Mail Order Towbar Specialists
01925 730 005

Home | About us | Towbars | Electrics | Accessories | FAQs | Deliveries | Collections | Price List | A member of the NTTA

Introduction | Single | Twin | 13 Pin | Universal | Specific (VSKs)

Electrics: Universal Electrics

Universal electrical kits are the way that towbars have been wired for years. They allow for connection to the vehicle lighting by use of snap connectors (eg Scotchlok's).

Using a tester you find the correct wire on the vehicle (for example the left hand side light) and then connect the black wire from the 12N socket to this using a snap connector. You would continue this with all the connections. Universal electrics can be used for single, twin and 13 pin electrical connections.

With universal electrics the fitting of certain monitors or relay(s) is required depending on what electrics you are fitting and the vehicle.


Audible Montior

UK law requires that notice is required on the status of the trailer indicators. With universal electrics an audible monitor (often called a buzzer) is connected in series with the indicator connections and when the trailer is connected and the indicators are on it makes a buzzing noise to confirm that the indicator is working correctly.

By-Pass Interface Relay

Since around the year 2000 vehicle manufacturers have been making vehicle lighting systems more complicated, requiring the use of by-pass relays when fitting universal electrical kits. It started with vehicles such as BMW, Mercedes and Audi and is now present in over 60% of new vehicles. Most of these are European vehicles, Far East and North American vehicles are now starting to require the by-pass relay.

Vehicles may have different reasons for requiring a by-pass relay. In fact many vehicles that we recommend be fitted with a by-pass relay will work without it. However if you get a fault on the trailer (and many people do) a high current in the trailer lighting system could cause very expensive faults to your vehicle.

However it is becoming more common for a by-pass relay to be required on vehicles. Even vehicles such as the current Vauxhall Vectra (2002-on) will not work with a universal electrical kit unless a by-pass relay is fitted.

There a small number of vehicles where a single by-pass is all that is required, but for most vehicles we recommend the fitting a 7 Way By-Pass relay. Some companies provide different types of 6 or 7 Way By-Pass relays, depending on the vehicle and whether single or twin electrics is being fitted, however we only use one.

7 Way By-Pass Relays include an Audible Monitor inside so no separate audible monitor is required.

Split Charge Relay

A self switching self combination relay is only used only when fitting 12S electrics. The relay monitors the voltage in the vehicle and only provides power to the switched connections (see below) when their is sufficant voltage in the vehicle. This is usually about 30 seconds after the vehicle has been started (and for about 30 seconds after the vehicle has been switched off). This means that when towing with your caravan if you pull into the services for a meal you do not have to worry about the caravan leisure battery being fully charged and your vehicle battery being dead.

Five connections are made to the relay they are:

  • Direct power feed (usually) from the battery
  • Earth
  • Switching Connection 1 (to 12S socket Pin 2) which charges the caravan leisure battery
  • Switching Connection 2 (to 12S socket Pin 6) which supplies the caravan fridge
  • Permanent Live Connection (to 12S socket Pin 4) for the caravan power supply


All prices include VAT. Towbar prices do not include towballs, electrics or delivery.
Towbars Direct, Norbrook Trailers Ltd, Tarporley Road (A49), Whitley, Warrington, Cheshire WA4 4DS
Telephone: 01925 730 005 Email:

Home | About us | Towbars | Electrics | Accessories | FAQs | Deliveries | Collections | Price List | A member of the NTTA