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I do this on every M939 truck I get my hands on. The interlock can be problematic due to faulty button, faulty wires, air solenoid not working, or sticking air cylinder. There's really no reason for it to be on there so I disable it.

Try shifting when the truck isn't running and has zero air pressure. If it will shift then, you're having problems with your interlock (which I suspect is the case). In my opinion you'd be better off to just do away with the interlock. Look toward the front top of the t-case and you'll see what...