Allison grade braking question....

[DuckhunterInTN]

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I was reading the FAQ's on www.allisontransmission.com (http://www.allisontransmission.com) at http://www.allisontransmission.com/...ategoryID=11#56

and it stated that "...the (down)shift will never occur without depressing the brake pedal..." regarding the grade braking feature.

I could have sworn that my truck would downshift throught the gears if I just simply let off the throttle while in tow/haul mode...

Which is right??

[Idle_Chatter]

I think that's a misprint in the manual. My transmission will definitely downshift as speed rolls off without touching the brake pedal. I think the thin distinction is that it is not really "grade braking" without the brake applied, but the downshift causes "grade braking" in the lower gear because of the transmission design. I got into a nearly contentious exchange over this a couple of years ago when I stated that I often shift to tow/haul as I'm coasting down a ramp off the interstate which causes my transmission to downshift at 45 mph or so and "grade brake" on the ramps. Another member practically called me a liar and stated quite rudely that I was "unconciously" applying my brake or "riding the pedal with my left foot" because the MANUAL SAYS that it won't happen. It does, very useful function when towing an unbraked trailer like mine or on wet and icy conditions.

[Jackmchome]

The "grade breaking" feature does require brake pedal appllication to engage. It will downshift at least one gear more than the normal downshifting that occurs in tow/haul. It makes use of the 3200 to 4600 rpm range for grade breaking. Scarry the first time you use it!

[et]

My Allison does not always need the brakes applied to engage grade braking. I use it all the time to slow down at a T intersection. I only apply the brakes to come to a complete stop.
I also use it to slow down when leaving freeways.

I believe it engages this way only when going up hill, at least in my experince. On the level or going down hill, I must at least tap the brakes to engage grade braking.

[DuckhunterInTN]

well that is good to know that my truck is not the only one that acts like that. It doesn't bother me either way, IMO.

I guess what you are saying Jackmchome is that it makes the "grade braking" more pronounced?

[Jackmchome]

I tow a 15k, three axle, trailer with my D/A. I can cross the sierra's with very little use of brakes thanks to grade breaking.

[DmaxMaverick]

Quackerwacker....

What you are experiencing is normal, but is not "Grade braking".

When in "Tow/Haul" mode, the transmission map is different than when in "normal" mode. The map defines the gear selection range (by the TCM) according to engine RPM.

The downshifts you experience while in T/H mode, with no brake input, is just gear/engine RPM matching. Watch the tach. It will happen at the same RPM/gear, every time.

Engaging the grade braking feature requires brake pedal input as well as other sensor inputs, such as speed increase, 0% APP, brake fluid pressure, braking response, and others.

The statement you quoted from the Allison site is correct, in context. It means that a downshift, other than a [mapped] downshift, will not occur without brake pedal input, when T/H mode is selected.

Hope this helps to 'splane it.

[DuckhunterInTN]

Quackerwacker...LOL.

Yeah, that clears it up a lot. So I guess with my truck being new and never towed anything I most likely have not actually felt "grade braking" occur. (the land around here is pretty flat, so no big hills to gain speed going down, at least while empty).