Mack GU813 S/N: 1M2AX16C9JM039991. When you're in 12th gear, roughly 1400 rpm and up, the transmission acts like it is trying to shift up another gear, when there isn't anymore gears to go. And when it does that, the rpm dial goes up higher in rpm, rather than lower, like how it would shift normally between gears. All other gears work fine. One code popped up as I was driving. P107492: Gears 2 and 3 engagement system - incorrect performance or operation. I was thinking of swapping TCM's with a sister truck I have. What else should I try?
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