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Old 04-15-2015, 12:12 PM   #1
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Duramax question

I received great feedback from this forum regarding our new (to us) Silverado 2500 HD Duramax. We are on our first cross country trip. So far the Silverado/Flying Cloud runs just great. A big improvement over our previous 2500 Suburban combination.

I have experienced one issue that results in this question. I've noticed when under load (up a hill for example) when the truck does downshift from overdrive 6th to 5th, it is what I would call, a "firm" shift. It does not happen into any other gear or when shifting up. Based on your experiences, is this normal? We never experienced anything like this with the "Blurb, but then again this is an Allison transmission. Thanks!

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Old 04-15-2015, 12:15 PM   #2
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Jack, as an about-to-be owner of a 2015 Sierra Duramax/Allison truck, I'm looking forward to the answer to your question, as well.

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Old 04-15-2015, 01:27 PM   #3
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I do not have a Duramax so I do not know the answer. But a question: are you running in some type of tow/haul mode when pulling? The shifts on our Dodge are firmer in tow/haul. At least I will bump your question back up in the portal.
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Old 04-15-2015, 01:28 PM   #4
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If you are using the Tow Haul Mode the transmission is reprogrammed to do that. I has a brain and that is done to extend the life of the trans. The long soft shifting that is normal for the Soccer Moms is not good for the trans while under load.
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Old 04-15-2015, 01:40 PM   #5
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I had a 2011 GMC 2500 duramax/allison combo pulling a 33ft travel trailer for over 20,000 miles in all kinds of conditions some in tow mode some not & never experienced what you're experiencing. I think you need to get that tranny looked at as it will probably get worse & cause real problems.
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Old 04-15-2015, 01:55 PM   #6
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I have used the Tow Haul mode from time to time and it seems to hold the truck in 5th gear a bit longer. We had the same feature in our Suburban and noticed that it had a positive impact on towing, but not sure I see much difference.

I read here previously that with the Duramax/Allison, some us the Tow Haul and some not. Thoughts? We will be back on the road tomorrow and I'll check then to see what if any difference the Tow mode has on the firm shift issue.

But to answer Howie's question and short of checking specifically, I would say that the shift issue is evident in either Tow Haul mode or not.
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Old 04-18-2015, 10:07 AM   #7
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Duramax question

If you're experiencing the same issue when not using the Tow/Haul mode, I would Google Duramax sites to see if anyone had a similar problem. It may be something as simple as a software update and the dealership never notified the PO.

Good Luck!!
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Old 04-18-2015, 11:31 AM   #8
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In case this thread is still active, I feel I have to tell you that we had a bad experience with our 2013 GMC 2500/D-Max.
After the first few thousand miles, we began to notice some "firm" shifting, going downhill, slowing down onto exit ramps, etc. I mentioned this to the GMC service mgr who said he hadn't heard of any (Allison)trans. problems.
A month or so later, we left on our first road trip with our FC28.
We alternated between tow-haul and non tow-haul modes, but always engaged the exhaust brake.
The shifting became less smooth, but not rough.
We made our way from MA thru PA into VA. By the time we got onto the Skyline Drive the shifting had become rough to the point we had to get down off the Drive. As we descended, the transmission shifted harder and harder and began to actually "bang" down thru the gears, sometimes even grinding through them.
We used OnStar who ran a diagnosis remotely and found some error codes, and then arranged for us to go to the Chev/GMC dealer in Charlottesville.
They could not find the same codes and had no idea what the problem was, but assured us that we could drive home without voiding the warranty.
We got home, and to the dealer, with a little over 7000 miles on the trans.
After ,/#(*&^%$ around for several weeks, they said they'd have to re-build the trans.
After I protested, GM finally agreed to replace it with a new one.
My point is that there could very well be an evolving issue with your trans., and I'm pretty sure my case wasn't the only one.
The new one has performed flawlessly for the 8000 miles we've since put on it.
Good luck with yours,
Wallace: 2013 GMC Sierra 2500 D-Max
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Old 04-18-2015, 11:34 AM   #9
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My 15 Duramax has a noticeable downshift in tow/haul but I wouldn't describe it as hard. 19 k miles. Love the truck.
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Old 04-18-2015, 11:53 AM   #10
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We have a 2011 Silverado 3500 Duramax. It now has 80,000 miles on it, and it has spent the majority of its life towing an Airstream while carrying a pop-up truck camper. It has been a n exemplary tow vehicle.

I have never noticed any hard down-shifting when trying to maintain speed on an upgrade. The Allison always seems to perform flawlessly.

