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How do you test a cooling fan thermoswitch ?

The missus's sick old Passat (RP engine) is now beginning to overheat. Seems like the temperature climbs too high, red light on the gauge on the dash comes on and then a few minutes later the temperature drops.

I guess the usual suspects are the thermoswitch for the fan and the thermostat. I seem to recall that there is a quick way to testing the thermoswitch by disconnecting wiring and then... errrr... what ?

Just like to eliminate or confirm that fault before draining and checking the thermostat.

Thanks for any tips.

-- JohnB

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

Reply to
Chris Street

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

Reply to
JohnB

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

Reply to
Duncanwood

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?

Reply to
Chris Street

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

Reply to
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?

Reply to
Chris Street

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