Wait-- look at the exhaust side of the turbo. Do you have an exhaust brake there?
What brand?
To prove if working/not working: Use the down arrow on the Allison shift pad to select, say, 4th gear. Leave it there for the duration.
Accelerate to close to governed RPM in that gear. Let off throttle, exhaust brake OFF. Turn on exhaust brake. Turn it off. Turn it on.
You have eliminated the transmission as a factor, since it is in the same gear for the whole test. There should be a significant increase in braking HP with the exhaust brake on vs off, particularly at higher engine RPM.
Let us know what you have and what you find.
And, yes, some newer Cummins engines have VARIABLE VANE TURBOS instead of exhausts brakes.
