CP Fitters' Phase 2 Apprenticeship Training

Bus: In the context of electronic engine and vehicle management, a bus is an electrical pathway over which data can be transmitted. Think of them as electronic roadways along which the data travels.

CAN stands for Controller Area Network. This is an electronic network over which the various vehicle systems can communicate with each other.

Each vehicle system is known as a station. For example, engine management is a station on the bus, the instrument panel on the dashboard is a station, transmission shift control is a station.

The CAN bus is a serial bus. In other words, data is relayed on the bus one bit at a time, one after the other.

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The CAN bus is simply a pair of wires, often twisted around each other, running around the vehicle and terminated at either end of the two-wire network with resistors of 120 Ohms. The only components connected to the CAN bus are the electronic control units (nodes). Other components, such as sensors, motors, light bulbs, switches, etc. are wired only to the electronic control units. 

A vehicle which uses CAN bus for on-board diagnostics can only respond to an OBD-II request from a tester which uses CAN bus. From model year 2008 vehicle manufacturers must use the OBD protocol specified in ISO 15765, also known as Diagnostics On CAN.

Two wires of CAN bus, CAN-H and CAN-L, will have the same voltage when idle (about 2.5V), or a voltage difference of 2V when a signal is placed on the CAN bus. When a signal is placed on the CAN bus the CAN-H line is at a higher voltage than the CAN-L line.

Each electronic control unit have its own CAN identity code, like an address (may respond to several CAN id codes). If an electronic control unit is to communicate to another it will need to know the CAN identity code of the recipient.

A simple check to see if the CAN bus is in use in a vehicle, and accessible via the OBD socket, is to connect a resistance meter across pin 6 and pin 14. Due to the combined resistance of the two termination resistors at 120 Ohms each the overall resistance should be read as 60 Ohms.

OBD-II provides access to numerous data from the ECU and offers a valuable source of information when troubleshooting problems inside a vehicle. The SAE J1979 standard defines a method for requesting various diagnostic data and a list of standard parameters that might be available from the ECU.

The various parameters that are available are addressed by parameter identification numbers or PIDs which are defined in J1979. For a list of basic PIDs, their definitions, and the formulae to convert raw OBD-II output to meaningful diagnostic units, see OBD-II PIDs.

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