ALLISON "GRADE BRAKING" QUESTION

[reelbob6.6]

I have a 2001 chevy duramax with allison transmission. Do these allisons downshift from 5th to 4th on grades when the brake is applied on steep hills in normal drive position?
Acorrding to a chevy notice I have it should but mine does not and the dealers I have taken it to say nothing is wrong with mine. It shifts down great in the tow/haul mode but not in the regular drive mode?? I could not fine anything in the site but I am new to the board. THANKS FOR THE INFO IN ADVANCE

2001 2500HD
EXTEND. CAB.
6.6 DURAMAX
ALLISON TRAN.
15,000MILES LITTLE TOWING

[Jim Dandy]

I recently went on a 450 mile unloaded trip that included Powder River Pass in N. Wyoming. On the way down off the pass there were a dozen or so corners which required slowing from 65 to 45 or less. On the first 4 or 5 corners my truck did not shift down but required me to brake all the way through the corners. After the first 4 or 5 corners the Allison had learned what to expect and dropped into 4th. It would shift back up into 5th after going through the corner and feeling throttle applied to speed back up. All this was with T/H off.

I was concerned about this since I was under the impression that the down shift would only occur with T/H on. I called my buddy/Gm service manager when I got off the pass. He told me my Allison was operating normally, that the downshift will occur in both modes. At the speeds I mentioned above this translated into about a 700 rpm jump when the downshift occured. Also, when it started downshifting it did so after about a 3-5 decrease in MPH when the brakes were applied. This is something worth keeping in mind when driving on ice/snow.

[ 05-21-2002: Message edited by: Jim Dandy ]</p>

[MudNurI]

one other thing to mention- when mine was "limp" for a week- it would not grade brake....I asked the tech about it, and he said it was a feature of the "limp"....


--Brandy

[reelbob6.6]

Thanks Jim Dandy. My truck has 15,000 miles and shifted down in normal drive for several months and then stopped. I have taken the Truck to two dealers who say nothing is wrong and that not shifting down on grades is normal?? Yet the GMC document id # 850627 states:
Does tow/haul have to be selected for "grade braking"? No.. However, grade braking will be less effective in normal mode than in tow/haul. in normal mode, the grade-braking feature will pre-select only the 5-4 downshift, whereas the tow/haul grade braking has the ability to preselect the 5-4, 4-3, 3-2 downshifts."
I can not make mine down shift at any speed from 5th to 4th by applying the brake and slowing to a safe speed on any type of grade??? However it does in t/h mode but stays in the lower gear longer than I would like.
How do I change this problem? The dealers are not going to do anything because their scan tools don't show a problem and we live in a valley no hills??

[reelbob6.6]

BRANDY: WHAT IS THE "LIMP"? THANKS BOB

[LA DMAX]

Mine will grade brake from 4-3 sometimes, I guess depending how long or hard I press the brake. I don't mind it. What is kinda weird is that in T/H mode it won't down shift 5-4 until I'm below 65mph or so. Again, hasn't bothered me much yet.

LA DMAX

[MudNurI]

Bob

it's basically a "safe" mode Allison goes in when you "throw a error message"...My fuel filter was pretty nasty, my truck would go from 1st to 2nd and 2nd to 3rd fine, but when trying to go to 4th, it would go to 40 mph, and 3,000 Rpm, and STAY THERE....eventually my SES light came on,,I shut off the truck for a bit, and it went back on, but until I had the new fuel filter installed, and the error message deleted, I was LIMP...basically means no power. I believe it is to protect the transmission- since you already have "something" wrong, it doesnt want you doing anything stupid to it...I hated the week I had it...It also had basically NO power..it was the 1 week when the power strokes could get away...

trust me, when your truck goes limp, you will know!

Brandy :D

[reelbob6.6]

Thanks Brandy: I should have known since my small block fuel injected chevy boat engine has two limp modes. The ecm cuts the rpms down to 2800 and then 600 if oil pressure or water temperture are abnormal and I have to limp home, and in the pacific that can be a long way/time. There is a way the boat owner can reset the code(s) ecm if the problem is solved but most boat owners don't know how. I plan to reread my manual. I wish the allison had a manual???

Thanks Bob

[MudNurI]

Bob- Some people here have an Allison Manual- where they got it I don't know..the only special book I got was the Diesel Supplement. My Allison is almost outta warranty- probably 2 more months, so to late for the book now LOL... smile.gif

FYI, I haven't had any shifting problems since the new fuel filter- going 25000 miles with the same filter is NOT a good idea!...

Brandy

[hoot]

http://www.allisontransmission.com/documents/product/publications/pubs/MT3190EN.pdf

This is the link to the "Mechanics Tips" manual. I belive it's the one many are referring to.

[ 05-22-2002: Message edited by: hoot ]</p>

[mdrag]

"On-Highway Operator's Manual" link, described as a usual glove box manual:

http://www.allisontransmission.com/documents/product/publications/pubs/OM3063EN.pdf