The aux fan typically pulls 12-15 amps, and is fused at 16A. Often those fuses show signs of heat damage after a few years. If the car is already using 20-25A, an extra 15A load pushes the stock alternator past the limit of it's idle output (which is only 35-40A at idle). Remember, the alternator rating is typically at 2000rpm (engine RPM). The older alternators will typically put out about half the rated current at idle. The newer, high-output ones are more like two thirds rated current.