Sounds like a broken vacumn line somewhere.

 

if you can push the plunger in and out and it dont go into 4 x 4, then there is something wrong inside te pumpkin, or the electric on the transfer case is not engagning that end. When in 2 x 4 you can turn the drive shaft by hand right?. Now put it in 4 x 4, shut the truck back off, and try to turn the drive shaft again. If the electric at the transfer case is working, you will not be able to turn the drive shaft, and the problem is in the front differential. if you can turn the driveshaft, then the transfer case has a problem.

 

Good Points Steve!, that is why you have that 190,000 of expierience!! Thank you for all the help you have given me also thus far..
Rich

 

you might want to leave the vacuum hoses off the axle for the test. i guess the front end could engage and the transfer case is the problem, and your drive shaft would then be locked by the front tires????

 

Now correct me if I am wrong,,,,97' to current F-150's are all,,,IFS front suspension,,,,there are NO Hubs ! It is a front axel dissconnect that the vacum actuator is moving,,,,,They have CV jointed axels,,,,The axels and front drive shaft are always turning,,even in 2wd,,,,Thats why when you either turn the ****( Not the best setup) or pull the MSOF manual lever,,,it does not grind,,,,all components are turning the same speed,,,,it shifts the transfer case into 4x4 mode,,and also activates the front axel dissconnect at the same time!!

Now on my sons 01' F-150,,,it has the MSOF (Manual lever),,,when we got the truck,,the 4x4 did not work,,,the real only problem was the small "oil switch" (elec switch) on the transfer case was bad,,when the transfer case shifter was moved to 4x4,,,this switched turns the dash 4x4 light on,,,like a $18 dollar part,,,,with the switch replaced,,,the 4x4 dash light would come on,,,and send the signal to the GEM modual which in turn sent a signal to the vacum relays(behind the battery on the firewall) which in turn sends vacum to the front axel disconnect,,,locking it in,,,and 4x4 then worked great,,,and it still does,,his truck is at about 230 K miles now,,,drives every day,,he said he was in like "20 inches of snow yesterday(we got alot of snow here in CT,,,,someplaces) and all worked good.

Make sure your "4x4" dash light come on when put into 4x4 !! If Dash 4x4 lite's up,,,I have heard the the Gem modual can cause troubles too !,,,,Good Luck !

 

m350...ah yes....I feel his pain with the snow....I live down in Fairfield County and we got about 15" My 01 worked really well in it though!!
Rich

 

im learning that mine I think is the vacuum pump on the firewall! the two vacuum hoses one blue and one pink, my blue does nothing !! either somthing telling it not to send vacuum or just bad pump hope this helps