quick question for you guys. Woke up this morning to an inch or two of snow. As I headed for work, I discovered the roads to be pretty slick, so I threw it in four wheel drive. I started hearing a slight grinding noise and had no 4 x 4. I started to pull over and as I almost got stopped, the four wheel drive engaged and worked fine. I tried several times and always the same thing, the 4 x 4 only engages at very low speed. I have had to replace both of the solenoid valves on the fire wall that control the vacuum motor on the front axle, but have had no other problems with the 4x4 in the past. Any ideas? The truck is a 2000 F150 with the floor shifter.
thanks

 

I would suspect a vacuum leak or a faulty actuator motor. It is not pulling on the shift fork lever hard enough to completely engage it and it is slipping back to the disengage mode when under stress. You will need a vacuum gauge to test for leaks. Install it on the blue tube end at the motor and engage 4x4. There should be no leak down on the gauge. You may also have a defective shift fork.
That will require opening up the axle to fix.

JMC

EDIT: I would also check the transfer case shift linkage for binding.

 

Ive seen 100's of stuck 4x4 motors give it a wrap with a hammer works alot of the time

 

does anyone know if you can put grease in the little boot on the vacume on the side of the front diff? the vacume pushes the little rod in and out? which pushes the lever to engauge the fork assembly

 

The shift on the fly grinding is a tip off where the failure is occurring.
There is an electromagnetic clutch that is used to spin up the front shaft to match the rear drive speed, then a spring loaded collar locks the two togather.
The grinding you heard should have been at this point.
Of course with the truck stopped the spinup doesnot have to take place.
It may be just an exersise deal from not using it very often.
The 4x4 lamp is signaled by manuel shifter and trans motor positions and is independent of the internal actions of the trans so can light and there still be a problem.

A 2W to 4H operation should not be done above 45 mph at temps below 32°.
Give it a workout a bunch of times and may be it will get loose and lubed enough to work properly. If not then a decision has to be made as to what has to be done to fix the situation.
Hope it works out easy for you.
group 3 powertrain operation msof ref.

 

it's not engauging i think its time to replace the shift fork assembly,,, its an 2003 f150 screw fx4... also why are so many people having problems with the 2003's 4x4? i owned a 2001 screw and never had any probs with the engaugement issue. was there a change in the design that made it worse?