So. If you use a wire tracer, you may be able to find the module. Since the DDEC II isnt activating the jake then throtte position has to be involved in some way. look at the throttle pedal and the throttle on the engine for a limit switch
I haven't wandered too much on the inside of the coach. I do know which wire comes from the back so I can separate that one and tone the other 3 wires at the relay.
It’s hard for me to believe that they are not using some signal from the DDEC for the zero throttle. It may not be the engine brake output maybe there is another output. I will look through my DDEC II docs tonight and see if I can find anything that they might be using for the actuation of the jakes. The jakes module I know of still gets it signal from the DDEC ECM.
It’s possible the relay is junk too. I will test it later to rule it out. So far one of four solenoids was dead and I replaced it. All four click when I apply power with a test lead.