4 Wheel Drive Engaging - Ford 7.3

[onsight]

Since your truck has the switch in the dash, It has shift on the fly. Leaving the hubs in the AUTO position, they will lock/unlock through vacuum as you shift into 2 or 4 wheel drive.(hopefully - the hubs themselves sometimes fail). Turning the hub to the LOCK position manually locks the hubs (wheel ends). You want to LOCK the hubs if you know that you will need 4WD (mudding or snow plowing). When your hubs are in AUTO - they dis-engage and engage with each change of direction (forward/reverse), which is pretty hard on the front axle.

 

[Diesel-ME]

One other note - if you get stuck prior to manually locking in these type of semi-auto hubs, be SURE to manually lock the hubs, else they will auto-engage during wheel spin - causing a huge CLUNK, and perhaps hasten their demise...??.... You only want to use the dash switch when you are moving.

~paul

 

[Flywheel]

FireMan has the best reply - i'll add a bit...

The dash switch will operate:
1) transfer case
2) Pulse vacuum hublock system

With the hubs in "Auto" or "Lock", you are able to shift the transfer case between 2WD and 4WD during normal driving (without wheels already spinning-out).

For 4Lo - you must be in park or neutral, foot on the brake and not moving - to get in and out.

The switch on the hubs allows for freewheeling in "Auto" if you are in two-wheel drive - or manually locked in the "Lock" position. Many folks just leave them locked for the winter to anable seamless angagement / disengagement.

You will be fine running with the hubs in lock on any road - however, you may notice some u-joint binding at full lock, since the spindle is turning.

When you leave the hubs in "Auto", and select 4Hi, a signal is sent to the 4WD module, which engages the transfer case, then the light on the dash lights. Another signal is also sent to the Pulse Vacuum Hub Solenoid. A vaccum pulse is then sent from the vacuum reservoir (you have a vacuum pump and reservoir on the passenger side above the wheel well (diesel)) to the hubs to engage them. A diaphragm in the hub will pull the gears together and they engage similar to a click type pen - a push to engage and a push to disengage. - So along those lines,, when you go back to 2Hi, a short pulse is sent to disengage the gears in teh hub to allow freewheeling.

There are inherent problems with the vacuum hub system - vacuum leaks. Symptoms of a vacuum leak are that the vents will revert to defrost for about 30-40 seconds. Since the vents run ont eh same vacuum system, they go to defrost as a safety - so you can see...

A good tip for your tranny is that when backing a heavy load or moving through a field at low speeds, disconnect the vacuum solenoid and put the truck in 4Low - this will reduce the stress on the tranny clutches - reverse is especially susceptable to overheating when backing - so 4Low can be a saver. This will engage 4Low in the transfer case, but not egange the hubs for easier maneuvering. There is a modification you can do for an in-cab switch to ever-ride the vacuum solenoid.

More information can be found on thedieselstop.com

Congrats on the "new" truck

-Flywheel

 

[rgood]

***There are inherent problems with the vacuum hub system - vacuum leaks. Symptoms of a vacuum leak are that the vents will revert to defrost for about 30-40 seconds. Since the vents run ont eh same vacuum system, they go to defrost as a safety - so you can see...****

This is very interesting - Thanks to everyone for the Posts! Flywheel, I have this problem you describe above - How hard/expensive is it to fix???

 

[Flywheel]

Rgood,

It may not be hard to fix at all - just takes a little inspection and troubleshooting.

The common leak occurs where the vacuum line attached to the nipple on the hub. Heat causes the rubber tubing to become brittle and crack. You can just cut off an inch and reattach.

Other than that, try to follow the lines from the vacuum solenoid. Plug the line going to the hubs with a golf tee. If the vents still revert to defrost, then the leak is in the solenoid. If the vents do not revert, then isolate each hub with the golf tee. THere are a couple of seals in the hubs that can go bad - especially if you have a bad hub. Check these by lifting the front axle and trying to "wiggle" the wheel back and forth at 12 and 6.

PM me if I can help more.

 

[rgood]

Thanks Flywheel ! You fixed me right up - I could feel with my hand that the vacuum hose to the right hub was cracked in the back - I need to replace the hose, but the quick fix was to just cut and reattach as you suggested.

Thanks Again !

 

[Blk88GT]

You may want to join TheDieselStop.Com - www.thedieselstop.com and ave a read.

 

[Flywheel]

rgood - just regular vacuum line hose from the auto parts store is what I put on my truck 4 years ago. A good indication the it is going bad is when it leaves your hands black.

Good tip again on thedieselstop.com - I'm addicted.

 

[ddivinia]

Guys, I recently bought a 2000 Ford F250 - 7.3

It has the In-Dash SWITCH to engage the 4 WD.

It also had the lock in - lock out hubs.

My question is - Does the Dash switch control the lock-in/lock out hubs too?
------

That switch does it all. XFER case and hubs. You can do the hubs manually in case of failure. Look on the hub, youwill see hoses going to operate the hubs.

D.

 

[horse7]

Also be sure the hub seals are in reasonably good condition. The vacuum can suck dirt and water into the hub with unpleasant results if the seals are bad. This is one of the main objections to the ESOF (electronic shift on the fly).

In my case I wanted ESOF since frequently the only need for 4wd is while on the driveway, and manually unlocking after getting out of the driveway (to avoid the slight reduction in mpg w/locked hubs, and to avoid the slight increase in wear) is an aggravation. Had to do that with my old Blazer with manual New Process/Spicers, tho mostly just stayed locked all winter due to the hassle.