Duramax Diesels Forum Truck of the Week
Old 07-21-2014, 12:47 PM
AKhdironman AKhdironman is offline
Join Date: Jul 2014
Location: Eagle River, AK
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Default Duramax allison tranny issues

I have a 2009 GMC Sierra 2500 HD Duramax with a Suncoast stage 5 allison transmission that is not working
I had a mechanic replace the IMS and internal harness as the P0708 code suggests this could be the problem. Did not fix it. When driving it after clearing the codes and disconnecting the battery, the park assist off message comes up it clicks loud under the dash, downshifts into limp mode and stays there until I clear the codes and disconnect the batteries. When depressing the brake it makes a loud click when this happens. ANY ideas....HELP. The next step is the TCM perhaps????
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Old 05-19-2018, 03:05 PM
JaYDiplomat8 JaYDiplomat8 is offline
Join Date: May 2018
Posts: 3

Hi AKhdironman,

I have the same issue as you had, did you ever resolve this issue?
I have done a search on this exact problem with the brake pedal clicking when the trans goes in Limp mode. Nobody else seems to have had this problem. I'm hoping you could share your experience with me as I am trying to figure this out.

Thanks in advance
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Old 05-19-2018, 07:11 PM
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Burn Down Burn Down is offline
Join Date: Sep 2008
Location: Boise Idaho
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He hasn’t been on since 7-22-14...

What mods to the truck are you running? Any codes when this happens? Just show up out of the blue or?

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Old 05-19-2018, 08:25 PM
JaYDiplomat8 JaYDiplomat8 is offline
Join Date: May 2018
Posts: 3

hey Burn Down,
super cool of you to chime in and thanks for the welcome!

This is my first Duramax. I bought this truck from auction so no history.
2011, 3500 flatbed work truck with 180,000 miles. LGH Engine, variant of the LML. Bone Stock, no mods. I was in the hospital, wife gave birth, so my father picked the truck up 1500 miles away. He was 50 miles from home and the Allison went into limp mode. It throws 34 codes associated with the trans all at once, lose the shift indicator and strangely enough the brake position sensor seems to click loud when depressed. The only other codes currently are some glow plugs. I replaced all 8 but still get the codes, I think its the glow plug controller.
For the trans, the fluid looks very healthy, correct level, I have cleaned both ends of the harness thoroughly, checked the pins at the TCM, checked all fuses, some grounds and replaced the TCM WITH ANOTHER ONE ACDelco and had the dealership Flash it. It Continues to do the EXACT same behavior after about 5 minutes of driving with either TCM. I believe it did the same after clearing codes and just idling as well. Doesn't seem to be the TCM. Any similar experiences or ideas?

Last edited by JaYDiplomat8; 05-19-2018 at 09:11 PM.
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Old 05-19-2018, 08:29 PM
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TheBac TheBac is offline
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Location: Mid Michigan
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Sounds like yet another 2011 with a TCM problem.
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"Kleshay" -- 2000 C5 Corvette -- the toy
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Old 05-19-2018, 08:35 PM
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AZlml AZlml is offline
Join Date: Jun 2016
Location: Goodyear, AZ
Posts: 217

Originally Posted by TheBac View Post
Sounds like yet another 2011 with a TCM problem.
Just replaced my TCM last week for similar issues. Didn't realize it was such a known issue till it was my issue

Op, did you get an Acdelco replacement for the TCM?
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Old 05-19-2018, 09:14 PM
JaYDiplomat8 JaYDiplomat8 is offline
Join Date: May 2018
Posts: 3

I did get a low mileage used warranted ACDelco TCM
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