My truck is always carrying a 3,300# truck camper in its bed while towing a 7,400# Airstream. Between the truck, the camper, and the Airstream, I am right at 19,000# coming down the road. I always run with the transmission in the
tow/haul mode.

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Old 04-18-2015, 06:17 PM   #11
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Jack read the manual on when to use it tow haul mode. I think you'll find that it's not needed with Duramax due to the weight
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Old 04-18-2015, 09:52 PM   #12
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2012 Dmax, 4x4 , Allison tranny. Towing 9800# triple axle. No shifting issue.

One thing the tranny will do is a "quick shift" so there is as little loss of speed while shifting. Under just right conditions I have felt a "solid" shift, especially in cruise. Usually, if grade increased slightly then had a strong rise. The "brain" is giving a little more fuel yet the RPM drops, so a bit more fuel, then the grade increases or the wind picks up to increase load... Then the truck will "shift hard/quick" and increase engine speed to maintain Speed. The combination does not always "match" for a "slick" shift...

Are you running in "cruise"?

Try this... Don't disengage the cruise... As you approach grade, start adding some fuel just before you get to the rise. The Dmax brain has no eyes to "know" the road is climbing. So you tell it. The cruise will ease off throttle but you do not. Maintain a "positive speed" at or above cruise slightly (1-2MPH)..

Once the "grade" stabilizes to a steady rise, ease off throttle until the Dmax brain takes over the throttle again.

Never failed for me.

If you want to duplicate the sensation, try manual shift under same hill climb. Shift is softer? Yup, because the Dmax brain isn't confused about What the outside road world is throwing.
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Old 04-19-2015, 08:46 AM   #13
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I have a 2014 GMC 2500 Dmax towing FC 30. Last summer was our first towing summer. I did some "level ground" towing without having the Tow/Haul engaged, but decided for me I prefer to have the Tow/Haul engaged with exhaust brake engaged for all of my towing. I am at 16,600 pounds, 68% of GCWR, not quite to the 75% specified in the owners manual for Tow/Haul mode. But I figure it is close enough.

with the T/H engaged, I never see 6th gear. It's not locked out by T/H, but you have to hit 67 mph or so to get the Allison to go from 5th to 6th gear, and I go slower than that. The gear shifts are more noticeable going downhill for me, especially if I have cruise set to say 60, the Allison downshifts to 4th gear and runs at 3500 rpm going down a 6% grade. First time I did that I thought the engine was going to explode before we got to the bottom of the mountain. But now I figure that the Allison is doing what it is supposed to, and I could drive in the Rocky mountains all day and almost never touch the brakes.

Most significant issue I had in moving from a gas TV to the Dmax was Hitch adjustments - that took me all summer to fine tune.

Some amount of shifting and noise is normal, and probably more in a Dmax than a gas TV. With the OP description of a "firm shift", it is probably normal. You could just "keep driving" and see if your experience stays the same or gets better. The gets better could be breaking in the engine, etc. Of course it could go the other way like "callmedave"'s experience - yikes!

I like the tests that cwf proposed. Also, what happens in solo? Can you replicate the "hard shift" solo? Also compare having T/H engaged, and not engaged. If you only experience the "hard shift" with T/H engaged, then I would attribute that to the normal operation of T/H, and you'll have to decide whether you prefer T/H or not with your total weight. You could also take a test drive with another truck from the dealer, does it feel the same or different? If you can get your dealer to allow you to do test drive tow that would be great - if you convince them to let you do that you must know your dealer really well! But they should be willing to do that since you bought a new truck, and to ease your mind if nothing else.
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Old 04-19-2015, 09:24 AM   #14
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Originally Posted by ghaynes755 View Post
Jack read the manual on when to use it tow haul mode. I think you'll find that it's not needed with Duramax due to the weight
My experience with our 2013 GMC 2500/D-max has been that the Allison transmission runs significantly cooler when tow-haul mode is engaged.

I could not find in my manual that tow-haul mode is not necessary when towing.

The Denali version allows me to toggle through system monitors to where I can observe the trans. temp.

I haven't been able to determine how much is too hot, but cooler must be better for the long term life of the trans.

My (limited) understanding of the tow-haul mode is that it changes the shift rpm points for more efficient performance, especially under load.

After having the trans. replaced (see my post above) I always engage both tow-haul and exhaust brake modes when towing. Over the 8000 miles we've driven with the new trans., the shifting has been solid but smooth.

I do find that the trans. gets "hung up" a bit and sometimes won't shift to 6th. In that case I rev up the engine and it will then shift. At that point, the rpm's drop to about 1500 which I believe is 6th or overdrive.

Wallace: 2013 GMC Sierra 2500 D-Max
Gromit: 2013 Flying Cloud 28
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