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Until recent years, vehicle electrics were fairly standard, with only a few high-end vehicles having a bulb-failure monitoring system fitted. Recent years have seen major advances in the technology of road lighting systems, designed to improve efficiency and safety. These include systems often referred to as “multiplexed” or “can-bus” systems. These advances, whilst welcome for many reasons, do mean that choosing the correct wiring kit for your vehicle is not always a simple task. This short guide is intended to aid you in choosing the correct wiring kit to suit both your towing requirements, and your vehicle’s systems. It is however, intended as a guide, and the fitter must bear responsibility for the installation of the correct equipment. If you are in any doubt as to the type of wiring system used in your vehicle, please consult your vehicle handbook or vehicle manufacturer..
What are Muliplexed or Can-Bus Wiring Systems?
The subject of multiplexed or can-bus wiring systems is not one which can be explained in depth in a short guide. However, they can be simply described, without too much technical detail, as a system in which only a few wires actually carry a significant power supply around the vehicle. Other wires carry a control signal, sent from an electronic control unit, to operate the vehicle functions. As these control signals are not intended to send any significant power, they are incapable of supplying the power required to operate your trailer or caravan lighting. This obviously presents the towbar fitter with a problem. On such systems, the wires from the towing socket cannot be simply connected into the vehicle lighting wires. Instead, we use a bypass relay, which is powered by installing a new 12 volt supply directly from the vehicle battery, power distribution block, fuse box or other suitable constant power supply. The bypass relay is then connected to the vehicle's lighting wires, and “reads” the control signal sent, and relays the power from it's 12 volt supply to the correct function on the towing socket.
Advanced systems
Many new vehicles now come with a slightly more advanced multiplex/can-bus system. In addition to operating as described above, the system also monitors all lights for bulb failure, and should it find an inoperative bulb, compensates by illuminating another bulb on the car, sometimes slightly dimmer than normally. For instance, should a sidelight fail, the brake light will illuminate at a reduced brightness, until the faulty bulb is replaced. This system operates by “bouncing” a pulsed signal to the light, which would make a standard bypass relay chatter as it picks up the pulsing signal, and also may confuse the bypass relay into sending a similar pulsed supply to your towing socket. This is avoided by fitting the “smart” bypass relay, which, in simple terms, “programs” itself to understand the vehicle's signals when first connected. Vehicles using this system include, but are not limited to, new BMWs, Mercedes and some new Vauxhall vehicles.
Simple Bulb-Failure Monitoring Systems
Prior to 1999, the most common bulb-failure warning system operated on the brake and tail lights only. To avoid overloading the circuitry of the bulb-failure monitor, a three- way bypass relay is used. This takes the supply voltage from a constant 12 volt supply, whilst the connections to the vehicle’s side and brake lights draw a negligible amount of power as a signal to instruct the relay to allow power to the correct function on the towing socket.
The information above is intended as a guide only, to help you identify the correct wiring kit for your vehicle. Exmoor Leisure will not accept any responsibility for the use of incorrect wiring kits. If you are in any doubt as to the type of wiring system used in your vehicle, please consult your vehicle handbook, or vehicle manufacturer.
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