97 f150 4x4 318,000km on it and purs like a kitten. However when I engage the 4x4 via electric switch it engages just fine however when you step on the gas/apply torque to it the 4x4 kicks out and I can hear a clicking noise until the drive train stops and then it will engage again. The weird part is it won't do it in reverse! I can engage the 4x4 and spin all 4 wheels no problem in reverse. I have spent hours reading forums about 4x4 stuff and can't seem to find a solution to this. My thoughts are either the shift motor, or a gear is worn. Please anything related, help.

 

when you engage 4 x 4, the first thing that happens is the shift motor moves to engage the front drive shaft......... second, vacuum is sent to the front axle pumpkin to a cylinder that slides an engagement arm inside the axle to tie the driveshaft to the axle half shafts ......... This may not be engaged all the way, or it may be worn out.. Look under truck at the pumpkin.. spray the cylinder and bump the arm in and out with a small hammer to make sure its free. Shift into 4 x 4 , then crawl under and see if the arm hast moved 100% or will go further........ If it is fully engaged, you make have internal spline wear. ................. vacuum power is what moves the arm.......... vacuum also moves the heater/ ac air doors in the HVAC system...Your air flow thru dash / floor heat, etc is all OK ?? Not that HVAC and 4 x 4 are related, but they have the vacuum system in common.

 

I am home on Friday for a few days, I'll get at it this weekend and let you know. Thanks for the info.

 

Ok so I took the shift motor off and had the wife shift it in and out of 4x4. The motor works and it is moving it towards the 4x4 hi position. However it doesn't seem to be going all the way to 4x4 hi. If you look at where the shift motor is mounted there are 3 markings 2hi in the upper 10 o'clock position (when looking at the face) then the motor has to rotate the shaft counter clockwise to 4x4 at the 7 o'clock position. (roughly) and 4x4 low seems to be all the way around to the 2 o'clock position. So when I shift it to 4x4 it turns the shaft to about 8 o'clock and doesn't seem far enough. However the front drive shaft engages and can't be turned. Perhaps the motor isn't spinning it far enough. Also I took the cover off the front hub and checked the vacuum and that positioner. The fork seems to be moving back and forth ok but I will lube it up and make sure it is going all the way. It has never been an issue of engaging into 4x4 just staying in 4x4 when adding torque. And like I said before in reverse it pulls like a freight train and won't come out of 4x4. Thoughts on the situation? Anything please?

 

Neither is a "COMMON" failure. I have heard of several guys that had the front axle not engage fully... I have heard a few that had to take unit apart and replace engagement pin ... I have heard of transfer case motors failing and some "stuck", but never heard of one that would not go full travel when setting on the floor under the truck.