Community Forums

AKOne · 08-01-2019, 01:23 PM

When Pack Brake is activated, transmission (3000 series) will automatically downshift or attempt to downshift to 4th gear. Is it possible to disengage the auto-downshift?

96 Wideglide · 08-01-2019, 02:00 PM

I believe an Allison or Cummins shop can do it for you.

RKins · 08-03-2019, 11:49 AM

X2 on what 96 said.
The Pac-Brake is really not that strong as a brake itself. The combination of the downshift with the PacBrake makes it work. You can also get the tranny reprogrammed to dive for 2nd.

AKOne · 08-03-2019, 04:38 PM

Thanks gentlemen. I am a religious Pack Brake user. The main issue I have is on a downhill grade about 3 to 5%. I want to be able to manually drop down to 5th gear and then, as needed, trim my speed with the Pack Brake without the transmission trying to drop down another gear. Strictly a personal preference.

lawjohnson · 08-03-2019, 05:16 PM

From what I understand Allison can configure the trans several different ways depending on your needs or preferences. Also it is my understanding that it has to be done by Allison though I do not know why Cummins could not.

A Traveler · 08-16-2019, 01:23 PM

I did this on our coach. Any Allison shop can do it. In my case it took about 30 minutes and $50 to get it done.

RKins · 08-16-2019, 04:41 PM

I have a WTEC II Allison and I think there is a way to do what you want.

When the Exhaust Brake (EB) enable switch is on it applies a 12V signal to the TCM which enables the tranny downshifting and rules about the enable/disable of the EB (on above 10mph and foot off fuel pedal for tranny downshifting) and EB activates via wire 132 with a ground from TCM to the VIM. (look at the attachment)
Readers Digest version is that the EB is activated by the TCM applying a ground to the relay that activates the air solenoid to activate the PacBrake (PB) cylinder.
So the key is the ground applied on wire 132. I cut that wire and attached a bullet male/female connector and added the appropriate connector to a 12v LED attached to 12V to watch what this pin was doing when the EB switch was on and when the EB was commanded via the TCM. This helped me understand if and when the EB was being commanded.
For you - if you switch the EB enable on, it engages the tranny to downshift. So that is not what you want but you have no way to engage the EB manually.
So I installed a foot pedal that applies a ground to the VIM relay via the bullet connector I installed on wire 132 and that activates the EB. That's great but if I want to use the automatic braking built into the system I couldn't, so I installed a single pole double throw switch and marked each throw "Auto" and "Manual" and connected the foot switch on one input and the TCM on the other.
The VIM doesn't care who sends the activate signal to it, so in manual mode, you stepping on the footswitch applies the ground to activate the EB. If you want to let the automatic EB do it's thing, flip the SPDT to "automatic".
Digest that and let me know what you think.

dbarton291 · 08-20-2019, 09:10 AM

It can be done. Cost depends on how the vehicle is configured.
If the brake enable signal to the transmission TCM is via a wire, just disconnect the wire.
If the brake enable signal to the transmission TCM is via the J1939 data link, the TCM has to be reprogrammed.
It can be done either way, just different costs.

RKins · 08-20-2019, 12:51 PM

I agree with DBarton.
If you follow my instructions, you have the best of both worlds - you can disable the TCM from downshifting (by not turning on the EB enable switch), yet still have the ability use the EB when you want. Also, if you want the TCM to work as designed, you turn on the EB enable and turn the selection switch to Auto. All without paying the man to reprogram the TCM (there was a $175 equipment fee when I had mine checked at an Allison shop - only to find out it was operating correctly)
BTW, wire 132 is easy to find, it's marked and is orange.
Let us know what you decide and how it's working for you.