Mystery gear.....

The existence of that gear is why large fleets stopped buying 13 speeds, it is a mechanical oversight that cannot be blocked off without abandoning an otherwise brilliant design.

When that gear is selected a very large gear is meshed with a very small gear and stresses an area of the transmission housing that is not strong enough to resist the flex, that means the gears try to move away from each other under a load and when they move far enough to break teeth it causes catastrophic failure with no parts reusable including the case; which means no core credit for a replacement.

No amount of education would get fleet drivers to stop using the "Monkey Hole" (that is what that gear was nicknamed back in the day because only those as dumb as monkeys would keep using it) so large fleets gave up and bought the idiot proof 10 speeds instead.

The 13 speed is a fantastic transmission but it is usually ordered by Owner Operators (guys who have to pay for the stuff they break) and smaller fleets who hire a better caliber of driver (guys who are teachable); this gear is not a "mystery" to those who have seen the inside of a transmission case.

 

That makes sense. I appreciate that in-depth answer. I will keep that in mind. I guess I've been lucky so far. You probably saved me and a bunch of people from a monkee hole catastrophe. Thanks

 

I've known drivers here to use that gear to climb the big hill leaving town at 100ton gross. not enough grunt for top box but they cant hold it full jandal on the party pedal in 4th OD so they slot bog cog high range direct.

the poor box!! here are 20918B and 22918B with 3.9, 4.11, 4.33, 4.56 or 4.89 diff ratio.