Hi,just a quick one. Does the small radiator fan come on as soon as air con is switched on? Or should it? Mine doesn't.  

No.

Ah,Ok,so when should the fan kick in?

At the same time as the other one.

Its nice when a question has a nice easy, straight forward set of answers

 

Not really a straight forward answer. I maybe didn't ask the right question. I'll try again. When I switch on the A/C should a fan at the radiator come on, be it the small one or the large one? Other cars I've owned, Audi TT and others, as soon as I turn on A/C the radiator fans turn on. 

At the risk of repeating myself, - No! (of the straightforward type)

 

Fan cuts in when high side pressure reaches 10 bar.

 

 

OK ta. Guess I'll have to go to an A/C specialist then.

Depends on a few things. Ambient temperature has a strong effect on the static pressure of the refrigerant, so on hot days or with a hot engine bay, the extra pressure from the compressor to reach the threshold for fan activation will be minimal, and so happen rapidly. On a cool day from a cold start, the compressor will need to run for longer before fan activation happens, maybe a minute or two, or even longer.

 

On your car the fans are wired in parallel, so they should do the same thing at the same time. Are you seeing the main one spool up or not?

Failure of the slow/low speed function of radiator fans is very common on cars of this era, due to an internal resistor fracturing. There's info on here but this is a good reference: https://uk-mkivs.net/topic/224371-mk4-golfbora-radiator-fan-fix/

 

By unplugging the fan motors from the loom, and measuring resistances up toward the motor of each between the relevant pins, you can establish what is broken or not.

 

If one or both fans have lost low speed function, it'll be much longer before the refrigerant pressure builds to the second, 16 bar threshold for high speed fan activation.

Hi Wino, brilliant thanks for that, exactly what I needed to know. Probably my fault for not asking the correct question in the first instance.