radiator fan switch

Please help,the fan switch at bottom of radiator,would someone please explain to me how this works and when it operates,and what are the symptoms of a faulty switch.
the fan on my vehicle only operates when the water temp is very high it frightens me.

 

When was your thermostat last changed.? Mine is a two speed fan and will usually cut in when the gauge goes just past half way and will bring the needle to just under half way.

If just 2 pins on the fan motor the fan should kick in at 3/4 on the temp gauge and lower it to half way.
Fans motors with 3 pins have 2 speeds for the fan motor so work slightly differently and fitted to later cars.

The switch in the radiator monitors the coolant temputure and when it reaches a certain temperature should make the fan work, to test the fan is working remove the brown earth wire and touch it on the other wire and the fan will start.

How old is your coolant and what colour is it? Any brown sludge in your coolant?

Sounds like its working OK, its only supposed to operate the fan at high temps. If its not boiling up or losing water its fine.

On MK1 and Mk2s I've driven the fan only operates on hot days whilst manoeuvring in traffic (3 point turns etc). In normal driving there is enough airflow thru the rad not to require the fan.

If the engine is getting too hot all the time, that is a different problem to a switch/fan issue which appears do be doing what its supposed to be doing.

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