The 4wd on my F-150 stopped working. It has the switch activated drive. When I flip the switch the light on the gauge cluster illuminates and I hear a click, but nothing engages. Typically there will be a clunk when it actually does engage, but that sound is not present. What should I look at in order to diagnose and fix this problem?

 

Actuator on the front diff assembly not working.
That's where the clunk comes from.
It's vacuum operated from a relay on the firewall.
Good luck.

 

So all I need to do is find and replace this relay and all is well? Where is this located on the firewall?

 

You need to find out what is causing the problem and repair it, not just run with anything hoping for luck.
I give you what is in the path.
I can't tell you just what it is.
You have to do the work and find it..
Good luck.

 

2 vacume lines run off the diff. and go to two actuators or relays that should be located behind the battery on pasenger firewall. you can test it by putting a vacume gauge on it an see if it pulls a vacume and start from there. just check the line for holes or anything that might have burnt through it. usually the cluck is the transfer case locking into 4wd not the front diff. inside the t-case are shift forks that lock in the gears thats what clunks.

 

I think I located the vacuum operated actuator behind the battery. It has both a blue and a white small plastic hose attached to it. Is this correct? When the truck is running should there be vacuum to both lines?

 

i had the same problem but it ended up being my transfer case that was bad. but there is a detailed trouble shooting post at the top of this four that would really help you wish it was there when i had my problem. check it out

 

I went down to the dealer and bought both actuators installed them and I have 4wd now. Hopefully that was the only problem. Thanks.

 

awesome glad to hear that good thang it was somethin simple