ISX 870 jake brake issue

[Chamberpains]

Those percentages show that the ECM is recognizing the 3 different stages you command. Your problem is probably gonna be in the engine (possible solenoid or rocker), jake wiring harness, turbo, or the turbo plumbing and actuator system.

What boost pressure readings are you getting under a full load pulling up a hill? Psi under load stage 3 full jake down hill? Stage 2 down hill? Stage 1 down hill?

Your problem sounds like a solenoid not responding. Has the solenoids or harness to the solenoids ever been looked at? Pop the valve cover and give the harness and connections a healthy look over. If you don't know how, go to https://quickserve.cummins.com/info/index.html and register your engine for free. It will have all the info you'll need to check out your problem. Step by step.