Abstract: In order to enhance the failure diagnosis and tolerant ability of automated mechanical transmission(AMT), improve its stableness and reliability, a pneumatic AMT with 12 gears which is used on heavy duty vehicles was studied and developed a failure diagnosis system following ISO15765 using the diagnosis method including signal detection, analytical redundancy, real-time tracking and tolerated fault method involving analytical redundancy,open-loop control and feedback replacement. Experimental results showed that this diagnosis system can monitor and judge the failure time, take effective measures to make the failure vehicle limp home.