18 speed transmission grinding

The splitter slide collar is most likely worn out. The back box is an under drive unit and only turns when the button is in the direct position.

Basically you need the back box rebuilt. I would look at all of it.

 

Not sure what kind of experience you have with 18 speeds however with the splitter you only flick this down if your approaching a hill and your losing revs but not enough to warrant changing down a whole gear, you flick the splitter down wait a couple of seconds then hit the accelerator to increase revs this will change it down. I'd have to be in the truck personally to really see if its just the way you are changing gears or whether there really is a mechanical issue with the Eaton gearbox. Its just unusual when you say the splitter is grinding as I've never once had a splitter grind. The only thing that grinds are the actual gears.

 

No a bad synchronizer, or low air pressure to the range shifter will size the gears to grind while changing ranges. The bears have to mesh or you couldn't change range. There is a lie dude and high.side syncro