M939 Front Axle Will Not Engage- Transfer case rebuilding error- solved


Spartanburg, SC
Recently, I obtained a M929 dump truck and quickly realized that the front drive shaft was never engaging. I could hear the engagement cylinder moving when I actuated the low-range auto-engagement switch. Somewhere on this forum I had read about the piston being installed backwards in some vehicles, so I removed
the cylinder to check the piston orientation. It was in fact installed backwards, probably when the transfer case was rebuilt in 2006. There is no way this truck had front wheel drive since then. The following photos show the parts involved.

The easiest way to check if the front axle is engaged or not is to jack up ONE front wheel and try to turn it by hand. If it turns (and the drive shaft will be turning) the front drive shaft clutch in the transfer case is NOT engaged. When engaged, you will not be able to turn the front tire with one tire raised.

1 front drive cylinder parts.jpg 2 lip seal.jpg 3 actuation shaft.jpg 4 front of piston.jpg 5 rear of piston.jpg 6 name tag.jpg

Swamp Donkey

The Engineer
Steel Soldiers Supporter
Gray, GA
Nice job with the pictures. I've wanted to take mine apart to look at the seal condition even though it works fine. My shift lock cylinder (the one on top) leaked air into the T-case, I assumed because of a bad seal. Now I wonder if the piston is just backwards. Worth a check to satisfy my curiosity I guess, even though I don't plan on hooking it back up.
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