Water temp warning !

[Gregory Deschodt <GXD@synon.com>]

Hi all,

I need some help here... 
My fiancee's Audi 90  (91model) started flashing the water temperature
digital icon (the one with the little waves!) about a month ago. I
checked it out, it still had coolant, the fan was fine.... We took it in
and they replaced a $400 electronic unit that was sending false alarms.
Argh, expensive false alarms ! 

This time, a month later, here we go again !  The thing beeps and
flashes the temps LCD. Once again, there is coolant, but unfortunately
the water temps gauge has died in the meantime so I really don't know if
it's a false alarm or not. I know, it might be a good idea to get it
fixed, but I am afraid to ask them given the cost of the last repair...
Anyway, I think the fan did not kick in this time, so the fan may be
dead/disconnected. 

Strange thing , though... the temps light flashed when I exceeded 70
mph/3000rpm or when stuck in traffic, and stopped when I parked the car
and subsequently drove under 50 mph.... Smack me if I'm stupid, but I
assumed that if you go faster you cool the engine more, so I didn't
really understand this "stop the car 10 seconds and the light goes off"
business... Anyway, the friendly audi mechanic only caught the " I
didn't hear the fan" part of my explanation and called my girlfriend
quoting a new fan + wiring for $500..Seems too easy to me. 

Ahem... Ouch.. Question : I cannot believe the fan is toast, is the any
test/hotwiring I can do to see if it's just a bad wire ? If so, could
you please explain this to me in dummy terms ? 
Also,  is that quote reasonable ?  Any other idea, suggestion, good
garage address in the bay area ? 

Many thanks ! 

Greg.