Audi Repair: Audi TT secondary cooling fan not coming on, coolant temp, audi tt

Question
I've been working out a cooling issue with my car, it runs with the needle in
the middle most of the time but any time I'm running the AC in heavy traffic
or going up a long hill it will go as high as the mark before "H". I've replaced
the water pump, thermostat and coolant temp sensor and just now noticed
the the secondary cooling fan never comes on. Are there any procedures to
determine what's causing that? Thanks in advance for your time!

Answer
It sounds like the fan itself may be bad, or the secondary temperature sesnor is faulty.  First thing I would try is to look at the connection to the fan, apply 12V to it and see if the fan works.  Secondly that fan is possibly controlled by a relay, when the engine gets that hot, the relay should supply voltage to that fan; check that you are getting 12V there when the engine is 3/4 towards H; don't let it get to H.  If not check your relays.  Let me know what you find,  Jan