Our Cummins_ISL03 with Allison_CBRAKE_Jacobs490 engine brake problem on our 2005 Winnebago Vectra on a Freightliner Chassis is solved. The engine brake works great now!
Problem was apparently incompatibility of engine ECM firmware revision from A to W when ECM replaced. I had to replace the Intake Manifold Pressure Sensor. The change I observed was that the old sensor was an Absolute Pressure Sensor, and the new one is a Relative Pressure Sensor. My boost gauge before changing started at 15 psi with the engine not running, and would get up to about 35-40 psi at full power. Now the boost gauge starts at 0 psi with the engine not running, and will go up to at least 50 psi at full power, and is much more active than it was.
Neither Cummins or Allison seemed to have a bulletin noting this incompatibility.
It seems that the Allison control to the engine is only to allow the engine brake when the converter is in lockup - all the other control is in the Engine ECM.