Purpose Of System

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Electronically controlled transmissions were introduced on BMW products in 1986 on 5 and 7 series vehicles. Currently EH (Electro-hydraulic) transmissions are offered on almost every production model (Except E46 M3 and E39 M5). EH transmissions offer the following benefits the to driver:

Increased driving safety by reducing fatigue. All shifts are automatic as opposed to manual transmissions which require more driver interaction.
Increased fuel economy through use of lock up torque converter.
Increased fuel economy through optimized shift points.
Improved shift comfort by use of "Overlap Shift" technology (ZF).
More available features through the use of CAN bus technology. See Fig 1 .

Fig 1: Identifying Transmission Control System
Courtesy of BMW OF NORTH AMERICA, INC.

The EH Control System is designed to work in conjunction with the engine electronics for precise shift control. The TCM receives information on engine RPM, load and throttle position to provide optimum shift points to maximize fuel economy and driver comfort. See Fig 2 . The function of an EGS System is to:

Monitor all operating conditions through input signals
Continually assess operating conditions by processing input data and select the appropriate operating program for current conditions.
Activate transmission system components and to communicate with other drivetrain control systems.
Respond to driver selected driving program (Economy, Sport or Manual).

Fig 2: Identifying Electrical, Mechanical & Hydraulic Systems
Courtesy of BMW OF NORTH AMERICA, INC.

In addition to providing shift control, the TCM also adapts to changing conditions within the transmission by monitoring slip ratios and modifying line pressure. This increases the life of the transmission and reduces maintenance and adjustments. The TCM controls the operation of the Lock-Up Torque Converter which further increases economy.

On current models, the TCM also has the capability of adapting to driver habits and responds to changing environmental conditions. Items such as rate of throttle input and kickdown requests are monitored to select the most appropriate shift program.

The EGS system is also required to maintain occupant safety, safeguard drivetrain damage, improve vehicle emissions and operate in failsafe mode when a malfunction occurs.