2009 GMC Sierra 2500HD LMM Shift inhibit after transmission warms up,no Doc's (codes) - Diesel Place : Chevrolet and GMC Diesel Truck Forums
 
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Allison OEM Discuss repairing & maintaining an OEM Allison automatic transmission.

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Old 01-20-2020, 07:12 PM   #1 (permalink)
Ah64gunpilot
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2009 GMC Sierra 2500HD LMM Shift inhibit after transmission warms up,no Doc's (codes)

Ok guys and girls. So initially when this problem started I had a Range sensor circuit high with a P-0706 and a P-0708. I have pulled the valve body and replaced the range (PRNDL) switch. This didn't fix the problem. I also would get an Electro,if brake control module code sometimes as well. So I bought an Allison transmission repair manual with good schematics etc... While troubleshooting with an ohm meter I found the resistance low and out of range on the output speed sensor on the transfer case which did get flagged once, so I replaced that as well. I no longer have any codes or Doc's. Now I can start the truck up cold (after sitting overnight) and it operates just fine reverse and all other gears until I stop and shut the truck completely off with the key removed. Then I restart truck and I get the good ole "Shift inhibited" message with the flashing range/position selection under either R or D depending on what was selected. After the restarting and sitting in Park I can also feel some pretty good surging in the transmission that stops after about 2 minutes. It is enough that you can feel it sitting in the seat. The transmission and truck only have 97,180 original miles. I am the original owner of the truck and never had any other issures. I have drained and replaced the fluid and all filters simply to inspect the inside of the pan for debris. Everything is clean and I haven't pulled anything heavy with the truck, only my boat which is about 4000 lbs. I'm out of ideas and looking for a little help or suggestions. Thanks in advance!

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Old 01-20-2020, 07:24 PM   #2 (permalink)
OkDually
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Welcome to DieselPlace
I moved your post over to the "Allison OEM" section for more appropriate answers to your issue



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Old 01-21-2020, 08:20 AM   #3 (permalink)
Rvn4me
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Your P0706, P0708 leads to your IMS. Did you use an OEM? Did you do the procedure for Adjusting your shift cable? Verified your oil level hot?
Don’t know this is the cause but for some reason the 2011’s Allison 6 spd TCM has a high rate of Failure. This year only.

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Old 01-21-2020, 08:42 AM   #4 (permalink)
Ah64gunpilot
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Yes Sir I did go with an OEM IMS. I have visually inspected all wiring to or from the Transmission as well. I have only found lots of brittle wire loom. No chaffing or broken wire. I'm leaning toward a TCM at this point.
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