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Junior Member
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Discussion Starter #1
Sorry guys this may be kind of a dumb question. I am new to the duramax, I just switched over i was always a 12-v cummins 5 speed guy. Anyway my question is; Last weekened I was pulling my horse trailer with a couple horses home and I had to go down a few big hill. Now my understanding and from what I have seen from work pickup (an 08 3500hd duramax with service bed) is that when you put it in tow haul it will downshift as you are trying to slow down. I have always known this that was why I really like the chevy automatic, but i was under the impression that when you turned tow haul off it would not longer downshift automatically? My 08 work truck as long as tow haul was off it would not downshift so i just left the button on at all times because other wise it was slow to stop because it was using all brake with a load. Trips the scales at about 16,000 empty with just my tools. So is the 5spd that is in my pickup suposed to downshift with tow haul off? My problem was i would want to just touch the brake every know and then and stay about 70 but every time i would touch the brake it would kick down and i would be at 3k rpm and at 60 miles an hour. It does downshift more agressively in tow mode.
 

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'Ol Builder guy
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7,051 Posts
Sorry guys this may be kind of a dumb question. I am new to the duramax, I just switched over i was always a 12-v cummins 5 speed guy. Anyway my question is; Last weekened I was pulling my horse trailer with a couple horses home and I had to go down a few big hill. Now my understanding and from what I have seen from work pickup (an 08 3500hd duramax with service bed) is that when you put it in tow haul it will downshift as you are trying to slow down. I have always known this that was why I really like the chevy automatic, but i was under the impression that when you turned tow haul off it would not longer downshift automatically? My 08 work truck as long as tow haul was off it would not downshift so i just left the button on at all times because other wise it was slow to stop because it was using all brake with a load. Trips the scales at about 16,000 empty with just my tools. So is the 5spd that is in my pickup suposed to downshift with tow haul off? My problem was i would want to just touch the brake every know and then and stay about 70 but every time i would touch the brake it would kick down and i would be at 3k rpm and at 60 miles an hour. It does downshift more agressively in tow mode.
I'm not 100% sure of your question, but the allison is not supposed to downshift like it does in tow haul. Mine seems to even when it's off maybe just one/two gears.

One thing I do know about tow/haul is the owner's manual suggests turning it "off" while you are empty otherwise you'll has unsatisfactory downshifting.

In other words, if you run empty with tow/haul "on" it won't downshift the way it should.

I still use tow/haul, even when lightly loaded and it downshifts great.

Sounds like your's is dropping into 4th or maybe 3rd when you tap the brakes. That doesn't sound right.
 

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Junior Member
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49 Posts
There was a change between the 5 and 6 speed trannies. The five speed would only downshift like you are talking about it tow/haul. The six speeds were set up down shift even when not in tow/haul if I'm not mistaken. I can tell the difference between My brothers 05 with a five speed and mine.

Duke if you still have your owners manual for your ally it explains the differences. I don't have access to mine at the moment to comfirm that exsactly, but I do know for sure that they changed something with the down shifting slash slowing down.
 

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'Ol Builder guy
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Yes, my truck does have some limited downshifting with tow/haul "off", but it's maybe just 1 or 2 gears.
 

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Junior Member
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Discussion Starter #5
Mine was only downshifting one gear but i had tow/haul off. And every time i hit the brake it would downshift like they do when tow/haul is on.
 
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