I was told to post this here also.
4X4 not working
2004 F150 Heritage 4x4. The lights on the dash for my 4 wheel drive are not working. When you turn the switch nothing engages. I checked the fuses and all seem to be working was wondering if it could be a relay?

 

You will have to do more observation on the parts of the system due to there being a series of operations that go on to get the front axle into lockup.
At dash switch on, it signals the GEM module to run the transfer case motor to it's first position. . Is that happening?
If yes, the motor control board operates a firewall solenoid to transfer engine vacuum to the front axle 'actuator'. The actuator moves a sliding sleeve to lock the axle together. .Is that happening?
Check it out and lets go from there.
Good luck.

 

Absolutely NOTHING is happening. No noise whatsoever....dash lights do not work indicating that system is in 2 or 4 wheel drive and when switch is turned to 4L or 4H....nothing! Silence.

 

The dash light does not come on until the chain of events completes.

First thing is to look under the dash behind the fuse panel at the GEM module for signs of water leakage on that part.
Be sure fuse #104 in the engine bay is good. Take it out and test it.
It powers the case motor through a relay.
If good, does the case motor run when the dash switch is operated?
If no, are you sure you do not hear the relay pull up located in about the center of the dash. The case motor plug etc?
If you don't hear it, the GEM is suspect.
If you do, the case motor, it's plug or harness is suspect.
Can you see the progression taking place in trouble shooting?
Work along the path to come to a conclusion on what to do for repairs.
Good luck.