Cummins isx cm870 jake questions

If it were a Detroit I would say check the switch or the wiring from the outside in. On a Detroit, low is the middle two cylinders doing the braking, medium is the front two and the rear two braking, high is all six cylinders braking.

As far as Cummins, I am not sure of the sequence but it sounds like a wiring issue. Every ISX I have driven has had 3 settings.

 

On that vintage of ISX, IIRC, the 3rd stage jake is all 6 cylinders plus the Variable geometry turbo being closed off for maximum backpressure. There should be a small control box on the lower left front side of the engine block above the oil pan rail. There is a steel braided air line that runs from it all the way over to the top of the turbo controller.

The ISX's we had like this sounded like a locomotive when they were first started with low air pressure in the morning. When the psi got to 90-100, you could hear the turbo change pitch almost instantaneously.

The control boxes can fail internally, not varying the pitch in the housing of the turbo due to air leaking by the seals internally.

Does the truck run like a dog as well? Or does it have good power throught the RPM range?

 

My 07 Volvo w/ISX Cummins had 6 levels on the engine brakes. If you get on youtube and search, you'll find a video on that. I think it's maybe a solenoid or a wiring problem. Hope that helps.