How do you test a cooling fan thermoswitch ?

Plug a piece of wire across the thermo switch connectors is one way - if the fan then runs the switch is faulty.

Thanks Chris - just to be clear - is it wire across the connectors on the switch on the radiator or on the wiring terminal ?

Also, I seemed to recall being told yonks ago that the switch was faulty if the fan "didnt" come on.

But then it's >10 years since I last did this :-)

-- JohnB

IF the switch is faulty the fan doesn't come on /until/ you short out the thermoswitch on th radiator.

Across the wiring loom. Generally the switch closes when the rad gets too hot and either switches the fan directly, or with modern 20A fans it more likely switches a relay.

If the fan fails to start then either the fan relay is stuffed or the fuse is blown. If the fan motor has jammed then the fuse won't be far from blowing - I take it the fan moves easily by hand?

Thanks Chris ( and Duncan)

Yes fan seems to move fine - and as far as I can tell from the missus's description it comes on - just a bit late ! (I havent had a chance to run it up to temp today).

Hence me suspecting the thermoswitch or thermostat.

Will set to it at the weekend.

Cheers again

-- JohnB

IME the switches just fail, if it's coming on late then the rad could be sludged up. Has it had it's 3 year coolant changes etc?