So, i have never had issues with my 4x4 before. I have a 1999 with the 5.4, and the manual 4x4 lever (manual shift on the fly). It was working two weeks ago, and now is not. This was our first big snow where i live and i was getting firewood on my property and almost got stuck, i made it back in 2wd but i shredded my yard in the process... The dash lights up saying "4x4" when i engage it, but i dont feel or hear anything actually engaging. any advice on how to diagnose this is appreciated, or if you can point me to a forum/thread that already has the answers would work also. thanks.

 

Check for vacuum & the electrical connections at the 2 solenoids at the firewall ( on the passenger side). If every things good there move down to the actuator on the axle and check for vacuum. If not then there's a leak in the lines.
I sure some one will give more specific info as I'm just learning this also.

 

 

After my post I got to thinking you should have a switch some where so when you manually shift into 4X4 it energizes the solenoid circuit. Look for one.
I have ESOF. Sorry I didn't bring that up before.

 

If you really need 4x4 go under the front of the truck and take a big flat-head screwdriver and move the arm out or in to lock and unlock when the shift vac ford **** does not work. Look at the chunk on the front side under the cover where those two little vac lines go and move the little arm that is clipped out to engage and back to unlock.
Best thing ever made is the mechanical cable to do just that and you don't have to have ford vac system for it. It locks the front-end in when you need it.

 

The common failure point is the actual shift fork in the front end, not the vacuum system. The shift fork has little plastic tabs on it that move the splined collar to connect the axle shaft, and those plastic tabs wear from contact with the spinning splined collar, break off,and then the shift fork does nothing. It is not a question of if it will break, it's when it will break. It's a horrible horrible design. Replacing with a mechanical cable makes no difference for this. People who have problems with this system are usually people with a wet GEM, although a cracked vacuum line also can lead to problems.

You can move the actuator by hand to engage front axle.
since there is some slop in movement, sometimes even a broken front shift fork will still work and sometimes it won't. sometimes when it breaks, it leaves the front wheels in the engaged position, turning the front differential/driveshaft for many thousands of miles, costing 1+ mpg

Jack up pasenger wheel. With vehicle off..and transfer case in 2wd..move shift fork, you might need to rotate tires slightly to get the splines to engage. If the axle dont connect, turning the diff and front driveshaft...the shift fork is bad. It's about a one-hour job to change.

Its a 2 sided vacuum actuator, there is no spring return, it holds whatever position it's in if there's no vacuum applied to either side. although it could drift and eventually disengage /engage from vibration and driveshaft forces. if the fork is okay, just observing the position of the actuator when you engage 4 wheel drive will tell you what side of the vacuum isn't working, and then you can troubleshoot that tubing or solenoid. So, you can also manually engage it anytime you want, just disconnect the vacuum line for 2wd, it will stay connected. In other words, the vacuum system can never let you down completely, as long as you realize you can get under there and just move it with your fingers.

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good to know, thats alot of good information, thanks mbb. On a side note, i never got around to looking into it, but today my 4x4 was working just fine...so its intermittent i guess

 

Another thing about that cable set up is that will give you 4 modes, 4w hi, 4w lo, 2w hi, & 2w lo. Because you can engage 4w lo and not engage the front end.

 

It is most likely the shift fork pad(s). Have a look at this, if nothing else then to get an idea of how it works and what is inside there.
Intermittent performance can also mean there is a vacuum leak in the lines to the actuator or to the solenoids (vacuum valves) on the firewall by the battery.


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Intermittent things get worse
and let you down when needed

I discovered my transfer case shift motor was bad once, when had no 4wd. I had one intermittent episode months before i disregarded.

I was parked on side of a road at kids football game. Steep embankment. Hard ground. 3" high asphalt road edge. With cars in front and behind me , had to turn wheels uphill to pull out onto road, couldnt drive forward or back. No dice, rear wheel spin, dug in . Put in 4wd... Nothing. Once rears dug in, wasnt going anywhere regardless. Had to wait for someone that could pull me out. Its amaxing how ez it is to get stuck on a mild incline.