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Allison Question

TPB

By TPB
in 2000-2014 Silverado HD & Sierra HD

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TPB Enthusiast

TPB

Posted
Posted

Howdy!

 

I'm fairly new to this site. After much research & reading I just traded my 2001 Chevy 2500HD Crew Cab 6.0 for this new 2005 GMC Sierra 2500HD with the Duramax & Allison. So far I'm extremely pleased. I tow a 9,000lb fifth wheel camper & a 2,500lb boat behind the fifth wheel, & it was a bit much for the 6.0.

 

My question is:

 

When I'm driving down the highway & move the shift lever from OD (5th) to the "3" position, the truck shifts down two gears - evidently into 3rd gear, & skips 4th gear. It appears the only way to manually shift into 4th is to disable the OD. Is this correct?

 

Todd

TPB Enthusiast

TPB

Posted
  • Author
Posted

I didn't want to shift into 3rd, but into 4th - just down one gear from overdrive. It automatically shifted me down two gears into 3rd. It's not a big deal - if I need to tow in 4th, I'll just disable OD.

Shooter_va Enthusiast

Shooter_va

Posted
Posted

A friend of mine has the same set-up, when he pushes and holds the tow/haul mode button on the shift lever for a few seconds, this will lock out the overdrive.

MS3DALE Senior Enthusiast

MS3DALE

Posted
Posted

Yes,just push and hold the TOW/HAUL button and it will disable OD and put you in 4th gear,it mentions it in the owners manual. :lol:

KyleBst264 Enthusiast

KyleBst264

Posted
Posted

Press the tow/haul button quickly to disable/enable tow/haul mode.

 

Press and hold the tow/haul button to disable/enable overdrive. With O/D off, it will not go beyond 4'th.

 

You can run tow/haul on and overdrive off simulataneously and one without the other.

 

Also, if you want downshifting action, put it in tow/haul mode and take your foot off the throttle and your Allison Transmission will down drag and downshift like a stick shift all by itself.

BlackSilver Senior Enthusiast

BlackSilver

Posted
Posted

Why would you WANT to shift out of 5th gear? The Allison is not your grandpa's DynaFlow, and in Tow/Haul mode it is designed to take care of itself when pulling serious loads.

 

I pull a lot more weight than you're talking about, including a lot of mountain grades, and the Allison is always in precisely the right gear, up hill or down. It's smarter than us --- let it play the way it wants without putting a muzzle on it. :thumbs:

KyleBst264 Enthusiast

KyleBst264

Posted
Posted

I see you have an '05. I assume there is barely any miles on it right now. The TCM for your Allison Transmission will adapt to your driving paterns over a course of time.

KyleBst264 Enthusiast

KyleBst264

Posted
Posted

Holding the tow/haul button down to disengage/engage O/D only works for the Allison on '03 and newer. This is because the there is still PRND321 on the selector and going from D to 3, you'd skip 4'th.

 

For the other automatic transmissions, I believe you just put the shifter in "3" to disengage overdrive.

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