The aux fan in front of the condensor turns on in response to two different circuits:

1. low-speed fan is tripped by the high-pressure switch on the receiver drier, in order to protect the A/C from excessive system pressures. This can be tested by jumpering the two wires coming from the red/green switch on the drier.

2. high-speed fan is tripped by a temperature sensor on the engine block, to protect the engine from overheat. This should trigger around 105 degrees, can be tested by jumpering the two-pole connector on the 3-pole switch on the top of the block.

If it never turns on, check both of these circuits.