Towbar Electrics Explained

Towbar Electrics Explained

Towbar electrics have changed considerably over recent years. No longer is it just a case of joining into the rear lamp circuits.
Modern cars have a maximum current draw of 10 milliamps from the lighting circuits.
Cars almost always have bulb monitoring systems that would be affected.
An increasing amount of cars use multiplex lighting which is one wire single filiment bulbs doing two jobs such as sidelights and brake lights
Canbus Systems are now the most common in which all the light signals are sent along two data cables, this is a two way information stream (sending messages back to the ECU about its operation).
Many cars also now have trailer stability programs built into the ECU's programming. As a safety feature of the vehicle these should be activated by coding of the ECU after installing a Canbus compatible system.

Many people assume that their car will have a multiplug under the car to simply plug their towing socket into. Unfortunately this is very very rare.
Due to the fact that technology has moved on, even for a 7 pin lighting socket it is likely that a power source, and ECU connections will be required at the front end of the car. This will normally mean that fitting of the towbar electrics will include removal of various interior panels of the car, such as boot panels, rear seats, side and under dash panels, sometimes even front seats and carpet.

Another worry is how the installation will effect your vehicles warranty.
Whether you choose a Dedicated or Universal electrics kit, your overall vehicle warranty will not be affected if it is correctly installed by a competent person. The vehicles warranty will not be extended to the additional electrics but this is covered by LNB Towbars under our 3 year guarantee. It is against the law for any dealer or manufacturer to invalidate your warranty if you have had an adequate aftermarket product fitted.

Dedicated Electrics

dedicated wiring kit

A dedicated towbar electrics kit is a wiring kit designed to fit  and integrate specifically with a given make and model of vehicle. It does not neccesarily mean that it is the manufacturers original equipment wiring kit.
Being dedicated as opposed to universal means that it will not fit another vehicle other than the one or ones it is specifically designed for. There are 3 or 4 different electrics kit manufacturers available which can use different styles of integration. Some will use displacement connectors to join to can data wires while others will insert new pins into ECU multiplugs.

Sometimes an electric kit supplied by the manufacturer will be a universal kit rather than a dedicated kit, depending on the requirements of the vehicle.
The sophistication of a dedicated electrics kit can vary from that of a simple buzzer universal wiring connection, a bypass relay universal kit, or a more complex CAN BUS module integrated system. For this reason the costs of these kits can also vary considerably.
The added functionality of using a dedicated kit over a universally wired kit may be nothing at all. However if your car has added functionality from using a dedicated kit, such as a Trailer Stability System then it is well worth the little extra investment. We can advise you on this.

Most dedicated kits need the cars ECU recoding to activate its functions. At LNB Towbars we invest in up to date diagnostic coding tools, so that we can code your vehicles ECU in house.

Universal Electrics
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When installing universal wiring systems we include the cost of the buzzer or bypass relay, sockets and cable etc. in our quotes making it by far the most cost effective solution. We carefully wire into your cars circuits using sophisticated full logic bypass relays to protect all of your cars systems and to independently power your trailer lights without drawing large currents from your cars lighting circuits.Using this system we can add functionality of a parking sensor cut out relay so that when you are in reverse with a trailer attached the buzzer is disabled. This can be added even if your parking sensors are an aftermarket product.

There are 4 main types of relays used: 

  • A Buzzer - This is used mainly on older cars when no bypass relay is needed but an audible indicator warning is still required.
  • A 7 Way Full Logic Bypass Relay - This is used on most modern cars with bulb failure systems, multiplex lighting limited curent draw circuits.
  • CANBUS Relay - This is a universal version of a dedicated kit, it self dedicates and can be used with cars with CANBUS and TSP and has a bulit in combi relay.
  • Combi Relay - This is a split charge relay used to protect the cars battery from drain and switches on caravan charging and fridge on when the car is charging enough. This is only used with 12s (grey) or 13 pin sockets.



There is often speculation as to which type of connection is best. The truth is they all have their place depending on the application. As an auto electrician it is not feasible to only use one type but to choose which one is best and when.

