Transmission Control Systems: Description and Operation

GENERAL DESCRIPTION
The AW4 electronic transmission is a conventional transmission in that it uses hydraulically applied clutches to shift a planetary gear train. However, the electronic control system replaces many of the mechanical and hydraulic components used in conventional transmission valve bodies.

FUNCTIONAL OPERATION
The AW4 system performs it's functions based on continuous real-time sensor feedback information. The Transmission Control Module (TCM) continuously checks for electrical problems, some mechanical problems, and some hydraulic problems. When a problem is sensed, the TCM stores a diagnostic trouble code. Some of these codes cause the transmission to go into "limp-in" or "default" mode. While in this mode, the TCM will not energize any of the solenoids in the transmission, and the only transmission function are:
- Park and Neutral
- Reverse
- Fourth Gear





There is one exception to the preceding statement. If one or more of the transmission solenoids are shorted to voltage or stuck open (therefore appearing energized to the transmission), the transmission will be stuck in the gear that would normally be engaged when a particular solenoid is energized. See chart illustrated.

No upshifts or downshifts are allowed. The position of the manual valve alone allows the ranges that are available. Although vehicle performance is seriously degraded while in this mode, it allows the owner to drive the vehicle in for service.