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Glossary towbar wiring kits


Adapters are used to fit different sockets (7 or 13 pin) to the trailer connection. The term Short Adapter indicates a more compact adapter.

Charging line (L, L1)

In accordance with DIN/ISO 11446, contact no 10 of the 13 pin socket is intended as the charging line for the trailer battery. Most 13 pin electrical systems come equipped with an appropriate connector (min 2.5mm 2 cross section, max 20A).

This charging line is connected via a cut-off relay to the vehicle battery. As soon as the ignition is turned on or the motor starts running it can be used for a 2nd battery or a refrigerator in the trailer.

Central plug

Connections available in the vehicle that can be used for connecting the trailer wire set.

Check Control (CC)

Check Control refers to the equipment in a vehicle that monitors the lights and indicates any malfunctions. These are optical or acoustic indicators that may be integrated with information displays or dash computers. Since trailers hitched to these vehicles should be operated separately from the rest of the vehicle lighting, many electrical kits feature CC or are expandable in order to prevent damage to the vehicle electronics. Electrical kits with a CC interface require a CC expansion kit in order to function with a CC system.

Continuous Positive (D, D1)

In accordance with DIN/ISO 11446, contact no 9 of the 13 pin socket is responsible for supplying continuous power directly from terminal 30 of the battery. Most of the 13 pin electrical systems come equipped with an appropriate connector (min 2.5mm2 cross-section, max. 20 A) and corresponding earth at contact no 13.

Control light (C2)

These are frequently used indicator lights that drivers use to monitor the trailer turn signals as vehicle regulations require. These are usually green indicator lights that are controlled by the flash unit. The indicators must be situated with the driver's visible range; with some vehicles these are already installed in the dashboard or they need only to be retrofitted. These indicate to the driver whether the trailer turn signals are properly functioning and work together with the original indicators. The function of the existing C2 control light needs to be coded free for various models.

Crimp socket

A crimp socket generally refers to sockets where the contacts are already crimped to the wire set and prefabricated in a contact holder. In this model, the especially flat versions (40mm) are available.

Data bus (CAN)

Designates a particular digital mode of data transmission in the vehicle. Usually most of the control units are connected to each other via the data bus and exchange the necessary signals. In some vehicles the trailer control unit is also connected by the data but to the vehicle's on-board electrical system. This type of transmission is often also known as CAN (Controller Area Network). The two lines, CAN High and CAN Low, are generally laid out in the vehicle, twisted with each other.

Disabling the acoustic parking system (E)

The acoustic parking system is sometimes referred to as APS, parking distance contro (PDC), Parktronics or EPH (in Germany). This parking system monitors the distance behind the vehicle for example and potential obstacles are indicated optically or acoustically whenever the driver switches into reverse gear. To avoid incorrect information, the acoustic parking system should be turned off when operating with a trailer. When hitching and unhitching the trailer, APS can be disabled and enabled automatically using the microswitch in the socket, or as with most Westfalia electrical kits, automatically using the control unit. Many vehicles are equipped with an appropriate switch in the cockpit. Important Note: When mounting permanently attached hitches, the ball may be located in the sensor range. Whether the desired function of the acoustic parking system is ensured in this combination needs to be checked in each case. This problem can be resolved by fitting a detachable towbar.

Disabling the rear fog lights (acc German traffic regulations)

In accordance with ECE R48, the rear fog light of the towing vehicle can be disabled when towing a trailer. When attaching racks (eg bicycle) with lighting units, they have to be turned off in order to reduce glare. This is automatically disabled and enabled on the towing vehicle as needed in one of the following ways:

  • By a mechanical disconnecting contact in the socket or also
  • By a microswitch
  • By an additional cut off relay
  • Automatically by the control unit as soon as the trailer is detected
Enabling the trailer monitoring

The monitoring of the trailer turn signal is prepared in the vehicle's on-board electrical system and needs to be enabled (coded) after the installation of the electrical kit.

Installation instructions

The installation instructions provide general information on how to install the electrical system of attach the trailer hitch for the given vehicle. The description is provided in the form of text, diagrams and / or photos.

LED Lights

A type of lighting that consumer significantly less current and can switch from dark to light quicker. In vehicle specific E-kits these special features are taken into account.

Legal requirements

Throughout Europe the regulations are specified by EC guidelines. The national regulations are still valid.

Load current relay

At least 4-pin relay with increased connected load (eg 70A) and larger flat pins. Use in the command centre eg with Volvo.


Small switch, partially built into the socket, that is used for functions such as rear fog light switch off or trailer auto-detect.

