I've seen threads about stuck in 4wd, and will shift into 4 low but not 4 high. The problem I am having is not shifting into 4 low. It shifts fine between 2 high and 4 high, but when I try to shift into 4 low, the light flashes for a while then quits. Ive tried it is neutral, drive, reverse, coasting in both directions, and shifting back and forth. I don't hear any noises like its trying to shift or anything. Could it be vacuum, electrical, or internal with the transfer case? Thanks for any help.

 

I am assuming that you have the 3-button, NV233 transfer case in your truck.

The problem is likely either in the transfer case itself or the encoder motor. You could attempt to shift the transfer case manually from 2HI to 4LO by removing the encoder motor and putting a wrench across the flats on the shift sector shaft that is sticking out of the side of the t-case. 4LO will be the furthest counterclockwise position when looking at the shift sector shaft from the driver side. You should feel three distinct detents from 2HI to 4LO. From 4LO, the next detent clockwise will be neutral, then 4HI, then 2HI. There shouldn't be much resistance moving between the detent positions.

If you find that the transfer case moves well between detents, then the encoder motor may be the problem. If you are careful enough, you could pull the encoder apart and have a look at the gears inside. The attached picture shows the internal gearing of the NV233 encoder motor, specifically a failed unit where the final drive sprocket has stripped teeth from either a manufacturing defect or an issue with a shift that sheared the teeth.

You could also back-probe the connections for the channels from the encoder to see if it is rotating through its proper range for the shifts. I have also attached the FSM section describing the encoder layout and associated schematic.

 

Thanks for the info. I will have a look around. hopefully it's easy to get to.