Working on my sons 1998 F-150 4.6L auto:
It has automatic hubs, electric shift Borg Warner 4406 transfer case

The problem is: whenever you really need the 4x4, or just flooring it, the
4-wheel drive disengages. Let up and it works for a bit then out again. Does it in HI or LOW range
You can jack up the front and it engages, can't spin the tire everything seams to be locked up like it should.
We have checked the vacuum lines and all appears fine. Changed the vacuum shift motor on the front diff & transfer case shift motor, didn't help.
Is there anything in the transfer case that can cause this or is it inthe front diffs vacuum system? Maybe one of the solenoids?

 

anyone have a good picture of the insides of a 4406 transfer case

 

any help please

 

are torque on demand transfer cases full time or AWD type? Thanks

 

Check 2 items. First put a vacuum gauge on the hose at the front diff that engages the axle and rev the engine and hold it for a second or 2, the vac should stay the same. If it drops you have a broken vac line, defective check valve, or cracked vac tank. try running a hose to the engage port (at the axle) from inside the truck. Use a cheap hand vac pump to engage and see if problem goes away. If not most likely a defective/damaged engagement collar in front diff

 

All the vacuum lines and valve check fine. I clamped the vacuum shift motor all the way engaged and it still acts up, leading me to believe maybe it is in the transfer case in the shift on the fly portion. It acts just like you flip the switch back to 2wheel drive then back into 4 high.

 

The transfer case uses shift forks and interlocking gears. If it is slipping it should make a grinding noise. I guess It is possible that a shift fork is bent or gears worn out. I don't have any other guesses