My gmc Sierra 2500 is in limp mode after replacing planetary gears in Allison 1000. Primary code is tcm(mil request). There are about 20 secondary transmission codes. Double checked all wiring harnesses. Any ideas?
First off: WELCOME
This site is filled with some great information and experienced/skilled members. Sadly one of OG transmission guru's recently passed, but there are still many on here that can help just as much.
That said, they are NOT on here all the time (some visit occasionally when they get a chance). They also can only help if/when there is enough information provided to help. Even then there are some issues which can only be diagnosed hands on or with a complete teardown.
I am very capable mechanically, I have completely restored an 80's pickup, done my own ring and pinion setups, transfer case rebuilds, assisted in building engines and even assembled a Turbo Hydromatic 400 for truck pulling. I learned enough on this and other Duramax forums (Dieselplace was big back in the day) that the Allison transmission is a much different design than transmissions I was used to. I read story after story of guys trying to build there own Allisons whether it be to save $$ or because they were just used to doing all their own work (or both), but quickly learned one SMALL error in assembling the transmission could lead to a really BIG repair fill to fix correctly along with the $$ spent to build it on your own in the first place. A torrington bearing facing the wrong way (happens a LOT) can wipe out a transmission. There are parts in the transmission that can cost as much (or more) than a complete running TH400 with warranty... I decided to drive 13 hours to Michigan to have mine built initially, and it saved me $ in the long run. I figured if I did my own transmission and got lucky it would cost me $X, and if I messed up it could cost me $X plus the cost of a total rebuild with Converter/cooler flush to remove debris, and it wasn't worth the gamble. Some day I may try my own, but it will not be on a daily driver or vehicle I can't afford to have down for some time while trying to see what I messed up.
Just fluid alone (if contaminated during a botched build) can be expensive.
Another thing I learned is having the proper scan tool is almost more important than having the correct hand tools. I believe you said you had code read at an Autozone. Not knocking that (I've done the same when I'm on the road and have had an issue in my car), but it does show you do not have the tools for proper diagnosis.
I'm no expert but have paid attention to posts over the last 20 years and I can say with a large degree of confidence that your transmission needs to be torn down, cleaned (at MINIMUM), and affected parts replaced. Even if you luck out and replace what you messed up and get it functioning again (not likely), you will have other issues soon after. The valve body is NOT very forgiving to foreign matter.
There's been plenty of posts of guys saying they "fixed" their transmissions only to come back a few months later begging for help.
The folks on here are more than willing to help, but ONLY if there is enough information. They've learned guessing only causes more issues and it's not like diagnosing a shifting issue with an old 700r4, C6, or TF727 where there were a lot less solenoids, computer controls, and more forgiving clearances/tolerances.
All that said, I do hope you get it figured out.