Motorhome electrics
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The CAN bus has numerous advantages in managing a vehicle’s many operational systems – here’s a brief outline of how it works
yes we can…
CAN bus is a term that seems to strike fear into the hearts of many motoring DIYers these days. I often see comments on internet forums about not fitting one thing or another because it might “upset the CAN bus system”. So what is the
CAN bus and why does it get upset?
‘CAN’ stands for Controller Area Network, and ‘bus’ in this case is a communication system that transfers data. So CAN bus is a communications network in a car, transporting digital messages between microcomputers, known as ‘nodes’.
These then carry out an action based on the message – switching on a brake light when the brake pedal is pressed, for example.
The CAN bus network consists of a pair of wires – the bus – and a series of microcomputers dotted around the vehicle, similar in concept to a phone network. The messages on the bus have a digital signature, which identifies the precise node the message is for, so only that node operates from that particular message.
The most common problem people encounter is when they fit non-standard bulbs, for example changing sidelights from filament lights to LEDs. This is not actually a CAN bus problem, though; what happens is that the lighting system carries out a self-test, which involves measuring the resistance in the circuit to make sure all bulbs are intact. An LED, or additional lights or loads, will result in an incorrect resistance reading and a fault light showing on the dashboard.
There are two flavours of CAN bus, high speed and low speed. The high-speed network carries data for the more critical systems – engine management, emissions control, transmission and antilock braking systems, for example.
The low-speed system carries less critical data, for entertainment systems, door switches and seatbelt sensors, internal lights and so on.
Robust systems
There are four main benefits to a CAN bus. It is relatively cheap and simple to design and work on, and very robust in terms of interference and electrical disturbances. It also allows for a single connection point for diagnostics to all systems in the vehicle, and it is a very efficient method of data transfer and manipulation.
So you don’t really ‘upset’ the CAN bus by fitting different bulbs or connecting a rear-view camera to the reverse light; all that happens is a fault message is displayed on the dash.
The fault message can usually be deleted, by refitting the standard light bulbs or removing the additional loads on the system. Sometimes the ignition needs to be cycled on and off after the fault is corrected, to clear the message, but on the whole, your vehicle’s CAN bus systems are really nothing to worry about.