4wd Engagement Issues
 

2001 F250 7.3 4x4 ~220,000 miles

four wheel drive worked fine in the spring. went to use it this past weekend and the transfer case shifts, indicator shows 4wd and 4wd low, but the front wheels are not engaging. I manually locked the hubs in as I always do when I want to use 4wd as I haven't used the auto lockers for years. Can anyone point me to where I should start? Most issues I'm reading about are leading to the switch, but the switch seems to be working as I can certainly tell the transfer case is shifting into low.

Thanks

I'm sure someone more knowledgeable than me will come along. But if everything indicates it's in 4x4 and you can feel it shift into 4x4, have you looked to see if the front shaft is turning? Maybe your wheel locks are no good and the plastic cages inside are broken apart? I have an 03 4x4 but lucky me the PO changed the lockouts over to WERNER manual hubs before I bought it.

The hubs are vacuum operated. There's a solenoid up in the engine compartment with vacuum lines going down the hubs. Typically it's the vacuum solenoid, a leak in the vacuum lines, or the hubs themselves aren't working. Easiest way to diagnose is remove vacuum line from solenoid and pull vacuum with a hand gauge. If both hubs engage then you've proven the hubs and vacuum lines are good and the solenoid is bad. If you can't pull vacuum, you need to dig further and look for bad hubs or leaking vacuum lines.

These hubs work like a clicky pen. Vacuum is used to engage them. Once engaged vacuum is released and the hubs are locked mechanically. Vacuum is added again to disengage and then released. The wheels have to be turning at each step to disengage/engage. I usually just reach behind the front wheel and turn the shaft by hand to lock it up.

I like this setup as it doesn't rely on vacuum at all times to engage/disengage the hubs like they do on the F150s with the IWE's.

On my 6.0 the 4x4 hub solenoid is right behind the passenger battery. Not sure where it is on a 7.3 but I imagine it's in the same general area near the vacuum pump.

if the hubs are locked manually at the hub, does it not negate the need for the vacuum line and pump?

Yes, if you are OK doing it manually that's a fine solution. A lot of guys convert to manual aftermarket units.

If you see above, that's the issue I'm running into - hubs are manually locked, and still no front end engagement....

So this is a different animal, but my parents used to have a 1990 GMC Suburban, 4WD (manual shift not electric, but other than the servo controlling the shift it's basically the same). One winter with the front hubs locked in, dad put it in 4WD but noticed it didn't feel like the front tires were spinning. We investigated everything, made sure u-joints were good, fluids were good, pulled, disassembled, and reassembled the hubs, couldn't find anything wrong.

ON A WHIM we jacked up one front tire and tried to turn it with the truck in park and locked in 4WD...and heard a hellatious grinding noise. Crawled under there while he spun the tire again, and the splines for the front driveshaft had sheared completely off. Ended up having to buy a whole new front driveshaft.

Not saying this is your issue with ANY kind of certainty, but if your hubs seem to be locking in, and the transfer case is going into 4WD, it might be worth it to try jacking up a front tire and trying to spin it - it shouldn't be able to if everything is locked in and working.

When you manually lock the hubs are the axle shafts engaging? You can reach around behind the wheel and check them. If not, then you've got issues with the hubs themselves.

To diagnose this:
Lock in your front hubs as you normally do according to your statement in your first post.
Leave the truck in 2H (Engine off)
Crawl under the truck and try to turn front driveshaft.
If you can not turn it, the issue is either in the servo motor or transfer case
If you can turn it, the issue is you front hubs, While turning said driveshaft, watch your front axles to see which side is turning, That will be the hub you have issues with. If neither side is turning you have issues with your front diff.

When I would change out the summer/winter tires on my 2006 or 2011 F350's, I would lock the front hubs and engage the 4wd so that the front tires would not spin on me as I was working on them.

so maybe do that as a simple test. Lock the hubs, put it in 4wd, then jack up each front tire one at a time (don't have to have them both in the air) and see if you can spin the wheel. If you can't, the the 4wd is engaged.