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

On Tue, 23 Aug 2005 22:18:39 +0100, JohnB wrote:


> 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.


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

Chris Street wrote:


> 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

On Tue, 23 Aug 2005 22:51:10 +0100, JohnB   
wrote:


> Chris Street wrote:
>
>> 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.

On Tue, 23 Aug 2005 22:51:10 +0100, JohnB wrote:


> Chris Street wrote:
> 
>> 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  :-)


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?

Chris Street wrote:


> 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

On Wed, 24 Aug 2005 00:36:49 +0100, JohnB wrote:


> Chris Street wrote:
> 
>> 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


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?