Description of operation, function chains: Notes

Fig 1: BUS System Communication Diagram

Courtesy of SAAB-SCANIA OF AMERICA, INC.

A large number of function chains can be identified in the bus system. Below are three examples of important ones.

1. Vehicle speed

Vehicle speed is sensed by the ESP system through a proximity sensor at each wheel. The sensor supplies the control module with alternating current, the amplitude and frequency of which increase with increasing wheel speed. The frequency is proportional to wheel speed, 29 Hz at a speed of one wheel revolution per second. The ESP control module uses the 4 wheel speeds to prevent wheel lockup on braking.

The ESP control module is connected to the P-bus and transmits the wheel speed for each of the four wheels. However, only the speed of the rear left and front right wheels are used by other systems.

Wheel speed RL: The ESP control module transmits the bus message "Wheel speed rear left xxx km/h". This message is used by the MIU (internal). The MIU sends "Vehicle speed" along the P-bus and I-bus.

Wheel speed FR: The ESP control module transmits the bus message "Wheel speed front right xxx km/h". This message is used by the engine control module.

Since the main instrument panel is connected to a bus, all control modules connected to the bus are supplied with information on the speed of the left-hand rear wheel and the odometer reading.

The table of transmitted bus messages shows that MIU transmits the vehicle speed and that it is used by: Trionic, SID, ACC, Audio and TWICE. MIU also transmits the odometer reading, which is used by SID for the service indicator.

The various systems use the vehicle speed for the following functions:

The PSG 16 receives information about the wheel speed of all four wheels via the bus and a separate lead from the ESP for front right wheel speed. The wheel speed signal on the separate lead is used by the cruise control function.
DICE uses the value to activate the radiator fans for A/C on, high outside temperature and low speed.
SID uses the value for trip computer functions.
ACC uses the value for activating recirculation in connection with high outside temperature and low speed.
The Audio system uses the value for increasing the volume at high speeds.
TWICE uses the value for automatic locking of the doors when driving off (certain markets).