This was a 9.9 on the pucker scale.


Rain and freezing rain this morning for the morning drive. On the highway, passing lane, going downhill, probably too fast for conditions. Brake lights flashing ahead, yup ice on the road. Tap the brake to see how much trouble I'm in and Ally decides to downshift like its in tow/haul. Don't know what gear it went into but saw over 3,000 on the tach. Backend starts to come around, fortunately no one slowed very much in front of me and I 'drove' it back in line. Bottom of the hill, back on the flat, the tranny still held the lower gear, I cycled it into tow/haul and back and it up shifted. No question it was in normal mode when it did the grade braking thing.


Is this a known problem? Does the TCM/ECM hold codes for something like this? If so, how long? Its going to be a few days before I can get it to the dealer.


PS its a '02 Dmax/Alli 4X4 ECSB. Have Hot Juice, but running stock for the winter.Edited by: WillowCreekStable

I'm not sure there is anything "wrong". Grade braking is always active, just much less pronounced in normal mode. In normal mode, you can still feel it downshift and gently pull you down when coasting to a stop. You don't free wheel like in a car as it would downshift while slowing. That said, it can comand a higher RPM downshift from 5th to 4th in normal mode if the TCM thinks it needs to. It's based on the same reasoning it downshifts in TOW/HAUL mode. However, in normal mode it will only make higher RPM downshifts from 5th to 4th and no lower unless in TOW/HAUL.


Yes, I've heard of others going through the same pucker scale for the same reasons. So far, I haven't been going down hill on ice at a higher speed and tapped the brakes yet.http://www.dieselplace.com/forum/smileys/Confused.gif


Steve

This has happened to me as well.


This function is active in both normal and tow/haul mode. I believe I read it in the manual. In theory, when it breaks your rear end loose, the ABS senses differential wheel speeds and proceeds to upshift the tranny. I doubt it happens fast enough though.


To be safe, I may be shifting into neutral before tapping my brakes under snowy or icy conditions. Then the question is how to shift back into drive without breaking the rear end loose?





Steve

SteveCA, your right. I remember reading that too. I also agree that it probably doesn't upshift fast enough.


Steve

I drive this route everyday, and I've tried to recreate the experience going down the hill the last two mornings without success. Not bragging about my driving skills, but I guaranty that if my wife was driving there would have been a bunch of vehicles all wadded up at the bottom of that hill.

I'll assume that this was an isolated brain fart for now but it certainly caught my attention.

Frankly, I'm really surprised that GM hasn't seen it fit to make that auto downshift switchable by the driver and warn folks about that problem in the manual. The manual warns you about every other driving hazard known to man...


This is an accident waiting to happen on snow and ice!


My old 1993 suburban had an opposite problem.. with the high torque at idle of the 454, it used to push me through the snow when I tried to stop, activating the cheap ABS, resulting in extremely long stopping distances on slippery stuff. One of my buddy's wife drove straight through an intersection that way in Lake Tahoe. Damn near kille a few folks.


I learned to shift into neutral when coming to a stop under those circumstances. It shortened the stopping distances by over 50%!


Scary stuff!


Steve