Allison Issue

[varmint2506]

So, 2006 GMC 2500 HD has been down-shifting very hard for a couple days. This AM went to leave house and the truck did not want to move forward. Acts like something is bound up. The truck will move if you basically floor it but it feels like you are power breaking it. Truck backs up fine and will roll forward in neutral just fine but once in drive it will not roll forward very easily. Tried to move it around in the yard now smells really bad of burnt clutch. All the trans shops that know Allison say they have never heard of anything remotely close to this issue. Any ideas?

[Doug_Scott]

Have you tried shifting manually? What you are describing sounds like your transmission is going into two forward gears at the same time. Basically a trans brake.  Typically in racing a trans brake applied high gear and low gear at the same time.  A transmission with a trans brake applied can stop engines with more than 2000 horsepower. 

 

Also try disconnecting the battery ground cable, wait 30 seconds, then reconnect it.  I would also disconnect the positive WHILE THE NEGATIVE IS DISCONNECTED. After you have the positive cable connected, reattach the negative cable. I have no experience with these computerized transmissions, but I am a firm believer in the mechanical portion is the same, they have just altered the valve body.  There should be a way to access the transmission control module (TCM) and see what it is doing as well as trouble codes.