Tranny is Down Shifting On Grades

flyinryne · 03-23-2009, 06:55 PM

On my way to a buddys I had to go over a pretty big mountain, And noticed that as i was going down that it downshifted. I never droped below about 50 Mph, It got to the point that on the last stretch it was turnin about 3,000 rpms and then when i got on level ground again and had to get back into the throttle it up shifted and everything was fine. It also did the same thing on my way back over it on my way home, This time i put it in neutral. I Never had it in Tow Haul Mode while i did this. One more thing I notcied. I havnt had this truck long and it been my first D-max, Its a 04 LLY 4dr crew cab short bed, When i go down the interstate it turns somewhere around 2500 rpms at about 75-80, Im gettin ready to haul our 30 foot enclosed about 400 miles to ky over sandstone hilton if anyone knows that mt. If i run it in tow haul then wont it cut out over drive? and in that case how fast am i going to be able to go with it in tow haul mode? And if it does cut out over drive then Why dont it, I mean when i go down the interstate and i put it into tow haul nothing changes. Is this normal or what?

**Tow Master** · 03-23-2009, 11:43 PM

Id say thats the alison acting normal! When I pull off the main road onto the gravel road I live on there is a small hill. Alot of times my truck downshifts into fisrt holding it there. But, I have towed with my truck down the interstate doing 70-80mph in tow haul mode. As far as I know it doesnt lock out over drive.

**ob_1jr** · 03-24-2009, 01:10 AM

it's called grade braking and it is completely normal. It will do it in normal and tow/haul.....just more pronounced in tow/haul. The only thing that tow/haul does is change the shift points and lock the torque converter in second through 5th.

Doug525388 · 03-24-2009, 04:21 AM

on my 05LLY the overdrive does not disengage in tow/haul unless I press and hold the tow button to make it disengage. just my 2 cents worth!

**bigdaddydiesel** · 03-24-2009, 05:03 AM

I agree, that is the Allison Tranny doing what it does best with the Grade Braking Feature. Also, there is no limit to the top speed just due to Tow/Haul Mode. Once you get into 6th gear you are there and you will hold that gear until you hit a grade where you need the extra RPM's to climb the hill.

**Hook'em_Horns!** · 03-24-2009, 05:11 AM

Yeah, normal stuff. The first time it happened to me (no tow/haul), I was running empty and going down a steep hill with a light at the bottom. Well, of course the light changed and a soccer mom on the phone immediately pulled in front of me, drastically reducing my stopping distance. (BE-OTCH!) Just as I'm eyeing the left shoulder to swerve to and keep from hitting her, Shazam!, the Allison kicked in like it had a load on and started downshifting.

flyinryne · 03-24-2009, 10:35 AM

Ok thats alsome, i agree with you guys then that it is normal. Just one more question though, I would like to get a exhaust brake in the commin year, How well does a exhaust brake work with allison down shifting?

**bigdaddydiesel** · 03-24-2009, 10:54 AM

Hear is a good example of how well it works....

Glad you asked!!!! LOL

duramax-rob · 03-24-2009, 11:19 AM

My truck shifts like the rest, only when going downhill at say 40-45 and I kick it in to neutral, the rpms come up to say 1600 as I roll down hill in neutral. Is this also normal?

Durallymax · 03-24-2009, 11:22 AM

Quote:

Originally Posted by duramax-rob

My truck shifts like the rest, only when going downhill at say 40-45 and I kick it in to neutral, the rpms come up to say 1600 as I roll down hill in neutral. Is this also normal?

yes it keeps the rpms up because it thinks you accidentally put it into nuetral, this way when you pull it back into drive while still rolling if you choose to there is less strain on parts.

which is one of the reasons it doesnt pay to shift into nuetral going down a hill you actually use more fuel. I never understood why people did this with a duramax. it is tuned to cut fuel to the motor when certain parameters are met. Its called the DFCO Deceleration Fuel Cutoff .

The Parameters for it to enable are less than 1.17% throttle, RPMs over 1,000RPM, Speed over 16mph and the engine has to be running for at least 60 seconds.

For it to shut off the TPS needs to go over 1.95% throttle, RPM needs to drop under 850RPM and speed needs to drop under 8mph.

those are on an LB7