My experience has been the same...once locked the wheels need to turn a bit before they are engaged. And the auto locks need periodic use and maintenance to function. I now have warn manual locking hubs...because they are reliable. I don't know if they lock right away or not because I always lock them well before I get into the rough stuff.

 

let me get this straight. once the hub is locked using the vacuum it will stay locked even if vacuum is lost? and to unlock it again it uses just less vacuum? I have been trying to figure this out cause my auto hubs dont work. gotta do it by hand. i found a small leak in the y splitter and I have vacuum to both wheels now. also the wheel hubs and seals were just done and I checked the barb on the spindles and it holds vacuum.
If i have vacuum coming from the control valve all of the time while its switched to 4x4 does that mean my valve is bad?

 

Sounds like it. Think of the vacuum hubs like a slightly more complex click pen. Click on, click off.

Attached below is a PDF with a great description of how the entire ESOF system works, including pulse vacuum levels and times.

You can do a lot with a vacuum gauge and hand pump. Pull a vacuum directly at the hub, for example, and make sure it holds. Pull a vacuum at the line coming off of the PVH solenoid and see if it holds. See if it locks the hub. Use the gauge to see if you get the proper vacuum pulses at the proper times, etc.

If you're getting the right vacuum pulses at the right times and the hubs and lines are all holding vacuum properly, then it's also possible that your hubs are gummed up and need cleaned and re-greased. Here is a good description of how to do that.

Welcome to guzzle's 4x4 Autolock Hub Lubrication Maintenance Web Page

 

Found it... It's in the PDF I just posted. The transfer case uses a "dual-cone" synchronous clutch to synchronize the front drive line to the rear.

 

Thanks for all the posts guys!!

Matt

 

Someone mentioned the hub switch being locked and had to use pliers to rotate it from Auto to Locked. One of my hubs has this same problem. The vac system works in both hubs so I've never messed with freeing this switch to rotate correctly.

Is the best way to free the switch with a pair of pliers / the brute force method. I like that method a lot but usually when I go that route something else breaks, so I haven't tried it on the hub switch.

 

I just read through the pdf dchamberlain posted. Reps given.
Wow, that is one complicated system.

The upshot?
Even in good working order, it can take over a minute to engage the t-case & hubs. So be rolling steady when you turn the dial & be patient.

In the snow, if you flip the dial, then blip the gas, expect lots of rear wheel spin.

 

I have a 2001 350 dually w/ 300 lbs of weight in back. Had REAL bad blizzard snow in Illinois tonight and even in 4wd had much trouble!! It acted like i was in a plain old 2wd with no weight! You can hear the transfer case thump but maybe not auto hubs. Will it hurt anything to manually lock the hubs even if in 2wd to avoid the possibility of bad vacuum till warmer weather?

 

It's done all the time. I started doing it also. It's a real good idea if your vacuum isn't 100% reliable. The worst that can happen is you loose a tiny bit of gas mileage.

 

Thanks EX!

I manually locked them anyway last night cuz I knew I'm gonna be in4wd for a week with this crap! And it made all the difference.

Now I have yet ANOTHER springtime project!