Hi Steve, Hi Dave, Yes I have verified that the Pacbrake is physically actuating when 12V actually gets to the exhaust brake solenoid. With the ignition OFF, if I apply 12V to wire 205 (see attached revised schematic), I have visually observed that the Pacbrake immediately snaps to the APPLIED position. As soon as I turn the ignition key one click right (to the RUN position but with the engine not running), the Pacbrake snaps back to the UNAPPLIED position. This is as it should be, because the Allison transmission control module (TCM) will activate the exhaust brake relay in the VIM and thus remove the 12V signal from the exhaust brake solenoid, since the engine is not running and the set of conditions for Pacbrake activation are not sensed by the TCM. Thus I know that the mechanical aspects of the Pacbrake are functional, and that the Pacbrake will activate if 12V is allowed to get to the exhaust brake solenoid coil. This test also shows that the associated relay in the VIM is functioning, as it does its function of opening the 12v circuit to the exhaust brake solenoid when commanded to do so by the TCM.
So, what I seem to be left with is the fact that the TCM is always holding the VIM exhaust brake relay in the coil-active state, which will prevent the exhaust brake from being applied. I have not yet found the physical location of the VIM, but I have not yet removed and looked behind the panels to the left of the driver's knee (below the shift console), or crawled under the front of the motorhome to look for it on the other side of the firewall. Once I locate the VIM, I intend to "diode OR-gate" a wire to the output of the relay to allow me to have direct access to the exhaust brake solenoid coil. This way I can hook up an indicator lamp to this line, and if necessary, a manual control override pushbutton. The indicator lamp will allow me to observe the Pacbrake status while doing a test drive to bring forth all the conditions required for the TCM to allow Pacbrake actuation.