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Undercar CAN Bus Diagnostics

If serial data buses did not exist, a wiring harness would have to be five times its normal size and use twice as many sensors to deliver the same level of functionality and safety we see in the modern vehicle. For example, take a brake pedal sensor. On a modern vehicle, the position of the brake pedal is used by the shift interlock, ABS system, cruise control, traction control, brake lights and electric emergency brake. If each system required its own switch and wiring, the complexity of the wiring harness and switches would be a diagnostic nightmare. 

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Serial data buses help to eliminate multiple sensors and wiring. One sensor can share information with multiple modules without having to connect directly to the multiple modules.

What is On The Serial Data Bus?

A serial data bus uses voltage to communicate. Modules toggle the signal off and on, making the 1s and 0s of digital binary language like Morse code. This code can communicate commands that allow something as simple as rolling up a window or as complex as stability control correction.

Zero volts on any serial data bus is translated into binary language as “1,” and when the voltage increases the voltage to a specified level, it equals “0.” Most electronic devices operate on signals toggling between 0 and 5 volts.

On most automotive serial data buses, the peak voltage level might be 7 volts. This extra voltage is to accommodate resistance in the wires and ground problems that may cause voltage drops. 

If a signal was on an equal length of time as it was off, you would have 0, 1, 0, 1, 0, 1 as the binary message being sent out. It could represent what the throttle position voltage is, a signal being sent from the airbag module to the BCM reporting the status of a sensor.

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Whatever the bus message, it’s comprised of 0s and 1s, or the states of highs and lows. Some systems use a variable pulse width that not only toggles between on/off, but can transmit additional information by varying the length of time the voltage is either on or off. This is how all serial data buses operate. You are never going to be able to look at the signals on a scope, decipher a series of 1s and 0s, and say that it is a command to turn on the brake light. What it can tell you is that a module is communicating and the bus is active. 

Reading the Wiring Diagram

As a technician in the modern vehicle era, you’re going to need to understand these “bus lines.” The dotted line at the edge of the component, node or module indicates where the CAN bus enters and exits.

Some schematics may include other information in the boxes with two arrows pointing in opposite directions. All two-wire CAN bus lines terminate in a resistor(s) of a known value. This is what produces the correct amount of voltage drop.

Voltages

One of the most critical aspects of networked modules is the requirement for healthy system voltage being supplied to the various modules. If the system voltage drops below 10 volts during a non-cranking event for modules on a hi-speed CAN bus, some modules will shut down and communication will cease. Some system like ABS and electric power steering may go into a limp- or safe-mode. 

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For some vehicles a sudden drop in voltage could be caused by a weak battery or an issue with a power distribution module. Often a low voltage event might occur after the engine is started and modules might be performing self-diagnostic tests when the loads on the alternator are the greatest. Also if the alternator or battery are not working properly, a sudden correction by the ABS, stability control or electric power steering system could put enough load on the system to cause the modules and CAN bus to shut down.

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