Allison, Braking, Overreving -

[peachin]

I would like someone to tell me the story of
Braking with the Allison and keeping it from
overreving on a steep decline (Full tow haul - full load) - while the tranny
is shifting down. Need a good discussion on this area - point the direction thanks

peachin@msn.com

[Idle_Chatter]

peachin, a lot has been discussed about the trans-braking "redline." I assume you mean the red cross-hatching on the tach. The Allison _will not_ trans brake at or above the redline. It will shift out to prevent exceeding the rpm limit, even if it means giving up on some braking. The system was working as programmed if it was selecting and holding at the cross hatched rpms. You have to have faith in the programming of the system and let it do its job.

[a64pilot]

Don't have my truck with me to look so I can't give you any numbers. As I understand it the not solid red area of the tach is for the allison or the six speed to use for Grade braking. The beginning of the not solid red area should be where the rev limiter is set for I.E. if you push the pedal to the floor in neutral the tach should not go any higher than this. I believe this RPM is a fuel limit and not a mechanical one. The solid red portion at the top of the tach should be the mechanical RPM limit of the engine. The engine should never be operated in the solid red area. I do not believe that a properly operating allison will drive the RPM into the solid area, as long as you don't have the ally manually selected into a lower gear that is. If you are in the position of the Allision driving the RPM deep into the dashed area frequently you may want to check into an exh brake.

[mdrag]

peachin,

Try a search on this topic and you should find plenty to keep you busy....

Or go to Kennedy's website and choose the TECH TIPS -> DURAMAX TIPS -> ALLISON GRADE BRAKING

www.kennedydiesel.com

[Jackmchome]

I kept the response to a question from long ago as it is one of the best explainations I have seen. Also, as said above, believe that the computer will not allow the engine to over-rev during grade breaking. (Approx 4700rpm)


The Allison Grade Brake Feature is a wonderful feature if you understand how it works and what inputs the computer needs to activate the downshift.

1. The Grade Brake Feature is intended to anticipate the need for a downshift so that the driver will not have to shift the transmission manually


2. The Grade Brake Feature is more aggressive in Tow/Haul Mode. By more aggressive I mean that the Allison will down shift without a brake apply if the computer reads a 0 from the foot feed, or TPS in tech speak, for 7 seconds while the vehicle speed is increasing or remaining constant, not losing speed. If further downshifts are needed the driver must tap the brakes, approximately for 1 to 2 seconds, to get more downshifts.


3. The Grade Brake Feature is not as aggressive when Tow/Haul is not engaged. This means that the Allison will only downshift to 4th gear when the computer reads 0 on the foot feed (TPS) and the vehicle speed is increasing, i.e. down a steep grade, or when the brake is applied for a 3 to 5 second duration.


Personal opinion and after some doubts I have come to depend on these downshifts, to the point that I drive in Tow/Haul all the time with or without a trailer or heavy load. Tow/Haul has other benefits like holding lower gears longer which in heavy stop and go traffic has helped keep a reasonable distance between me and the vehicle in front of me. The Grade Brake Feature is helpful when exiting the freeway, the Alison downshifts to slow the vehicle with very little input from me. I must admit that when I first tested the Alison and the Grade Brake I was skeptical, until I used it the way it was intended. Now when I climb back in my Buick, yes I do have a car, I miss the Alison and the Grade Brake. Just another reason to have the truck I guess.

[Jimamatic]

Peachin,
Through experimentation, I've found a way to sort
of trick the grade bracking feature into doing
what I want it to do intstead of what the computer wants it to do. I pull a 7500lb Jayco
trailer and when on a grade I simply let the
tranny downshift (tow haul mode) to where I want it, then I simply get out of tow haul mode. The torqe converter will stay locked and the tranny will not downshift any further. This allows me to maintain freeway speed with occasional braking, and remain out of the hatched red area on the tack. For me this provides a nice balance of engine and manual braking. I just don't like seeing and hearing my diesel rev that high period. hope this helps.

--------------

2500hd Chevy d/a cc sb pewter 4x4
Bilstein shocks

[chipper]

jimamatic
I have learned xactly the same trick, it works well with my 12,500 lb 5er.