I'm looking for a description / schematic of the 4 wheel drive system on my 2003 F150 FX4.

I've done some research and understand what happens when you command a shift into 4 wheel drive. Either moving the manual shift lever or turning the selector switch engages or disengages the front drive shaft and changes range gearing in the t-case.

From there the GEM module activates the appropriate vacuum solenoid which supplies vacuum to a vacuum motor on the front axle that moves a switch fork. What happens from there? I'd like to know because I like to know things and it may help with future diagnosis.

Thanks!

 

Well your right that the vacuum in supplied to the front hubs and that engages the auto hubs. Thats all the vacuum does though it just stops you from having to get out and engage the hubs manually. Everything else just takes place in the transfer case, where basically clutches engage and make the front drive shafts turn. Manual or electric 4x4 switch it all does the same thing except the electric switch engages solenoids to do the work and the manual shift you do it yourself.

Theres a bunch of BS in the newer systems that there is no reason to get into.