I guess the TCM didn't come with the transmission? If you want 6th gear, you'll need to visit an Allison dealer. Expect them to be very weird about this, and for it to be fairly expensive.
Just like everything else, identify a parts and repair center near you and get to know the people. Hire a Hino-specific mechanic for a few hours to give you as comprehensive an analysis as possible. Investigate parts availability before buying. I was very impressed with how smooth these two...
I've only worked on one pair of Hino marine engines. I've heard from others that they're very good if maintained correctly. They are not the same as in the trucks, the architecture is different.
Avoid the temptation to get the turbo rebuilt and reusing the old vnt controller. Avoid aftermarket 'rebuilt' controllers. We've seen plenty of these cause myriad problems.
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Most of the trucks we see are from the same customers, and we retain customers for years when we can show them that we were able to fix a $4k part for a few hundred bucks worth of labor on the same day the truck showed up for repair.