Pac Brake Gear Selection

[John Demartini]

I thought my PAC brake was not functioning, come to find it's activating on in 2nd gear.  I now understand this is a preselection and can be changed.(?)  Does anyone have experience with this, and any suggestion.  I'd actually like it to come on in 6th, and then work it's way down all the way to a stop.

Does the preselection need to be done by an Allison tech, or can I do this myself?

If it's set to 2nd from production, why?  And if so, what's the best way to effectively utilize it when set to 2nd gear?

Thanks in advance for any suggestions or insight...

[David T. Richelderfer]

I am not sure if your issue is somewhat the same as ours but we have a Jake Brake that was programmed to downshift our transmission to 4th gear when the accelerator is released while the Jake Brake toggle switch is in the ON position.  I did not like the automatic downshift so had it reprogrammed at the Kenworth branch facility in Pasco, WA.  Now when the accelerator is released with the Jake Brake ON, the transmission does not automatically downshift.

The Kenworth technician connected his laptop computer to the engine/transmission interface plug under the dash plate in front of the steering wheel to make the programming change.
 

[Steve Huber Co-Admin]

John,
I'm surprised that your tranny shifts to 2nd when the Pac Brake is activated. Most shift to 4th. Key is that the max recommended RPM for the 3126 is 2900 (see pg 49 of 1998 CAT 3126 Op Manual in Coach Assist) so if dropping to 2nd increases it close to or above that, I would consider having Allison reprogram the tranny to drop to 4th or above as David noted in his post. (IMHO 2500 RPM would be the max I would consider to ensure long engine life). I'm curious if another 90s and 2000 era coach owner can confirm what his unit shifts to when the Pac is applied.
Steve

[Eric Maclean]

John
It sounds like the programming is correct
First the pack brake has to be turned on w hich will automatically turn off the cruise control.
The pack brake when activated will show the second bear select in the left hand window of the shift pad .this is the lowest gear that the pack brake will operate in.
If you are driving with the pack brake turned on at highway speed say 55 mph and release the throttle completely the pack brake should activate and the transmission control will then shift down one get at a time to maintain an effective rpm for the pack brake to work when the vehicle slows down to a point where the transmission control module thinks it can shift down one more gear it commands that shift untill the vehicle drops to about 20 mph where the transmission would shift to 1st gear at this time the pack brake shuts off or drops out to avoid stalling the engine.
At any time during pack brake operation the pack brake will shut off and return to normal operation if you press the accelerator

As Steve pointed out the fourth gear down shift seems to be a little premature and causes higher  rpm than we'd like but that is the way they were programmed from the factory.

Hope this helps
Eric

[John Demartini]

Thanks Steve and Eric:

I talking with an Allison tech who's willing to help out.  Simply put, the PAC brake does not come on(activate) until the coach slows enough to downshift to 2nd gear.  With the system turned on, there is no change in the gear position read out either. 

I do need dome help otherwise, I need to locate the TCM (trans control module).  Can either one of you help me with a possible location on my coach?

Thanks again...in advance.

[Eric Maclean]

John
The TCM is located in the console below the shift pad .
The VIM is located in your electrical bay below the driver's window.

Eric