Originally Posted by Mike Canter
It can be a Jacobs exhaust brake. Jacobs makes them.
Sure do. We have a Jacobs exhaust brake on our ISB.
I agree that the easiest way to figure things out is to get out on a highway, take your foot off the fuel pedal and see what happens. If there is an exhaust brake and it is in the "on" position, things should start to happen right away. On our dash, there is a green light that comes on when the exhaust brake is engaged. Just because the switch is in the ON position does not mean that the brake is working at that moment. A number of conditions have to be correct before it does work.