BMW Repair: FAN OPERATION, fan clutch, temp gauge

Question
MY PROBLEM IS WHEN I TURN MY AIR COND, THE TEMP GAUGE RAISE TO THE 3/4 POINT. I LOOKED AT THE OWENERS MANUAL AND ITS NOT ADDRESSED. COOLENT IN RAID AND OVER FLOW TANK ARE FULL, REGULAR OPER WITHOUT AC IS FINE.FAN AND FAN CLUTCH SEEM FINE. I SEEN THAT THIER IS TWO FANS,ONE ON OUTSIDE OF RADIATOR ONE ON INSIDE. OUTSIDE ONE NEVER COMES ON.IS THE OUTSIDE FAN FOR HIGH COOLANT TEMPS OR WHATS ITS PURPOSE? ITS A 1990 BMW 325I JUST PURCHSED VECHICLE SO REPAIR HISTORY IS N/A  MEL

Answer
The aux fan (one in the front) should come on everytime you turn on the AC.  it should speed up when the car gets to about 3/4 or more.  you either have a bad fan, bad fuse or a bad coolant sensor (upper left side of the radiator with 3 wires leading to it).  Pull that plug off the coolant sensor and jump the terminals on the wire (2 at a time) with the engine running, if the fan comes on, it's the sensor, if not it's something else in the circuit.
-cheers