Dedicated Towbar Wiring Kits
This guide will explain the need for these dedicated/specific wiring kits to be fitted on newer vehicles when installing a towbar, so the next time you are planning to install a towbar stop and think about your vehicle's electrical system or you may be walking into a giant of a problem.

Did you know that vehicles fitted with Electronic Stability Programmes (ESP) as part of their Adaptive Driver Assistance features, in many instances offer within ESP a Trailer Stability Programme (TSP) which alerts the vehicles ESP facility to counter the weaving and snaking effects of a caravan or trailer when towing. What you didn't possibly know is that this TSP systems lie undetected in the vhicle software until one of these kits is fitted and bought into operation when the vehicle detects a trailer or caravan being connected.

How does the vehicle know a trailer/caravan is connected?
Detection of a trailer or caravan happens in a number of ways but the most common is via signals being sent to the vehicles CAN bus system generated by a unique towing module which is recognised by the central computer and becomes active as soon as a trailer or caravan plug is inserted into the electrical socket.

Be Aware of Bypass Relays
If a Standard wiring kit or bypass relay based wiring kit is used on a vehicle with these complex wiring systems, the vehicles ESP facility will not function at all as it has been bypassed hence the bypass relay therefore the TSP and other safety feature systems are unable to be activated and will NOT function.

CAN Bus Systems
CAN Bussed Systems (Controlled Area Network) allow the manufacturers to reduce the cost of construction by replacing yards of wiring harnesses, a single pair of wires became sufficent as the transmission medium and because CAN's are now used in hugh volumes, all the components are extremely affordable. Its high transfer rate and high transmitter reliability, as well as the ability to recognise data error makes it ideal in electrical noisy environments, where earthing can be a problem.

The idea behind CAN is simple. Instead of connecting everything to a central control unit through a main wiring harness, each component in the network has its own processing and communication capabilities with one data channel connecting all units. In automotive CAN applications, the instrument panel, power windows, body accessories and even man sensors and actuators all have their own individual mini control unit. All the units, called "nodes" communicate with each other through a single pair of wires. These are know as the "data bus".

All the data on the bus is addressed to a specific node or group of nodes by function, not location. The relevent nodes respond to a particular pre-programmed protocol. For example the vehicle speed sensor will put its data on the bus, and every other controlled unit that needs vehicle speed information will recieve it directly without having to go to the PCM (Pulse Code Modulator) for a VSS (Vehicle Speed Sensor) signal.
Because there are multiple nodes sharing a single bus, every data package includes a priority code. This will ensure for instance that if the ABS control unit and the lighting unit both send a message to the PCM, the critical ABS system message gets to it first.

CAN technology will significantly reduce the number of connector pins and wires in the vehicle. With extra blank connectors built into the network loop, or with a splice kit for adding connectors, nodes can be added on the production line (or aftermarket) without running a new wiring harness. However designers have built in more than one network on the vehicle, linked by gateways where appropriate. The mission-critical nodes will be linked on a high speed class C network that can operate in virtual real-time. Less critical items such as the instrument panel lighting systems will use a slower class B network, and occasionally used body accessories such as power windows are likely to use a class A network.

Activated Driver Assistance Systems inform the driver about all the relevent data being transferred around the vehicle - the corresponding display is then activated either in the instrument display or on a centre panel display. Computers for engines and transmission control or for lighting system management monitor the vehicle using data transmitted via the CAN bus. The systems no longer operate in isolation alongside one another, but can now quickly react to the information from electrical and electronic functions.

As a result, effective fault monitoring and diagnosis of the systems are intergrated into the CAN bus, defects can be localized more quickly and even temporarily compensated for by corresponding secondary systems. For example, a faulty tail light function can be temporarily replaced by a brake light bulb. The driver would instantly be informed of the failure to address the problem.

Dedicated Towbar Wirimg Kit Functions

Trailer/Caravan Detection
This is simply an element of CAN protocol which is activated usally by the car acknowledging a trialer is connected. This is not done by any mechanical means but by simply inserting the trailer plug into the towing socket. The car relises it has a trailer/caravan and now stimulates certain operating systems to wake up. These systems include:

Trailer Stability Programme (TSP)
A change to the parameters of the vehicle's Electronic Stability System - the car detects the 'Snaking' of a trailer/caravan ctivating the ESP sensors in the vehicle, it brings the trailer/caravan back under control.

C2 Function
The C2 function of the trailer/caravan is activated, usally by a double frequency flash rate of the cars indicators, should a trailer/caravan indicator fault be detected.

Fog Lamp Cut Off
The rear fog lamp on the car is switched off when towing if a fog lamp is fitted to the railer/caravan.

These important features and operations will NOT function if the car is fitted with bypass technology used in standard towing electrics. They will only work when they detect a towing module which is sending and recieving signals through the CAN Bus system.