7-Pin socket in accordance with DIN/ISO 1724

Simple connection (N version), not watertight, sufficiently fulfils the original legal requirements. It is not possible to include continuous positive or reverse lamp signals, because all of the lines are already in use. As of the 1st January 1991 reverse lamps are mandatory on all vehicles and racks (eg bicycle) with reverse lamps also have to include all of the other light signals, thus meaning that the 7-pin socket is no longer sufficient.

7-pin socket in accordance with DIN/ISO 3732

Simple connection (S version) as an extension to standard 7-pin socket for reverse lamps and continuous positive, pin connections not identical to standard 7-pin socket (sometimes used on GB).

13-pin socket in accordance with DIN/ISO 11446

Simple connection, watertight with simple handling. Corresponds to automobile industry standard. Signals such as reverse lamp and continuous positive can be suitable transferred and reverse lines are available allowing for additional options that are especially useful in camping applications.

Pole reversal kit

A pole reversal kit is used if the switched signal, for instance of the rear fog light, is applied as a negative signal at the light, but is needed as a positive signal for the trailer and the switch-off. Using the relay, the signal's pole is reversed before the switch-off. The same principle is valid for individual systems of the acoustic parking system. In addition, the circuit logic can be adapted using the kit.

Rear fog light switch-off relay

Usually a 3-pin relay with special circuitry, without extra earth connection (eg Daimler E-class W/S210).

Socket west

Special socket with 7-pin connection acc. DIN/ISO 1724 and 6 additional contacts for signals such as RFS, KI.30, etc.

Substitute light function (eg Mercedes W203)

If there is a defect for eg of a turn signal lamp on the vehicle, then the reverse lamp flashes as a substitute. The fault is indicated in the vehicle and ought not influence the function at the trailer. On many vehicles the brake light is switched on and dimmed as a substitute light in case a side lamp fails. As a rule here, an error signal is sent to the driver.

Tail lights dimmed

If various brightness's are shown by a single bulb, then a dimmed signal is meant. The signals come from the vehicle's on-board electrical system and can generate for eg with a conventional 21 watt single thread bulb various brightness's and thus show functions such as side lamp and brake light. This equipment usually requires special trailer control units that are included in vehicle specific electrical systems.

Trailer auto-detect

The trailer auto-detect signals to the towing vehicle or control unit whether or not a trailer is hitched. This information is used to control the turn signals, acoustic parking system and other control functions. A microswitch installed in the socket selects whether to transfer the information to the control unit or directly to the electronic system of the vehicle. In the current version an attached trailer is usually detected electronically by the electrical load of the trailer lighting and the required information is passed on accordingly.

Trailer control unit

This is the designation for control units that partially or even completely supply the trailer lighting. This is controlled generally via the tail lights and the continuous positive to be attached to the control unit supplies the trailer. In vehicles without Check Control the turn signals and partially the rear fog lights are supplied via the control unit. In vehicles with Check Control the entire trailer is supplied via the control unit so that the vehicle electronics are only loaded minimally. The original turn signals do not have to be replaced.

Turn signals

The flash unit, often referred to as a 'flash relay', regulates the flash pulses sent to the vehicle and, if hitched, also to the trailer. Malfunctions are indicated by changes to the flashing rate of the control lights. In most cases, the flash unit is integrated with the main electrical system. It can however also be combined with the warning blinker switch and other control units. For operation with trailers, the original flash units were replaced in most cases by flash units with C2 function and an additional control was installed. Flash units are available in a wide range of pin configurations, some of which require additional adaptors in order to install a C2 flash unit.

TSP ½ Trailer Stability Program

This system is also known as the following:

  • TSP ½ Trailer stabilisation program
  • TSM ½ Trailer Sway Mitigation
  • TSA ½ Trailer Stability Assist

This system monitors the drive situation of the rig by sensors on the vehicle. The vehicle detects after the connection of the trailer plug that the vehicle is in trailer mode and monitors the velocity-dynamic movements that are relevant to this operating mode. In case the rig gets into critical driving state, for eg because of oscillation/fishtailing of the trailer, the vehicle's systems support the driver until the rig returns to a stable condition. Such systems are offered by many automobile manufacturers now as an option or as a standard feature. The activation of the TSP is done by linking to the vehicle's on-board electrical system using co-ordinated control units, usually by CAN bus.

Westfalia now provides a number of electrical cable looms with this safety-relevant system.

Vehicle specific connection (S)

All of the vehicle connections are made using the available connections so as to avoid connection errors and save time. The wire lengths are adapted to the vehicle.


This is the designation for electrical systems that work with a trailer control unit and usually do not require modifications to the dashboard electronics. Faults in the trailer turn signal system are indicated by an increased flashing rate of the turn signal indicators in the vehicle. No additional C2 lights are required.