DD15 Engine Brake Issue

[armana87]

One of my trucks were bypassed in the past and everything has been fine after that (2-300k miles until recently drivers been complaining about his brakes so finally I took it for a test drive and discovered that engine brake is always engaged even at standstill if you hit the throttle you can hear the engine brake cutting the rpm’s drop quick even driving between shifts at low speed..and if you turn the engine brake off from the steering switch everything is normal when driving. This happens when engine warms up and jakes get activated. When he got back to our yard the engine was misfiring because he was driving it with the jakes holding the truck back, after it cooled down it never misfired again.

The only code I show is

SPN 973 FMI 9
SPN 986 FMI 9 (on and off)
SPN 2791 FMI 7 (inactive)