Hi guys I just replied to a similar thread,just gonna copy paste.
I know this post is old but I also see replies a few years after initial post.
I kinda have same problem,I have a 1996 Golf Mk3 Gti. Long story short,the thick 6mm Red/Green wire on mine is not connected and does not go to the fan thermoswitch (screwed into radiator)as another guy posted here.Yes my car also has an ecu.
Im not saying he is wrong,but on my car,my thermoswitch has all 3 of it's wires already connected and I have tested that they work by jumping the pins.
I have either the slow or med speed working. Let me explain,acc to Haynes manual I have a 3 speed fan (has to do with my second inline {also 4 pin} coolant temp sensor)
Im no auto elec but my fan works regarding 4 wire to 3pin cooling fan ,with 6mm Red/Green stripe "blanked off".
Do u guys also have a 3 speed fan like mine is supposed to be? That 4th wire of mine is bothering me because I know I am supposed to have a faster speed which should kick in when the temp gets very hot.
Just to add to this,someone said here that the 4 pin is ground,low,med an high which makes sense.
So basically if I don't buy a 4 pin fan I won't have the high speed working? (Unless I swop the wires on my thermoswitch like it was before,then I had high speed but not low/med )
Another thing is that I have read about a 3 speed fan thermoswitch (I have a 2 speed). Will probably have to get that as well.
Anybody shed some light?