Crimp connections are an effective join that can have good insulating qualities. They should be checked by pulling after each crimp. The drawbacks are that they can be that they can be pulled out with force or if poorly crimped, but are very good for removeable joints. If joined to a wiring loom, the loom will have to be cut. These are best used on ends of wires, to join identically sized wires or to join in something that can be removed if needed.

Soldered connections are often favoured as they are very strong against pulling. A proper solder join should be insulated and air tight, normally by using a heat shrink sleeve (pvc insulating tape is not sufficient). This means that a join to a wiring loom will have to be cut. The drawbacks are possible sharp edges that could short out, they are very difficult to remove and dry joints that cause too much resistance in the circuit. This is the best join for different wire sizes.

Displacement connections are the only join to not cut the original wire. They join by using a blade of metal that displaces the isulation around the copper wire and makes a connection bridge between the copper wire of one cable to that of another. These come in a variety of different sizes and it is important that the correct size is used to avoid bad connections or damage to the wire. Avoid using cheap unbranded connectors as these are often mis-sized or too sharp and can either cut the wire or cause bad connections. These are best used for joining identically sized wires when you do not want to damage the integrity of the original wire.

Universal Relays

Smart Can Universal Can Bus Module

The Smart alternative to a dedicated Can Bus wiring kit. It is a true Can Bus Module.

It interfaces directly to the vehicles Can Bus
It recognises the host and self dedicates to that vehicle
It fits a wide range of vehicles
It sends data back to the car about the connected trailer
It enables towing related functions such as Trailer Stability Programs
It has a built in Caravan function, controlled via the Can  by the vehicles engine revs
Offers an alternative to a dedicated wiring kit

22187 new

Ultra Smart Bypass Relay

7 lamp bypass with programmed microchip controller, intelligent switching on all circuitsmakes the trailer lights work correctly at all times, however complex the car''s lighting control system.

The advanced relay uses logic to control the switching of the trailer lighting circuits. The ultra smart bypass, with its enhanced logic will work without adjustment on virtually all current 12v cars. It bypasses any vehicle bulb failure warning systems.

• Comfortably handles the most complex signals and pulses in advanced cars
• Maintains correct trailer lights even when car “swaps” functions if bulb fails
• Monitors the trailer flashers with both audible and terminal outputs.
• Self tests on fitting.
• Totally transparent: the car does not detect it.
• Quiescent current* less than one milliamp.
• Fully snubbed against “spikes” etc. Type-approved for EMC compliance
• Protected: stands rough treatment, accidental current reversals, etc.
• Future-proof: new in-vehicle scenarios will continue to be accommodated.


Flasher Circuit Buzzer

Rear flasher buzzer: a legal requirement, and a useful reminder when you are about to turn a corner that you are towing. Joins to both indicator circuits separately.



Combination Relay

Used with twin sockets or a 13 pin socket for caravan towers.
This relay provides charging power to the caravan''s leisure battery and to the caravan''s fridge when the vehicle engine is running and the vehicles altenator provides enough charge.
The relay stops the caravan from drawing any power and flattening the car battery if there is not enough output or the vehicle is not running.

Accessory (Add on) Relay

Parking Sensor Cut Off Relay

When used in conjunction with our logic bypass relay it can  cut off the parking sensor buzzer when the trailer plug is attached to the socket and allow the parking sensors to work as normal when the trailer plug is not attached.
This avoids an annoying constant tone when reversing a trailer on a vehicle with parking sensors fitted.
This relay works with both factory fitted or aftermarket parking sensors. There is a small additional cost for this relay and fitting.

Y lead

13 Pin to Twin 7 Pin Y Lead

For use on vehicles fitted with a 13 pin socket attaching to a caravan with twin 7 pin sockets


13 pin short adapter

13 Pin to (12n) 7 Pin Adapter

This is a short adapter that fits into a 13 pin socket and converts it to a simple 12n 7 pin socket for lighting boards or accesories. Small simple and easy to use.