Originally posted by 400E
Start with a search on "aux fans." You'll find a wealth of info since this system often seems to break down. The plus side is that it's an easy system to troubleshoot and fix.
Also, to save some common possible mis-diagnosis , ie- low fan switch..
The common practice of jumpering the a/c drier high pressure sw. and looking for low /ac fan operation is NOT a switch test...
It is a quick tech trick to verify both the fan Motor/s and the integrity of the low fan circuit [ fuse.relay, dropping resistor, etc]
This test tells the tech that the fan motors of BOTH high/low circuits are Ok as both circuits share the same fan motors.
SO, if you get low fan when jumping, then condemning the sw is way off the mark... There is a way higher possibility of the a/c system being low on freon [ not enough freon pressure to trip the switches cut-in spec.]
It is also possible to have NO low fan called for at low amient temps. The fan is there only to help/assist controlling high side pressure [when they are triggered by high thermal load]
The ONLY way to test the high pressure sw is with gauge reading of the high side pressures and comparing the readings to sw specs...
Quick fan motor/circuit test ? -Yes..
Switch test ?.. No