Transmission Control Unit

Electronic control system, AW 30-40 and AW 30-43

To determine the exact gearshifting and lock-up engagement points, based on the selected driving mode, the TCM (4) is supplied with signals describing:

1 selected gear position, from gear position sensor (5)
2 selected driving mode, from driving mode selector (3)
3 car speed, from speedometer (27)
4 speed of front planetary train sun wheel, from speed sensor (12)
5 engine speed and load, from engine control module module (ECM) (28)
6 throttle opening, from ECM (28)
7 brake pedal operation, from brake light switch (29)
8 full accelerator pedal operation, from kickdown switch (30)
9 transmission fluid temperature, from temperature sensor

The information is processed by the TCM, which computes which gear will afford the best driving performance for the selected driving mode.

The TCM executes gearshifting by transmitting the appropriate signals to two solenoid valves (S1 and S2) in the transmission. Based on the input signals, the TCM also determines if and when the torque converter lock-up function should be activated. Lock-up is controlled by solenoid valve (SL).

Unlike earlier automatic transmissions, lock-up is available in the AW 30-40 and AW 30-43 in three gears, 2nd, 3rd and 4th. When and how the function is engaged depends on factors such as the transmission fluid temperature and the ambient temperature.

During gearshifting, the transmission line pressure is controlled indirectly by solenoid valve (STH). Torque reduction takes place in the following sequence:

- the TCM transmits a signal to the Motronic 1.8 control (28);
- the engine torque is reduced (depending on the prevailing value);
- Motronic 1.8 transmits an acknowledgement signal;
- the transmission line pressure is adjusted;
- gearshifting is executed.

This procedure affords smoother gearshifting and reduces mechanical wear. The TCM is also connected to the DLC (6). Any control system fault is recorded in the TCM memory for subsequent display in the form of a diagnostic trouble code (DTC). If the fault is sufficiently serious to warrant immediate attention, the MIL (7) on the combined instrument will start to flash.

The TCM also enables the operation of individual components to be tested and certain input signals to be read using the DLC, all with the aim of simplifying troubleshooting.