Did you check the vacuum lines running to the modulator? Those hard plastic lines will crack in no time being that they run right next to the exhaust manifold (what the heck was ford thinking??). Best thing to do is to put it in 4x4 and listen for any air leaks through the path of the lines.

Check to see if you can move the lever that is attached to the vacuum modulator easily. You should have no problem manually moving the lever in and out of 4x4 (one thing you could do in the immediate time frame is to pull that lever into the lock position and restrain it there. At least you will have 4x4 until you figure out what the problem is).

If the lever moves freely, the vacuum diaprahm is replaced, the lines are good, then your problem must be one of the two solenoids on the firewall or the 4x4 relay (of course check the fuse for the 4x4 relay under the hood).

Buy, rent, or borrow a vacuum gauge and use it to check the operation of the solenoids. The ford service manual and even the hilton's service manual explains what pressure you should get. Basically the one line provides vacuum all the time but increases it by about 5 times the initial value (10" to ~50") to put it into 4x4. Kinda confusing to understand but once you get a gauge on there and spend a few moments figuring out how the diaphram actually works, it will make sence.

Good luck,

kev

 

Kev,

After more troubleshooting/experimentation, I have isolated the problem to the transfer case.

Following very good advice I have received on this forum, I raised the front two wheels off the ground. With the truck running in Park, I switched from 2 hi to 4 hi. I could crawl under the truck and rotate the front drive shaft by hand, which would, in turn rotate the front wheels. I heard/felt the vacuum driven center disconnect engage when I first turned the shaft.

I am assuming from this that the front end assy is working fine and the problem is with the transfer case.

My theory, thus far unsupported, is that since I can observe the electric shift motor switching thru its 3 positions, 2hi, 4hi, 4lo, and the truck actually goes into low (but no 4X4), then perhaps the problem may be with the contact plates in inside the electric shift motor assy. I plan to remove the cover and inspect it to see if anything seems misaligned or dirty.

It is my understanding that until the gear position sensor contacts are made, from within the motor, the magnetic clutch solenoid within the transfer case will not release the clutch separating the front driveshaft from the transfer case.

I'll post when I know more...

G

 

Thanks for all the info on this post, guys. My driveshaft was turning, so I checked the vacuum lines & ended up replacing the differential shift motor, which is actually a double-acting vacuum cylinder (an air cylinder that works on vacuum instead). Easy fix for $52+.
THANKS,

Dan

 

I too am having a problem with 4x4 engaging in my '96.

Haven't even looked at it yet, but will use some of the ideas shared here.

Any other info would be helpful too.

 

Seigel, If you have hubs on the outside of your axles (by the wheels), some of this info won't apply to your vehicle. I don't know when they changed from that system to the lever on the right upper side of the front differential.
Good Luck,

Dan