My 99 sub 2500 with 7.4 has developed an a/c problem , i worked on this weekend , put gauges on everything checks out like normal , my problem is the a/c is cool - but not cold at idle , after looking thing over I noticed that the electric cooling fan is not coming on when the a/c is turned on , I checked the fan by running wires from the battery to the fan wires and the fan runs fine , my question is what turns on the fan ? Im pretty sure its not controlled by a temp switch because the used to come on as soon as the a/c was turned on . I did check all the fuses , all are good . The owners manual shows the the aux. fan / a/c clutch relay as being the same relay but im not sure on that because the a/c relay turns the compressor on and off , the electric fan should run all the time when the a/c is on . I dont remember the fan ever turning on and off with the a/c compessor but im sure on that , maybe it did . Any help with this would be great . Thanks .

Scotts: Looked at my '98 manual. The VCM turns the aux cooling fan on when:

1. Certains DTC's are set????
2. Engine Coolant Temp is above 225F (107C).
3. A/C head pressure is above 300 psi. (Off @ 250 psi)

Hope this helps.......Steve