My 2008 Chev Duramax 6.6L with Allison Trans is stuck in 2nd or 3rd gear. My superchip that I use to read DTC's comes up with " lost comm with TCM" I have another TCM from same year, make & model...do I need to program it some how ? Need help.
You may or may not need a new/repaired TCM. The 1st thing to do is to make sure the TCM has power and is grounded. You will need a wiring diagram and a test light and DVM. If you aren't comfortable with doing the work yourself, you will need a shop to determine if the TCM is really dead. It is not at all uncommon to have circuit problems that cause this and you'd sure wouldn't want to pony up for a TCM repair if you only have a bare wire somewhere.
