We are asked all the time about unlocking 6th gear on the Allison MD3060 or Allison 2000 series. Is this possible?
Buses are built to the specs that the engineers approved. Many buses don't have 6th gear unlocked, because they wouldn't have enough power to use it. The bus would constantly be shifting between 5th & 6th, trying to figure out what gear it wanted to be in.
The transmissions mentioned, usually have a 6th gear inside of them. It is a computer (TCM) program that is telling the transmission not to use it. A shop with the proper tools can reprogram the TCM to new parameters, if they can get permission from the manufacturer.
So how do you get permission to change the parameters?
You will need to contact the manufacturer of your bus. This will generally be Bluebird, Thomas or International. They will be able to tell you the steps needed. Once you get an engineer to sign off on your use of the bus, you take the approval to an authorized Allison dealer. It is then a reprogramming (flashing) of your TCM. It can be a long process, if it even gets approved. A shop will charge you a few hundred dollars usually to re-flash your TCM. They will not be liable if the re-flash ruins your TCM. A new TCM can cost around $2500 in today's world.
So the simple answer is YES your transmission can be unlocked. The real answer is it will take you a long time, if not forever, to get approval from a school bus manufacturer engineer to approve your unlocking.
