'64 C60 freshly overhauled and rebuild with 292, SM420, and 2 speed rear.
The issue at hand: I can split shift up to 3 hi, and then I cannot shift to 4 lo, it just grinds away without wanting to engage, regardless of rpm range. I am stuck with 3 hi to 4 hi shift which is a big gap and bogs like a son of a gun, or 3 lo to 4 lo and then 4 hi. The 3 lo to 4 lo again is a big jump, but it doesn't seem to be as bad as the 3 hi to 4 hi jump. Its weird because all the gears work great, just not in proper order.
Up to this point it has been a "whatever" issue, but with the new log bed complete at now empty +1k lbs, it makes a noticeable difference, so it can be assumed that at actual GVW it will be significant. That and, I just overhauled everything else, so the axle not cooperating correctly leaves me wanting to fix it.
At this point I have ruled out every possibility outside of the diff. All new air lines on every square inch of the truck. Brand new rebuilt 292 and SM420. I cleaned and rebuilt the shifter with a new diaphragm and ensured all was sealed well. I checked the actuator and all appeared to function perfectly. So what I am left with is that it seems to be something mechanical in the diff, but that seems counter intuitive as the axle doesn't care what gear the trans is in, simply that the rpms are where they need to be to shift properly. So before I open up the diff in the coming 2 weeks (need to anyways for a cleaning and refresh of gear oil and seals), does anyone have any hypothesis on what the problem could be or what I need to pay close attention to?
As a random thought, do I need to try and shift the trans from 3 hi to neutral, shift the axle to low, and then shift the trans to 4 lo and see if that works? Is it possible that someone who previously worked on the truck could have reassembled something incorrectly to cause this?
