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Discussion Starter #1
Truck Details
2008 Chevy 2500HD
Crew Cab 8 foot bed
No modification completely stock

So I am really scratching my head at this one. This all started about a week ago. I was driving down the highway around 70 mph and all of a sudden the transmission went into neutral I coasted to the side of the highway. The shift inhibited message was displayed on the DIC. No CEL was displayed and all gears acted like neutral. So I turned the truck on and off and disconnected the battery reconnected the battery then the truck went into gear only with the manual shift mode and with a horrible screeching noise for about a half of a mile before I decided to stop to prevent damage. Still no CEL. To this point I have done the following with no luck:

  • Checked the TCM for codes
    • None
  • Checked for collapsed pan filter
    • Filter was in tact, perfectly clean oil and no metal on the magnetic drain plug
  • Checked the main harness plug on the transmission
    • was coated in oil plug was clean and grease with dielectric grease
  • Checked TCM plug and pins
    • no loose pins
Any ideas I'm currently leaning towards a fried TCM any thought really pulling my hair out on this one. I will trade a six pack for the solution!!!!:p
 

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Certainly LMM's do have faulty TCMs from time to time, but check the wiring harness carefully before you spring for a new TCM and programming, only to find out that your problem continues. The wiring harnesses between the transmission and the TCM have problems also and are much easier to repair, once the 'bad spot' is located. Take a look at this video
 

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Discussion Starter #4
Certainly LMM's do have faulty TCMs from time to time, but check the wiring harness carefully before you spring for a new TCM and programming, only to find out that your problem continues. The wiring harnesses between the transmission and the TCM have problems also and are much easier to repair, once the 'bad spot' is located. Take a look at this video
Are there any key locations they fail or where would you start?
 

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Are there any key locations they fail or where would you start?
If you watched the video, it shows you where this mechanic checked for problems. Start at one end of the harness and keep going until you find the problem or that the problems is elsewhere.
 

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Discussion Starter #7
So a little update here. Still not up and running yet:(

To date, I have done the following.

I followed the video and diagnosed the wiring harness. I found no breaks or chafed wires. every wire was connected and operating between each component. I went a step further and pulled the entire harness apart and also inspected every wire. No issues. Redid the split loom but now the transmission is showing me codes and the shift indicator is now gone. I listed the codes below.
P2536
P0708
P2671
P0713
So after this, I decided that has to be a bad TCM. So I ordered a used one off eBay. Plugged it in and no start. So I started to fiddle with the TCM plug and it finally started and I continued to get codes but different ones.
P0708
P2671
P0762
P2536
Now I switched back to the old TCM and it also wouldn't start without fiddling the plug and the indicator would return with codes and the original shift inhibited. I failed to mention that when I mess with the TCM the door locks and gauges go crazy. Every time I start the truck I get some form of these sets of codes.

At this point I talked to a local diesel guy here in central PA he stated it may be the NSBU switch so I replaced that and still the same issue. So I decided to order the crimp tool the pin removal tool a new TCM plug housing and some pins to rebuild the plug. I may have screwed up some of the pins when I was probing the plug but that may also be the original problem as well. The all data for all these codes says incorrect pin tension but I can't find any information on setting pin tension.

Any thoughts on this or tips on replacing pins/rebuilding a TCM plug or other possible routes to take.

I really miss my truck currently I'm driving the extended families spare pool vehicle the ugly yellow H3. I compare it to the chase vehicle in the old top gear specials that if your vehicle broke down you had the shame of driving it and all your buddy make fun of you.
 

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You asked about 'setting pin tension'. That is something you really can't do. You may be able to squeeze the female-side pins a little, make the inside size a little smaller, but you're as likely to make it worse vs better, and that assumes you knew which pins to work on. Using new pins is definitely the best route. What you can do is so use a new male side pin and insert it into each female pin and there should be a slight drag on the pin. Most of your pins will be A-OK, so those are the 'good' tension examples, now you just need to look for the 'bad' tension pins. Since you wil be opening your connectors, look carefully for any that are not in line with al the other connectors, not sticking out far enough to properly to contact the male pin.

I have attached information about P0762, your non-common new DTC.

Do you have a subscription to ALLDATADIY or MITCHELDIY ? Might be a great time to have one.
 

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Discussion Starter #10
Thank you Ron. Yes I have a subscription to alldata. I don't know how i did without it for all these years.

So I successfully rebuilt the plug and I am back at square one. I have some theories and thoughts.

As a recap square, one is shift inhibited with no codes displayed.

So I was reading and shift inhibited occurs can occur when transmission senses that the input speed is to high (What RPM is too high?) and will lock the transmission out. I was thinking maybe it was an input speed sensor (the tech 2 says it is operating correctly within the correct value) but then I was reading some more and some mentioned something about a stuck trim solenoid not allowing the C1 clutch to operate or something wrong with the C1 clutch. If I was going to replace any solenoid what solenoid would this be?

Maybe the engine is not throttling down and if it was why would it be doing that.

What causes the shift inhibited to trigger?
 

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It was made people like me who go from reverse to drive while still rolling. Because we grew up driving TH400s. So Allison will lock out say drive gear if out put sensor is telling TCM it is rolling backwards or vs versa.
 

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Discussion Starter #12
Replaced both speed sensor still nothing. Really confused here any more suggestions.
 

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Perhaps a Mike L conversation would be of help to you. It's a transmission problem, to be sure.
 

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When you see P0708 ( internal mode switch ) among other codes it usualy indicates a bad TCM on a 6 speed.
 
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