I am unable to shift on the fly, yet 4x4 works in lockout mode. Any help where to start looking??? to repair or ideas of issues. I do notice that when I shift on the fly heat comes out through defrost only for a little bit then returns to floor.

 

It sounds like one or both of the vaccum lines have come apart on the top of the spindle. the second time mine did it i used just a little rtv on it. very comon problem.

 

Magilla,

Be careful with the locking hubs. With the hubs manually locked, the front axle will rotate, but not via engine power. Also, you won't have the front half of the driveshaft moving so you really don't have 4 wheel drive (just locked front hubs). The ESOTF dash switch engages the transfer case and results in engine power turning the front axle and driveshaft.

I'm also guessing a vaccum problem if the inside dash switch isn't locking the hubs, but they do lock manually.

Good luck!

 

not all of them. mine are auto ONLY. no manual lockout. cant wait to replace them with the warns..

 

Ok, checked all vacuum lines and no leaks, measured vacuum at pump on pass. side fenderwell at 12psi?? What is the correct psi that the pump is suppose to be putting out? 18 ias a number that my mech is tossing out can't find it shop manuals. I do lose aC/heat whenever I engage 4X4. (short time, comes back). At hubs when engaged also have 12 psi. system will hold the 12 but takes 2-3 min to recover. IS IT A WEAK VACUUM PUMP???

 

i've heard of o rings failing in the hub assemblies.

this is beyond my knowledge

 

It sounds like you've ruled out a vaccum system leak (at least between the pump and hub assembly) as you have 12psi at both ends. I also do not know the proper amount of vac pressure, but it does sound like you have insufficient vac pressure based on the loss of of vent control (the vac pump controls the open/close of the vent flaps). My '99 350 does not have any vent control issues when I engage the dash switch for 4x4.

Just to be sure, do your hubs turn easily by hand? Have you also taken close look at the auto-lock hub to rule-out a bad hub (I don't know if the manual side of the hub would still work if the auto side was shot, but I'm guessing not).

Finally, check the o-ring on the auto-lock hub - if vaccum is good up through the knuckle, and you have checked the fitting on the knuckle and the main axle seal, the last place you can be loosing vaccum pressure is the o-ring on the hub (inexpensive at the dealer and easy to change) or the o-ring on the hub assembly (also inexpensive, but harder to get to - requires that you pull of the hub assembly).

As I think and type (always dangerous) perhaps you could test the vac system for a pressure loss at the hub by sealing the two vac lines at the knuckle (Just take the hose off the nipple on the knuckle, and plug the line temporarily - even some duct tape would probably do the trick for a few minutes). Then turn on the a/c or heat at the floor and engage the dash switch for 4x4 and see if all the air goes to the windshield defrost vents. If so, I would guess the vac pump is bad - easy to replace. If NOT, then I would guess you are loosing vac pressure inside the hub assembly. The next step would be to plug only one of the vac lines and reconnect the other to try and isolate which hub assembly is leaking (maybe both, so try each side with the opposite side plugged). If you determine that the vac leak is inside the hub assembly, you can check the auto-lock hub o-ring first (easiest) then move on to the hub assembly o-ring (yellow) and then the main axle seal.

I know when I replaced my wheel bearings and all these seals recently, the hub assembly seal was in the worst condition of them all (it was completely disentigrated on one hub assembly). If any of these seals are leaking, I don't think 4x4 would engage via the switch - at least not consistently.

Good luck!

 

If the front hubs are locked the drive shaft does turn, You are able to put it in 4wd as long as you arent spinning the rear wheels. It should go right in. I have been having problems with my auto hubs so with the snow flying I just keep the locked in.