Model: All from E38 to Present
Production: All
Up until the introduction of the E31, all of the information transferred between control units was transmitted on dedicated signal lines. As the various electronic systems grew more complex, the size of the wiring harness increased beyond practical limits.
Signals such as engine RPM, coolant temperature, throttle position, road speed etc. each used a dedicated signal line going to the control module that required this information. Each of these lines differed in the method of signal transmission. Some of the methods used were variable duty cycle, switched DC signals and signals with variable frequencies. This created a need for larger and more complex wiring harnesses.
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A solution to this problem was found by introducing bus networks to BMW Group vehicles. A bus system uses multiplexing technology similar to that used in the electronics and telecommunications industry. Multiplexing is a system of transmitting several messages on the same circuit or channel.
This technology allows control modules to transfer data bi-directionally at high speed and enables control modules to share sensor information. This also allows control modules to send and receive control commands at a faster rate than with conventional methods.
With the amount and complexity of features now available in modern vehicles, multiplexing is a necessary technology. There are numerous benefits to in-vehicle bus networks such as
- A reduction in the size of the wiring harness by decreasing the number of interfaces between control units to one or two wires.
- Greater system reliability by reducing the number of connectors and components.
- A reduction in the number of redundant sensors by allowing the sharing of sensor information.
- Reduction of costs for components, assembly and troubleshooting.
- Flexibility in system configuration for addition of new systems.
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CAN-Bus Operation
The primary function of the CAN-Bus is to exchange data at a high transfer rate between CAN subscribers. This is accomplished using two signal lines referred to as CAN-High and CAN-Low. Both of these signal lines transfer the same data at the same time. Two signal lines are used for redundancy in the event of a signal line failure.
Voltage on the CAN-Bus is divided between the two data lines. for an average of 2.5 volts per line. Voltage is measured from each data line to ground. Each module on CAN contributes to this voltage.
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When viewing the CAN-Bus signals on an oscilloscope, CAN-Low with be approximately 2.5 volts. The signal will be "pulled" low during communication. CAN-High will be at 2.5 volts, but the signal will be "pulled" high during communication. However, the fact that 2.5 volts are present does not indicate that the CAN-Bus is fault free, it just means that the voltage level is sufficient to support communication.