Cut through the jargon

Technical expert James Dillon offers technical tips for network analysis

Published:  24 February, 2014

By James Dillon

CAN is simply a set of rules by which the means of communication between a number of vehicle computers takes place. There are other standards such as LIN, VAN, MOST, D2B and Flexray. CAN is commonly experienced by automotive technicians as it is the dominant communication system for the modern vehicle's powertrain network (the computers which make up the powertrain control system). Individual control computers such as engine, ABS, ESP, parking brake, terrain modules, transmission and steering are clustered and share information to enable seamless control of the vehicle's power and dynamics.

The biggest change when considering networked systems is the fact that a component which is causing a problem may not be physically connected to the computer which uses its data. Consider an automatic transmission that is stuck in limp mode; the diagnostic trouble code may refer to a vehicle speed reference problem. The vehicle speed signal generator may be physically connected to the ABS system (wheel speed sensor) and pass its data over the network to the transmission computer. So the fault and the symptom may be 'separated' by the network. This makes fault finding interesting, particularly when a number of faults are seen in a number of computers. Working out which are 'symptom' codes and which are 'cause' codes is important so that time isn't wasted.

One technique I use in these cases is an analytical tool called destination/origination. When performing a global vehicle scan (or scanning a cluster of vehicle computers) a number of faults may be seen in a number of different ECUs. Trying to define the exact nature of the fault when you've been presented with a big list of codes can be difficult and grouping fault codes by ECU will begin to build a picture of what's going on. The next step is to take the list and draw a diagram - then draw a line between the ECU which contains the code and the ECU which the code relates to. Draw a small circle on the line on the ECU which contains the code (origination) and a small arrow on the line on the ECU which is mentioned (destination). Write the code description along the line (e.g. no communication). This process will really provide a clear overview of what's happening within the system. Usually, the ECU which contains the most arrows is the place to start your investigation, as this is being 'complained' about most by the other ECUs. Where multiple ECUs have an equal number of arrows and dots, an intermittent communication fault (wiring or gateway) is likely to be the cause.

www.diagnostic-assistance.co.uk

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