I have a 2001 f-150 4x4. My 4x4 was working fine one day and then the very next day it would not engage. It shows on the dash that it is engaged and when I put it in 4x4 lo the gear ratio reduces as it should, but the front wheels are not engaged. I have checked my vacuum lines and they work like they should all the way down to the front end. I have not heard any noises from under the truck. I'm not sure if it is a solenoid that the vacuum lines connect to on the front end or not, but it is round with an arm sticking out from the side. Could this be bad? Any suggestions?

 

Make sure that the fork is moving when you put it in 4wheel. Sometimes they get clogged with mud and won't fully engage.

 

where is the fork that engages the four wheel drive? How do you check it? My 99 4x4 won't engage. If I jack it up and turn the front tires with 4wd engaged, the front shaft turns but there in no power to the front wheels.

 

The shift fork for the front hubs is located on the passenger side of the front diff it should have a sheild around it with two vacuum lines running to it. When you start the truck you will have vacuum on the hose nearest the back of the truck, this will disengage 4x4, when you shift into 4x4 vacuum is taken away and lets the hubs engage.

 

More than likely you have a bad shift fork in front diff not a bad job to do but it has plastic ears that break off and ford has a revised part for that.