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Old 08-27-2021, 03:04 PM   #1
twindman
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Unhappy Allison tranny shifting different now

I have a Chevy 2500 with 125,000 miles on it. Up until about 5K miles ago it always shifted into 6th gear when towing at 55mph. Suddenly it now shifts at 67 mph.

Dealer says no codes and that is the way it is! Allison dealer says it is info from the engine that says it isn't putting out enough power to shift at 55.

Sounds to me like some sensor or engine computer part that is sending the wrong info.
Any ideas out there?????????????
 
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Old 08-27-2021, 03:11 PM   #2
DadsHemi
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Has it been serviced regularly
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Old 08-27-2021, 05:52 PM   #3
RBGALL
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I have a friend his truck started shifting different and he had to change fuel filter. Less power lower gear.
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Old 08-28-2021, 12:38 AM   #4
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With our rig, now at a bit over 260,000 miles, it has always shifted at the lower mph but when in Tow/Haul mode it holds back to shift into 6th gear until around 65-58 mph. Some years ago, I replaced the tranny fluid with the Alison Transynd full synthetic and it still shifts great but still shifts at the higher speed into 6th gear. I do a drain and fill of the tranny fluid every couple of years, and replace the spin on filter every 30,000 miles.
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Old 08-28-2021, 11:43 AM   #5
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I would rather have it that way instead of lugging the engine.
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Old 08-29-2021, 12:48 AM   #6
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Rohrman, is your comment 65-68 or 55-58 on shift speed??? Thanks
Also, if it is 65+, do you drive that fast, or just run in 5th gear?
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Old 08-29-2021, 01:41 AM   #7
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Yeah, 65 to 68, and when not in California, and on good interstates, we cruise at around 68 mph and the tranny will be in 6th gear. I never tow without being in Tow/Haul mode and always have the exhaust brake enabled. We run 14 ply rated tires, previously G614’s and now Sailuns.
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Old 08-29-2021, 07:06 PM   #8
twindman
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Originally Posted by rohrmann View Post
Yeah, 65 to 68, and when not in California, and on good interstates, we cruise at around 68 mph and the tranny will be in 6th gear. I never tow without being in Tow/Haul mode and always have the exhaust brake enabled. We run 14 ply rated tires, previously G614ís and now Sailuns.
I have michelins so I can drive 120MPH (LOL). I once read that the optimum speed for these Duramax is around 62 for good mpg and also power. I may have to bump up after this.
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Old 08-29-2021, 07:43 PM   #9
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Peak torque is right at 1600 RPMs and with my 3.73 gears 1600 RPM is just about right at 65 and in Tow-Haul the Allison won't **** into 6th until 62 or 63 MPH. So it gets me into 6th gear and peak torque at 65MPH and can cruise all day long.
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Old 08-29-2021, 08:19 PM   #10
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Originally Posted by twindman View Post
I have michelins so I can drive 120MPH (LOL). I once read that the optimum speed for these Duramax is around 62 for good mpg and also power. I may have to bump up after this.

I guess I got cheated mine are only speed rated for a 106. Darn.
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Old 09-01-2021, 02:41 PM   #11
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Originally Posted by twindman View Post
I have a Chevy 2500 with 125,000 miles on it. Up until about 5K miles ago it always shifted into 6th gear when towing at 55mph. Suddenly it now shifts at 67 mph.

Dealer says no codes and that is the way it is! Allison dealer says it is info from the engine that says it isn't putting out enough power to shift at 55.

Sounds to me like some sensor or engine computer part that is sending the wrong info.
Any ideas out there?????????????
My 2015 GMC 3500HD does the same thing as well as "clunking" on the downshift when not towing and coming to a stop. It's not been an issue other than the concern it shouldn't do that. I've seen numerous attorney ads for lemon law suits on Chevy & GMC transmissions. I'm hoping there's nothing to it but but will keep this in mind if I spend $$$$ for repairs.
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Old 09-02-2021, 10:06 AM   #12
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I had a 2500 chev allison tranny and it didn't shift into 6th until 67-68mph. I ran 64 most of the time and tested the fuel mileage at 67-68 and it was slightly higher consumption.
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Old 09-02-2021, 02:46 PM   #13
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A lot depends on the year of your truck and the weight you are pulling. Pulling 16000# and in tow haul, my 2016 3500 Duramax would change into 6th at 67mph. This became the speed I set cruise. My 2018 has more power and changes at a lower speed, but I continue to set cruise at 67.
You can force an earlier change into 6th by momentarily dropping out of cruise.
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Old 09-16-2021, 09:58 PM   #14
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In T/H mode, pulling my High Country, my old 07.5 would spend a lot of time in 5th, and I would typically drive between 60 and 65. Any incline would put it into 5th. With my current 2020 I really haven't paid attention since I can hardly feel it shift. Might be in 6th, or maybe 7th, 8th, 9th, or 10th. :-D It does like to keep it in the 1800-2000 RPM range.
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Old 09-17-2021, 08:32 AM   #15
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Well I've had a 2011 and now a 2018 Duramax Dually and when not towing or in Tow Haul mode will shift into 6th in the low 50s. When towing heavy 19.5K and in Tow Haul neither would shift into 6th until 62 - 63, so I cruise at 65 which is right at 1600 - 1640 RPMs.
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