I've had problems on my 13-speed ever since I bought the truck: when shifting from 'low' to 'high' it grinds. I tried various rpms, and even bought the 'gearmaster' - a gadget that actually tells you when to shift ... all to no avail. i recently had some work done on the transmission, and when it was out, i asked the mechanic to check the synchro. he said the disks were "like-new", so either something is wrong with the air shifter (not sure what the correct term is) or i have to swap this tranny for a 10-speed. if anyone has any ideas, please post.

Grinding when you flip the button to hi range means the range shift synchro is shot.
If it keeps on doing that long enough the teeth will get worn down to the point where the range shift will jump out of gear when you put the power on.

What teeth? The dealer took my tranny apart and showed me the inside of the synchro box. It's just a couple of disks pressing against each other. They have lining and when that lining gets worn out, that's when the synchro is bad or "shot". I didn't see any teeth there... The whole tranny was rebuilt, and the only faulty part of its operation is shifting from 'low' to 'high', usually in Gears 5 and 6.

I do not know much about it, but is there a vent to relaease the little puff of air??? Might it be plugged?

Just thinking out loud here...........none of our 13 speeds do that...........and I think you do not want a 10 sped

I think you need to clarify your question. Are you talking about your low to hi range shift or are you talking about the splitter (as in splitting gears)? the splitter syncro is at the upper rear of the trans and should be discs like you described. The hi-low shifter is lower on the main part or the trans (around the 6 o'clock position) and should be a cast iron housing with an air line going to it. You shouldn't be able to see any moving parts in that unit. This info is based on a 18 but should be similar to yours. Any good mechanic should be able to walk you though this to check it. It's rare that anything goes wrong with the controls on the gear shift itself. You'd be foolish to go to a 10 speed because of this problem.

When you flip the lever to change the gear range, the air from the shifter valve(shift knob) preloads one end of a spool valve, mounted high on the side of the trans. When that spool moves, the air is directed to one side or the other of the range shift piston on the back of the trans. There is an interlock built into the lid of the transmission that is supposed to keep the spool from moving until the shift lever is in the neutral gate. When some of the parts start to wear, the spool can move as soon as you start moving the shifter and it will try to shift the gears while the front section of the trans is still in gear, causing gear grinding. Try waiting to shift the range button until the stick is centered in the neutral gate and see if the grinding stops. If it stops grinding when you shift it that way, you need to work on the cover, good news since you don't need to pull the transmission out to do it. Behind the valve assembly on the edge of the trans cover is a hole with a plunger in it, one end of the plunger rides against the lower end of the shift lever and the other end fits into a notch in the back of the spool. All these parts will come out without taking the trans out of the truck.I fixed a 10-speed once by replacing the shift lever, the groove in the side of the pivot ball was worn too deep. Possibly, just changing out the plunger in the lid for a new one might be enough to fix it.

The teeth on the hi/lo range shifter sliding clutches.

Those discs are designed to bring the 2 sliding clutches into synchronization so they will engage without grinding. If those discs don't do their job, the range shift will grind violently and then slam into engagement.

After that happens enough times, the teeth get worn down and will push apart under power, rendering the transmission useless.

Seen it happen numerous times.

The grinding occurs when I'm shifting from 'low' to 'high' in the same gear using the side button eg 5 low to 5 high, 6 low to 6 high. So, I guess that's a "gear splitter" using your terms. It's okay when I"m empty but once I"m loaded I can never find the correct RPM to get it shift smoothly.

Does it grind if you pull it into neutral make the shift and go back into gear? If not just shift it like that. You should be able to preselect it and just lift off the fuel pedal but if not try it the other way and see.

I had a guy from Eaton tell me you're supposed to lift off the gas and push the clutch in real quick to make it shift.