Brake Tap Grade Braking

[Jasondt2001]

This is just a theory; but, in my truck unless I 'stab' brake it it's useless 90% of the time.
I'm wondering if it has something to do with the G sensor (stop laughing, I'm serious!) before it goes into grade braking downshifting.

I can go down a STEEP hill w/ tow haul on and let the truck start running away, and the MINUTE I stab the brakes it downshifts...if I go easy on it, no deal. (The cruise braking works at 5mph over posted CC setpoint though).

[killerbee]

I have an idea that the turbo brake that I put in the tune, keeps the truck from accelerating enough for the tap feature to work sometimes. It does seem to be a complicated algorithm as Ben said, and one of the factors that goes into it, is known acceleration. I am basing this on some reading, and the fact that I am able to get it to kick in on the flats after an acceleration.

[DURAtotheMAX]

Crafty1 (who knows his allison stuff) stated that grade braking was made less aggressive in later years because people who didnt understand it complained about it....but I have found the complete opposite??

On my 01 TCM that I run in the summer (no auto high idle or overdrive lockout on the shifter, but it shifts reallllyyyy nicely), grade braking is literally not there. Maybe once in a blue moon ill see it work.

Then when I reload my stock 05 TCM in the winter so I have auto high idle, I suddenly have much more aggressive grade braking.

Another thing ive noticed is IF you can finally get the TCM to do a grade braking downshift, all subsequent grade braking downshifts become MUCH easier to invoke (is that the right word?).

Like if Im towing a trailer and grade braking sucks, I do my fancy foot work of accellerating at WOT, then let off for 3 seconds, then dip into the brake for 1 second, THEN it will downshift...then I accellerate again and it upshifts......well from that point on as long as I leave tow/haul engaged, all I have to do is tap the brake and it downshifts. Its like once you get it to do it the first time, after that it throws the algorithm out the window (or severely dumbs it down) and basically just downshifts whenever you step on the brake for more than a second.

Oh if that allison TCM could talk...all the stories it would tell...

ben

[Sparky8370]

I am wondering if the taps adjust the grade braking as well. I have an 01, which Ben was just saying grade braking is virtually non-existant in, and the grade braking works. It def kicked in a lot more when I had been towing a 10,000lb load very frequently. Most of the time that I drove my truck one particular summer. I also noticed that on this one particular hill it would always kick in and rev it right to the moon. Same spot with my buddy's ford it did the same thing. It wasn't a fast hill and it wasn't any steeper than some of the other hills, so I have no idea why it was so aggressive there. The grade braking has actually kicked in there without it in tow/haul. So, now that I am thinking of it, maybe that is just a point it would naturally downshift and that just made the grade braking more aggressive??

[killerbee]

I have a tune with around 130 changed tables. I have somehow managed to disable the brake tap feature of the allison.

I fixed it, and this weekend, for the Cub Scout Trip, Tap braking worked like a champ.

What I believe was happening is there was slight fuel commanded at many of the lower rpm's, at zero throttle. It seems fair to assume that grade braking is dis-allowed with any commanded fuel.

[DURAtotheMAX]

I fixed it, and this weekend, for the Cub Scout Trip, Tap braking worked like a champ.

What I believe was happening is there was slight fuel commanded at many of the lower rpm's, at zero throttle. It seems fair to assume that grade braking is dis-allowed with any commanded fuel.

michael I dont suppose you would be able to give a little more advice on what basic changes to make to make the grade braking actually work?

I have been screwing around with this from time to time ever since I beta tested allison TCM support years ago and I could never figure it out.

ben

[killerbee]

I am not by any stretch versed on it.

It works when conditions exist that would suggest it is beneficial. I don't mean to be presumptuous...are there any real grades in CT? IIRC that's where you are.

I do believe that you need a minimum amount of forward acceleration (with zero fueling) for it to kick in. That means tap may not work on mild grades. It's more of a gut feeling from the experiences I have had. Speed plays a role also (I believe), and even the number of times or duration of brake applications.

So with that, I would not expect it to work on mild grades where the physical load is not turning the truck into a runaway train.


That's all I got.

[killerbee]

I did find that several times, more than one brake actuation was required. If there is more than one brake pedal sensor (assumption), maybe one of them is for threshold braking for grade brake feature.