I found that thank you. Does air coming out of the bottom constantly mean its bad?
Yes. They feed air from tge supply inlet port to the output port to perform an action. They stop supplying air from inlet to outlet port and vent/release the outlet port pressure when done performing the action. Commonly known as a 3 way 2 position valve.
If leaking, supply/inlet is leaking to outlet and passing out thru the open vent. Or if the solenoid is active, simy leaking past the supposedly closed vent.
If it is the fan solenoid is it leaking with ignition on or off? If it is the axle power divider does it still leak when mode is selected?
It really doesnt matter, it should vent once briefly when de-energized, like the fan control does when you shut off the truck, or de-select mode, but should never leak steadilly.
Havnt had one apart but could have crud inside, but just as likley are failing 20 year old rubber seals...
Thats why i researched an inexpensive replacement...