The 4wd has quit working and I get a check 4wd light when I engage the switch. The solenoid allows vacuum through when the 4wd switch is engaged. I changed the solenoid and same scenario. The plug for the solenoid shows 14 volts in 4wd or 2wd.

If I understand the system, when switched to 4wd the solenoid closes restricting the vacuum to the iwe. When switched to 4wd should there still be voltage at the plug? What tells the solenoid to close, and how?

 

Sounds like its your Transfer case shifting motor that isn't shifting.

Check the code.

Also...check to see if your front CV axles turn freely when engine running in 2WD and then after shifting in 4WD check if they are locked.

 

With truck running, in 2wd the wheels spin free.
Truck running, in 4wd, wheels spin free because there's still a vacuum on the iwe. This is the problem, I think.
With the truck shut off the cv's lock up, no vacuum.

 

F-150 4x4 Tests

You Tube

Not Sure if this video helps or not...

 

Thanks, yes I did see the YouTube vid and it helped me get this far, but I think my problem is farther up from the solenoid. I believe the solenoid reacts to a voltage change when the switch is moved to 4wd and the voltage on mine stays the same.

 

I think your 4WD switch isn't working right.

 

Ahh... Fuses all good for the 4x4 system?

 

Correct.

I don't think that that failure sets a code. It never has on mine.

The vacuum doesn't matter, the computer prevents the hubs from engaging until the x-fer case is in 4 lo.

 

I believe that's what I'll try next.

All fuses marked as pertaining to the 4x4 in the manual are good.

 

Vacuum is what keeps the hubs from engaging while in 2wd. So if the vacuum isn't isolated by the solenoid when switched to 4wd, the hubs can't engage. That's my elementary understanding anyway...