CAN bus explained

When you have a problem with your car, a part could be to blame. Yet, we have heard of many instances, where customers have wasted money having parts changed but the problem persists. A reason for this tends to be incorrect, or incomplete diagnosis, resulting in components being replaced blindly, much to the annoyance of the car owner. In reality, the car's data communication network might have been at fault.

As terminology between mechanical components and computers is so different, even professional technicians can struggle. So, because car networks are so misunderstood, we hope this introduction blog on the topic is useful by car owners and professional technicians alike.

Why do cars have network?

As modern cars are fitted with so many electrical comfort and safety features, the number of computer control units has ballooned. This meant that car wiring harnesses became larger, heavier and more expensive for the car manufacturers. Garages also found them extremely difficult to fault diagnose. The need for computers not just to communicate with the components they control but also with each other made the wiring even more complicated.

The simplest solution was to link the various computers together (most commonly with a wire) to form a network - known also as a 'bus'. This would not carry electricity to power the components but data communication signals. Each computer would receive data signals relevant to itself and ignore the others, before passing them on.

What is CAN bus?

The data language (protocol) also varies between networks (buses). The speed at which data is transferred also differs. Therefore, a modern car can possess multiple

buses, each running different protocols and data speeds. One bus may be for the engine and gearbox, another for the interior comfort features, another for the multimedia equipment and so on.

Various protocols (i.e. data languages) are employed, the majority of which use copper wires. The most common is LIN (Local Interconnected Network), which uses a single wire to receive data. Should the wire break, communication fails and the components on that particular network will not work. Most features using the LIN protocol, therefore, have a manual override switch. Typical examples include automatic headlamps, or windscreen wipers.

CAN (Control Area Network) is a protocol that tends to be used for the engine, gearbox and safety features. It uses a pair of wires that are twisted together (as pictured), meaning that control units can both receive and transmit data. Should one wire develop a fault, the other one will take over. CAN bus, therefore, is extremely dependable. Most problems tend to be caused by modifications being performed to the vehicle, such as electrical accessories being fitted.

Therefore, if you are told that your car has CAN bus, the correct terminology is that you have a networked vehicle that uses CAN bus protocol for one, or more, of its buses.

CAN bus was invented by Bosch but some car manufacturers use their own twin-wire protocols. Typical examples include Ford's SCP (Standard Corporate Protocol) and General Motors-era Vauxhalls using GM_CAN. Another alternative to CAN bus is FlexRay.

MOST (Media Oriented Systems Transport) is another protocol. Instead of using copper wires, it employs fibre-optic cables. The most common application is in multimedia systems, such as satellite navigation units. Diagnosing MOST problems is not easy but a kinked fibre optic cable will cause communication issues. Byteflight is a fibre-optics-based data bus, used commonly by BMW, for certain safety critical items, including airbags.

What happens when a network fails?

When a communication network fails, symptoms vary from nothing at all to the engine failing to start. The engine may even continue to run. We have heard of some Battery Electric Vehicles not coming out of their 'Ready' mode, meaning that the vehicle cannot be locked. Most commonly, components on the affected network will cease to function, which provides a useful diagnostic clue. Therefore, if many components stop working at the same time, the network might be responsible.

However, low-voltage situations cause communication faults, too. Therefore, ensure that your 12v battery has been tested for optimum state of charge and state of health.

How can networks be checked?

Detailed checks are not really possible for a typical car owner. One reason is that you will need detailed information about the appropriate network(s) fitted to your car, including its layouts, protocols used and data speeds. A proficient garage should have access to this data and be able to identify whether the appropriate network is capable of transmitting messages, whether any signals are being transmitted and, if so, what data is being transmitted.

This diagnostic work takes time and is more involved than a straight-forward check for fault codes, with a basic diagnostic tool. Some car owners think that paying for diagnostic time is a waste of money. Yet, very often, it is preferable to throwing cash away, by replacing perfectly serviceable parts.

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