While I was towing my toyhauler last week I was coming off a 6% grade and the allison was downshifting then, the tranny downshifted again and the tach jumped up into the redline. The tach hit 3200rpm, As soon as it downshifted and noticed that it was reving to high I immediately hit the brakes. It was only in the red for about 3-4 seconds. Has anyone else had this kind of experience while descending a hill, having the tranny downshifting into too low of a gear??





Any help is greatly appreciated!


thanx,


JLewhttp://www.dieselplace.com/forum/smileys/Clap.gif

its grade breaking and its a feature of your tranny... do a search and you'll see its normal.

Yes, DieselPower I realize that but, I didn't think that it should have downshifted into a lower gear being that it had already downshifted and I was decending the hill and the tach was already @ 2500rpms and then thats when it decided to downshift again and I didn't think it should have. Maybe I need to apply more brake next time?





JLewhttp://www.dieselplace.com/forum/smileys/Embarrased.gif

You can expect Allison engine braking to get the RPMs into the 4000's so that engine backpressure builds. That's by design so you don't need to waste or heat up your brakes.

if ya read the manual - it's ok to delcelrate in the hashmarked red zone, not the solid red... so you can grade break up to like 4000 rpm's but only accelerate to like 3200.


don't sweat a thing - it's ok and normal. http://www.dieselplace.com/forum/smileys/Big Smile.gif

yep, what he said - first towing trip over the sierra nevadas into reno i saw it get high into rpms and i was like oh crap what happened to my new truck, but quickly learned it was all good - how nice of a tranny is that though in all honesty, i love that grade braking http://www.dieselplace.com/forum/smileys/Clap.gif

if ya read the manual - it's ok to delcelrate in the hashmarked red zone, not the solid red... so you can grade break up to like 4000 rpm's but only accelerate to like 3200.


don't sweat a thing - it's ok and normal. http://www.dieselplace.com/forum/smileys/Big Smile.gif





Don't tell that to the pullers and racers since they hit the 4000 rpm zone on acceleration http://www.dieselplace.com/forum/smileys/Big Smile.gif

Actually the Solid Red starts at 4800 RPM. I don't get surprised by shifts in mine, since I do the shifting . . . I understand that the 4-3 shift on a down grade will spill your coffee and wake up your passengers.http://www.dieselplace.com/forum/smileys/HiHi.gif Just imagine if it had done it with the fan engaged.http://www.dieselplace.com/forum/smileys/Pinch.gif

The grade braking is a retarder in the trans, the engine is just along for the ride. The diesel supplies virtually no retardation because it doesn't restrict air like a gasser. The trans actually is doing the braking, not the engine. Big allisons have done this for years, now we finally have it in pickups. The book says that it will retard on the diesel up to 4800 RPMs, then it will upshift automatically.

this happened to me once coming south on hwy 101 down the cuesta grade into SLO...scared the crap outta me and i put it back in O/D and the rpms went back down...but now that i know its normal.......its all good

Norcal 2500hd That was the same hill that it did it on also!! I was on my was to Pismo for LAbor day wkend.


JLewhttp://www.dieselplace.com/forum/smileys/Big Smile.gif

How was pismo labor day? We decided not to go just for the fact that all the idiots come out on 3 day weekends....better safe then sorry...


Coming home up the questa is a fun one too....edge level 2....70mph up the whole thing towing the trailer and 4 quads.....http://www.dieselplace.com/forum/smileys/Clap.gif

Pismo was pretty good, my daughters boyfriend wasn't solucky. He rolled his Jeep and broke the windshield, everybody in the jeep was ok though. then on sunday nite he was going up comp and broke his driveline. So I had to tow him back to camp. He got fixed b4 we came home, and the ride home was not fun at all...driving in 102 degree heat w/ no windshieldhttp://www.dieselplace.com/forum/smileys/Cry.gif





JLewhttp://www.dieselplace.com/forum/smileys/HiHi.gif