TPS throttle readout will not go below 3% when foot is off throttle.

[Lonne Mays]

The readout from my VMSII system indicates that my throttle never goes below 3%, and I believe that this is why my the Allison transimission ECU is preventing the exhaust brake from coming on.  Does anyone else have their throttle reading more than zero when their foot is not on the pedal?

[Dave Atherton]

Lonnie, the throttle position sensor the correct reading in specs and readout with Cat ET in
Between 3 % to 5 % your problem is not at that location. Dave Atherton Retired Cat Mechanic

[Lonne Mays]

Thanks Dave, for confirming that the 3% is acceptable as a closed throttle position. I did see in the Cat MVP-ER SERVICE MANUAL for Caterpillar 3126 HEUI Engines (on page 13-7, second paragraph), that it gives 3%-100% as the range for the throttle.  I'm just trying to relegate that information with the information I found on a Pacbrake wiring supplement "FOR PACBRAKE EXHAUST RETARDER IN CONJUNCTION WITH ALLISON WORLD TRANSMISSION" which stated:
     "When the Pacbrake dash switch is actuated a 12 volt signal is sent to the Vehicle Interface Module (VIM) retarder relay to activate the brake solenoid and also sends a 12 volt input signal to the ECU via wire #119 of the vehicle interface harness. .... The Retarder Enable output signal is wire #132 from the ECU which activates a relay in the VIM to prevent engagement of the Retarder with the throttle>0 or lock-up off."

This description from Pacbrake seemed to correlate with the wiring diagram I got from Larry Fritz (for his 98 Patriot, which I believe is also a Magnum Chassis).  (See the attached pic that shows the area in question.)  I was just thinking that maybe they were being literal when they said "throttle>0", and that 3% is greater than zero.  All I know is that the relay in the Allison VIM is preventing 12V from getting to the exhaust solenoid when I have the ignition ON, and still preventing the exhaust brake from turning even when all the proper conditions are present for the brake to be allowed to activate (i.e., rpm>1000, transmission in 2nd, exhaust brake switch ON, foot off from accellerator, speed>30mph).   I just haven't figured out why yet, but I'll keep troubleshooting.

[Dave Atherton]

Lonnie, have you tried and applied 12 volt to solenoid at pac brake with jumper and check
to see if movement at pac brake than work back to Allison. Reason I’m bringing  that up after
troubleshooting system found problem at pac brake frozen at Actuation arm. Dave

[Lonne Mays]

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.