Coaster1 is correct. Your C9 Cat will not down shift at any rpm which would result in the engine over revving and damaging its self. The exhaust brake will not be effective without the downshift as they produce their best retarding force when the engine is close to its max rpm.
If you are regularly not able too achieve a steady control speed on downgrades with the exhaust brake on then you will have to start your decent at a lower speed when the Allison can shift into a lower gear. It's most effective when in 4th or below.
Full time since 2007
2003 Monaco Signature