would
dramatically drop, at least until the engine was hot enough to actually
need cooling down.

?

Maybe the oil would run into the 'drive' position when idle and get
moved back into the reservoir after a few seconds of engine revs?

I don’t think it was different in principle from any other, it just had a
high start up drag.

Understood.

Cheers, T i m