Safety Functions

Safety Functions

Safety functions are designed to safeguard against inappropriate actions by the operator as well as against system malfunctions. The system prevents reverse gear from being engaged at high forward speeds and prevents manual down shifting at excessive engine speeds.

The functions are not operational in mechanical 'limp-home' mode.

The TCM constantly monitors the transmission for faults. In the event of a problem the TCM will adopt a 'limp-home' mode. If a transmission fault is detected, the PCM generates a DIM warning message.

Safety concept is based on the following:

1. The hydraulic system has 'fail-safe' characteristics regarding its electrical operation, such that should the power supply be lost to the electro-hydraulic actuators, the transmission will initiate a 'limp-home' mode.
2. Recognition of critical shift operation by monitoring the last element in the signal path, i.e. the solenoid valve, and checking by means of redundant measured variables, i.e. engine speed, input speed and output speed.
3. Each time the vehicle is started there is a check on the entire safety hardware and the associated program parts and signal paths. A malfunction in this part of the system, or triggering of the safety circuit, is communicated to the operator through the DIM message centre right.

Limp-home Modes

If a fault is detected in the 'Shift by Wire' system the vehicle will go into 'Limp Home' mode. There are three types of 'Limp Home'.

Electrical - 'Gearbox Fault Reduced Function' will be displayed in the message centre right. Touchtronic and Sport modes will be disabled. Gearshifts will still be possible but shift quality will be degraded.

In certain circumstances 'D' will be restricted to 3rd gear. If travelling at speed 3rd gear will engage when an appropriate speed has been reached.

Reduced Engine Performance - 'Reduced Engine Perform' will display in the message centre right. Engine performance will be restricted.

Mechanical - 'Limphome No Gear Change Possible' will display in the message centre right and a audible warning will sound. If travelling forwards in 'D' or 'D Touch' the vehicle will go into a locked gear (3rd or 5th depending on vehicle speed). If entering mechanical limp home mode in any position other than 'D' or 'D Touch' (forward gears) the park lock will engage.

Do not attempt to change position while in mechanical limp home mode, i.e. pressing either P, R, N, D or pulling back on both gear shift paddles simultaneously. If this request is detected at a speed below approx. 20 km (12.5 mph) the engine will stop and the parking lock will engage.

At a speed above approx. 20 km (12.5 mph) the request and any other transmission request will be rejected and the vehicle will continue in 3rd or 5th.

Torque Converter Operation

The TCM continually monitors the state of the torque converter.

There are three operating modes for the converter:

Torque Converter Clutch

Hydraulic Pressure Regulation

The TCM supplies an internally switched +12V supply to No.4 pressure regulator. The regulator is operated by switching the other side of the operating winding to ground. Hydraulic pressure is controlled by 'pulse width modulation' (PWM) of the ground switching signal i.e. the duty cycle: the time that the pressure regulator is switched on.

TCM Supply Voltage

The TCM monitors battery and ignition switched supply voltages.

A permanent supply is used to maintain a battery backed 'memory'. Should this supply be cut, due to battery disconnection or power interruption, the shift quality adaptations (made due to the transmission's changing characteristics over time) will be lost. This will result in a small reduction in shift quality for a period until the shift adaptations are re-learned.

The TCM will adopt 'limp-home' mode as a result of the supply voltage being greater than 16 V or below 7 V with an engine speed of greater than 1600 rpm.

Should the ignition supply be between 7 and 9 volts, the TCM will hold the gear that it has currently selected. If after 2.5 seconds, with the engine speed more than 1600 rpm, the voltage remains at this level, 'limp-home' mode will be adopted. The 2.5 second delay is built in to prevent reaction to a momentary voltage fluctuation.