My 2008 Silverado HD (Diesel / Allison 6-speed) has decided to stop working. Not completely, but mostly. A couple of days ago, I went out to take it to town and the trans seemed to be slipping. The further I got up the hill to the main road, the worse it got, so I turned around and parked it. All had been well a few days ago, so I suspect electronics. Fluid is recent and looks clean.
This morning it wasn't raining, so I decided to see what was up. Hooked up my scanner and got these codes:
P0-700-00 Transmission Control Module Requested MIL Illumination
P0-751-00 1-2 Shift Solenoid Valve Performance - No First Or Fourth Gear
P0-761-00 4-5 Shift Solenoid Valve Performance - Stuck Off
P0848-00 Transmission Fluid Pressure Switch 2 Circuit High Voltage
P0873-00 Transmission Fluid Pressure Switch 3 Circuit High Voltage
P0877-00 Transmission Fluid Pressure Switch 4 Circuit Low Voltage
P0701-00 Transmission Control System Performance
I did some research and found several instances where the fluid had leaked though the plug that connects to the Transmission Control Module and caused similar symptoms. Got under the truck, pulled the plug and found it full of oil. Cleaned it with mass air sensor cleaner, blew it out with compressed air, plugged it back in, and then hooked up the scanner and cleared the codes.
I had convinced myself that the truck was now fixed, but boy was I wrong. Trans shifts late and hard. I can shift trough all 6 gears (manually), but the truck won't go over 40mph and surges at that speed.
It looks to me like the TCM wiring is berkeleyed, and that the TCM may be as well. I would be tempted to take it to the dealer, but our local GM dealer went out of business. The closest one is now about 2 hours away.
So what is the wisdom of GRM? I have done the normal stuff on automatics - installed rebuilds with torque converters, changed fluid and filters, and changed out a couple of vacuum modulator valves.
Above is all the experience I have with an Allison.
Went through this with my motorhome this year.
Changed whole valve body and wiring connector through the pan.
Ended up being the TCM. $1,100. 
Best to go to a real Allison dealer, but you will go into shock when you learn their labor rate.
TCM is a common failure. Dad's 08 Dmax had to have it done under warranty.
Suspected that there was no one near me who had the tools and experience to work on the Allison; verified that today.
Called an authorized Allison service center two hours away - they told me that they could work on the Allison stuff, but not on the GM interface side. They suggested a GMC dealer near them.
After discussing the problem with the dealer, contacted AAA and had the truck towed to them - 87 mile tow and we had coverage for 100 miles. Win!
Will find out what is wrong next week.
Right makes me scratch my head with the Allison trans in all the new Silverado 2500's and 3500's. Hopefully someone can work on them down the road.
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