If you would like I can send you the instructions from my 124 CD on how to remove the auxilary fans.
I would recommend supplying power and ground directly to the suspect fan to make sure it is faulty. That is the easiest way to tell.
Also the aux fan(s) should come on when the high pressure gets somewhere between 250- 300 psi. By simply connecting a ohm meter to the switch you can tell if the switch is completing the circuit or not. You need to be sure the system is properly charged though.
HTH
