General Information

FUNCTIONAL OPERATION

The NAG1 electronic transmission has a fully adaptive control system. The system performs its functions based on continuous real-time sensor and switch feedback information. In addition the TCM receives information from the Shift Lever Assembly, ECM (engine management) and ABS (chassis systems) controllers over the CAN bus. The CAN bus is a high speed communication bus that allows real time control capability between various controllers. Most messages are sent every 20 milliseconds, this means critical information can be shared between the Transmission, Shifter, Engine and ABS controllers. The CAN bus is a two wire bus with a CAN C Bus (+) circuit and a CAN C Bus (-) circuit. The CAN bus uses a twisted pair of wires in the harness to reduce the potential of radio and noise interference. The CAN bus also uses a 120 ohm terminating resistor in both the ECM and Sentry Key Remote Entry Module (SKREEM) modules. The module terminating resistance is measured across both CAN bus circuits at the ECM or SKREEM module. The control system automatically adapts to changes in engine performance, vehicle speed, and transmission temperature variations to provide consistent shift quality. The control system ensures that clutch operation during upshifting and downshifting is more responsive without increased harshness. The TCM controls the actuation of solenoid valves for modulating shift pressure and gear change. The required pressure level is calculated from the load condition, engine speed. Power for the transmission system is supplied through the Transmission Relay. The TCM is located in the under the drivers seat of the vehicle.

The Transmission Control Module (TCM) continuously checks for electrical problems, mechanical problems, and some hydraulic problems. When a problem is sensed, the TCM stores a diagnostic trouble code (DTC). Some of these codes cause the transmission to go into "limp-in" or "default" mode.

The NAG1 will default in the current gear position if a DTC is detected, then after a key cycle or shift to park the transmission will go into Limp-in, which is mechanical 2nd gear. Some DTCs may allow the transmission to resume normal operation (recover) if the detected problem goes away. Permanent limp-in DTC will recover when the key is cycled, but if the same DTC is detected for three key cycles the system will not recover and the DTC must be cleared from the TCM with the DRBIII scan tool.

Once the DRBIII is in the Transmission portion of the diagnostic program, it constantly monitors the TCM to see if the system is in limp-in mode. If the transmission is in limp-in mode, the DRBIII will flash the red LED.