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Old 06-28-2019, 04:38 PM   #1
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Allison transmission shifting

Just got back from a 550 mile maiden trip to the mountains in northeast Tennessee with the new truck. I understand that the Allison transmission “learns” shift points and driving style. The truck shifts smoothly while not towing and under what I would consider the easier towing situations. But when I’m faced with a need to hit highway speeds quickly, especially if there’s substantial grade to pull, the transmission shift very hard. At one start from a red light, I immediately hit about a half mile 6-8% grade, and was leaning on it pretty good. The shifts felt like a clunk and sounded like one too.

Those of you that have the Duramax Allison setup, did the transmission eventually learn to shift better under heavy strain, or do you think there may be an issue that needs to be addressed? The truck (2019 Chevy 2500) has almost 2k miles on it now. Fluid level is good. Trans temp very low, less than 150.

EDIT: After thinking back on a little hydraulic experience I had, I would describe the shift as “under full pressure.” When we used to start the pump or shift paths with hydraulics, we oftentimes took the pressure control loop through a smoothing ramp so that the new path wouldn’t get hammered so hard with full pressure. When that wasn’t done, it could cause the type of sound and feel that I was experiencing on this trip.
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Old 06-28-2019, 05:56 PM   #2
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My 2012 3500HD does that. When I'm anticipating that there might be a harsh down shift, I'll manually down shift the trans just before I think it's going to do it on it's own and the shifts are not as harsh. I watch the tach and make the shift.
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Old 06-28-2019, 06:50 PM   #3
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Allison transmission shifting

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Originally Posted by BessieHunter View Post
My 2012 3500HD does that. When I'm anticipating that there might be a harsh down shift, I'll manually down shift the trans just before I think it's going to do it on it's own and the shifts are not as harsh. I watch the tach and make the shift.


My shifts were hard up and down, primarily gears 2-4, probably because when I had reached the speed I needed, I backed out of the throttle some. Didn’t notice as much when the exhaust brake was working.
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Old 06-28-2019, 07:01 PM   #4
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I've honestly not had an unusually hard shift in my 16 and I've towed all over western NC, eastern TN mountains. Both interstate and backroads. My truck shifts better than the wife's Camry. Maybe I'm lucky and my truck learns quick!
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Old 06-29-2019, 08:45 AM   #5
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From what I have read, the Allison tranny's are known to clunk in gear pulling weight, and at other times. I know my '16 will do it, and even after having the dealer erase the "memory" on the tranny, early in its life, it still will do it. The good thing is that apparently it does not affect the life of it.
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Old 06-29-2019, 08:53 AM   #6
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Originally Posted by Wileykid View Post
From what I have read, the Allison tranny's are known to clunk in gear pulling weight, and at other times. I know my '16 will do it, and even after having the dealer erase the "memory" on the tranny, early in its life, it still will do it. The good thing is that apparently it does not affect the life of it.
IT does affect the life of it;it extends the life by not wearing any parts caused be slippage
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Old 06-29-2019, 09:29 AM   #7
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It's very interesting this conversation just started. I arrived home yesterday from a cross-country trip with my new-to-me 07 Duramax and it began doing this half way home. I have been googling this problem this morning before I came to FRF.

Many sites (duramax forum; dieselplace dot com; gm-trucks dot com; etc.) have many complaints about this. Some have the issue only at stopping, some at start-up, and some in both instances. More than one said they had the problem since new. There appears to be 2 main issues causing this problem.

1. (when stopping - my problem) for some reason - damage, dirt, bad installation, etc - the front wheel abs sensors activate early and cause the clunk or lurch when stopping. Fix is to clean/replace the sensors.

2. the splines where the slip yoke connects the drive shaft to the transmission becomes dried out. Guys with 2WD rigs pull the rear drive shaft and the slip yoke is behind the carrier bearing. Guys with 4x4 rigs say the problem is at the front of the driveshaft where in connects to the transfer case. In both cases, drop the driveshaft and clean/regrease (with hi-pressure moly), re-assemble. In every case they say the problem goes away - - for a while. Many claim that replacing the slip yoke with an updated nickel plated one solves the problem permanently.

In almost every case, one or the other of these fixes resolved the problem.

From what I have read, the LBZ Duramax had a transfer case pump rub problem that would cause a leak and eventually trash the transfer case. The best fix is to install an upgraded rear housing. I had already planned on having that done so when they drop the driveshaft for that issue, I'll have them install the updated slip yoke at the same time. If the problem persists, I'll look at the abs sensors.
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