A wire is still a wire...

Snap-on's Diagnostic Product Specialist explains vehicle electronics systems

Published:  04 March, 2014

By Lee Collins

All modern cars rely heavily on electrical systems: be they for running the engine, operating stability control systems or for the lighting and air conditioning. As a result, these systems have become complex and highly integrated. Vehicles now typically have a number of electronic control units (ECUs) exchanging large amounts of information with each other across a Controller Area Network (CAN Bus) to operate. CAN enables multiplex wiring systems to be used, significantly reducing the amount and size of cabling in the wiring loom and therefore weight.

Conventional wiring systems use separate wires to control each electrical function. In contrast, multiplex wiring systems allow multiple electronic messages to travel back and forth through the same data link wire, just as your broadband cable at home allows telephone, TV and Internet connections to travel through the same line at the same time. The multiplex wiring system's electronic control modules send information back and forth, monitoring vehicle components and interpreting messages transmitted through the wires. The modules are remotely controlled by signals, rather than by separate wires, fewer relays and connectors are required, reducing the number of possible failure points.

This perceived complexity understandably has a tendency to put a lot of vehicle technicians into a cold sweat and send them running for the hills when faced with tackling faults in such systems but this shouldn't be the case. After all, a wire is still a wire and the basic principle of how we test one, by examining the voltage at various points, hasn't changed in all the time vehicles have been using electrics. It is the control signals in those wires through the use of CAN and multiplexing that have become more complex in the various areas of a motor vehicle. By contrast, this can actually speed up some diagnostic processes.

Lighting systems provide a good example of how vehicle electrics have advanced in even the most basic yet fundamental areas. Did you know most lighting systems are no longer fed with a straight supply from the vehicle battery via a relay? Even with standard bulbs, some systems now use a duty cycle to operate them. Pulsing at a frequency of 100Hz or more, the benefit of this is an overall reduction in power consumption but this also means the same electronics systems can monitor the circuit and warn the driver of a bulb failure. Scan tools, such as Snap-on's MODIS Ultra or VERUS Pro, are more than capable of testing these circuits, not only through a powerful scope meter but also a dual channel graphing multimeter via the frequency and duty cycle functions.

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