New Pacbrake wont disengage

[mel s]

Originally Posted by Wingman1

Sounds like a speed issue. Some owners have reprogrammed their transmission to engage the pacbrake in 6th gear and then they manually downshift the transmission. This prevents slamming the transmission into 4th gear at too high of a speed. It sounds like your brake works at lower speed because you have not reached higher speeds yet.

Wingman1
An Allison automatic, (as designed), can not "slam" into any gear that could damage either the engine or the transmission.
(If your transmission slams into any gear..... something needs fixing)!

My gear selector, (aka: Allison electronic shift pad), displays "2nd gear selected" when the PacBrake is activated.... but the transmission doesn't actually downshift until/unless the speed AND the engine RPM drop to a point that's safe for both the engine and the transmission.

BTW, an exhaust brake, when used properly, will save wear and tear on the service brakes.....(at 133k miles my service brakes are all original ...and only half worn).

Mel
'96 Safari

[CAPTBOB26]

WINGMAN, My shift pad does the same thing and I understand the advantage of using the exhaust brake to save wear & tear on my service brakes. Maybe it's my driving technique, but when cruising with the cruise control on & I approach a situation where I would like to use the exhaust brake, I turn the cruise control off, slow to around 60 MPH using my service brakes if I'm going faster, then engage the exhaust brake. That's when it won't come on most of the time. I called PACBRAKE and spoke with a tech who mentioned a solenoid that activates the brake mechanism as a possible cause.
Thanks everyone for your inputs and advice! I will continue to pursue the gremlins that are keeping me awake at night!

[mel s]

Originally Posted by CAPTBOB26

I turn the cruise control off, slow to around 60 MPH using my service brakes if I'm going faster, then engage the exhaust brake. That's when it won't come on most of the time.

CAPTBOB26
When the PacBrake is activated at 60MPH it is difficult to "feel" the braking because at 60MPH the Allison transmission is in 6th gear....(at 60MPH your PacBrake may be "coming on".... but you may not be able to "feel" it).
However, as your speed decreases the Allison automatically downshifts....
the lower the gear the Allison attains, (aka: shifts into), the more noticeable the "braking effect" of the PacBrake becomes.

In other words: the lower the gear the more you can "feel" the PacBrake).
Could it be you have only a perceived/imagined "PacBrake problem"?
Good luck.
Mel
'96 Safari

[CAPTBOB26]

Mel, I'm starting to think the same thing! Maybe it's my driving technique! I will be taking a short trip in a few weeks & I plan on "playing with the system", so to speak. Tomorrow I plan on checking out my accelerator pedal movement (TPS). Thanks again for the info.!

[Dr4Film]

Whether you have the OEM PacBrake or the upgraded PacBrake PRXB model, neither one is very effective at speeds over 55 mph and when the Allison is in 5th or 6th gear.

Reduce your speed PRIOR to cresting a grade such that the Allison with be in 3rd or 4th gear and your PacBrake will be more effective.

The PRXB has far better horse-braking power than the OEM PacBrake.

Operation | Pacbrake

Dr4Film ----- Richard

[docjust4kids]

Originally Posted by Skip426

The air pressure dump side of the control solenoid may be plugged ; at least partially ; because the activation air does bleed out eventually.
Follow the air line back from the brake to find the solenoid; Remove and clean. While there, remove the supply line from the solenoid to the cylinder and check for collapsed wall on the line, many members report their lines melted, from being run too close to the exhaust, and install longer line to route them properly.

Skip, if you are still available, thanks for this post of 6 years ago. I'm brand new to this site, and was having PacBrake issues, where the brake activated quickly and properly, but took nearly 2 minutes to disengage. The actual baffle moved very easily by fingers, but during the two minutes of painfully slow disengagement of the exhaust brake, the turbo wouldn't function and my 30 foot, CAT 3126 powered 1998 Safari Sahara barely moved -- neither safe nor acceptable. After cleaning and lubricating the 5 moving parts of the visible brake, there was no change. I read that the quick exhaust (Humphrey) valve might be bad, so I cleaned a bit of dirt out of it -- no improvement. After reading your post, and studying how the entire system works, I took apart the solenoid and attached valve assembly. The exhaust port of it was totally clogged with dirt! Once that was cleaned out, I replaced the parts and the result is a perfectly functioning PacBrake with IMMEDIATE return of the baffle to the open position once power to the solenoid is released!! So satisfying! My wife can't really comprehend how satisfying it is to WIN the battle against mechanical adversaries!! Thanks for your insight into this -- it helped a lot. Happy traveling, and blessings! David