Technical Data: Receiving: Notes

WARNING: This page is about a different car, the 2002 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Control Module Message Transmitting Circuit Diagram (Receiving)

Courtesy of SAAB-SCANIA OF AMERICA, INC.

When a control module is transmitting, all the other control modules listen to it. They do this by looking at the difference in voltage between bus+ and bus-. (A voltage difference close to 0 V is interpreted as a one and a difference close to 5 V as a zero.)

What the various combinations of ones and zeros mean is determined in advance so that all control modules interpret the information identically. The message contains a description of what is sent and its value, e.g. "Coolant temperature 100°C" or "A/C On".

Electrical interference, such as from a relay, will affect the voltage on both leads in the same direction. The difference in voltage will not be affected, however, and this makes the bus extremely insensitive to interference.

Below is a summary of the messages which are sent out by all the car's systems that are connected to a bus. It also shows which systems use the transmitted information. The purpose of the summary is to help you gain an understanding of the car's functional relationships, which is a fundamental prerequisite for rapid fault diagnosis.