872152
CHAPTER 2
B50D Mk III 6X6 ADT
TESTING THE “CANBUS”
CHECKING THE CABLES
The construction of the “CAN” line is of a high quality cable consisting of a CAN_H and a CAN_L cable. These two cables carry the information in Series to and from the Control units in the system. CAN_H and CAN_L Both send the same messages and basically check on each other.
When testing the “CANBUS” the most important factor will be the 60 Ohm reading across the CAN_H and CAN_L.
The O’s and 1’s are created by voltages that are produced inside the control units.The two cables are protected by a shield around the outside to prevent in EMI (Electro Magnetic Interferences).This EMI can cause false or incorrect signals in the system. The “CANBUS” has two resistors connected to the end of the “CAN line.” These Resistors are fitted to Terminate the signals in the “BUS”. If these Resistors where not fitted the signals would reflect in the “CAN” line and the System would not work.The Resistors also play an important Role in the system for the technician,The resistors are connected across the CAN_H and CAN_L lines.If You measure the Resistance across any of the two lines at any of the control units you will measure 60 Ohms. A short circuit or open circuit will cause the resistances to change. There are numerous errors that can occur within the system. 1. CAN_H Short to ground. 2. CAN_L Short to ground.
NOTE: A digital Multi meter Must be used to check any readings on the D Series Machines. Before beginning the troubleshooting process, read and understand the following: Wire identification presents the Connector, the terminal on the connector and the colour. Shut off the engine and ignition before any harness connectors are disconnected or connected. When disconnecting a harness connector, be sure that pulling force is applied to the connector itself and not the wires extending from the connector. Inspect all connector terminals for damage. Terminals may have bent or lost the necessary tension to maintain firm contact. Clean dirty terminals or connectors with isopropyl alcohol and a cotton swab, or a good quality, non-residue, non-lubricating solvent. When checking the CAN harness at the connectors, it is important to remember to check the CAN from each of the pins on the different plugs that are used in the CAN circuit.
3. CAN_H Short to CAN_L. 4. CAN_H Open circuit. 5. CAN_L Open circuit 6. CAN_H and CAN_L Open circuit. 7. CAN _H or CAN_L shorted to a Arbitrary Potential (Voltages) Any of these errors will cause a Error code to come up on the MDU Display. The Display will show For example “CCU CAN ERROR” on the display.
62
REVISION 0