The CAN bus is a multi-master data bus, which means that any device connected to the network can take control of the bus and transmit data to any other device connected to the data bus.
The CAN bus consists of basically a two-wire circuit with devices connected to the wires and terminating 120 ohm resistors at each end. It has a bus topology which consists of a main harness where each device is daisy chained into the main harness.
The two signal wires are referred to as CAN high and CAN low. The voltage measured on these two wires is 2.5 volts when no data is being broadcasted. When data is being broadcast, the volt measured is 1.5-2-5 volt on CAN Low and 2.5-3.5 volt on CAN High. An oscilloscope best illustrates these signals. When measuring with a digital volt meter (DVOM), most of the time the voltage is at 2.5 volt with periodic transition on CAN high to 3.5 volt and CAN low to 1.5 volts. This will average out to slightly above 2.5 volt for CAN high and slightly below 2.5 volt for CAN low on a digital volt meter. The variance from 2.5 volt will depend on the load on the bus, but typically CAN high will be somewhat greater than CAN low. This measurement is done between the CAN High or Low signal wire and ground.
As an example, on a low load CAN bus the measurement for CAN high was 2.56 volt and 2.495 volt for CAN low. For a high load CAN bus the measurement was for CAN high was 2.95 volt and 2.15 volt for CAN low using a digital volt meter.
The CAN bus is a high speed data bus which requires 120 ohm termination at each end. As the signal propagated down the line, if it is not terminated, some of the signal will reflect back and interfere with the next data signal. Hence the termination at each end ensures the communication is not disrupted and the signal is propagated cleanly from the source device to the target device. This can be verified with an ohm meter, measured between CAN low and CAN high wire, anywhere on the data bus, and the result shown should be close to 60 ohms, which equal to a parallel combination of 2x120 ohms resistors. The physical resistors are typically embedded into the two end devices connected to the CAN data bus.
To troubleshoot the CAN bus, a OBD2 Break Out box is used. This box connects to the OBD2 port of the vehicle and provide 16 measuring points for the CAN bus. Terminal 6 is CAN high and terminal 14 is CAN low. Terminal 16 is power and terminal 4 (chassis) and 5 (signal) is ground.
Basic troubleshooting of the CAN bus contains the following steps:
The failure of the CAN bus may be due to the harness itself or a device connected to the bus. By unplugging the suspicious device, or all devices on the bus, the exact fault location can be pinpointed.