FAQ - Explanation of terms


The AFC technology (Automatic Flash Control) is a trailer flashing monitoring system (control unit) which ensures the legally prescribed monitoring of the trailer flashing lights without an additional optical (C2 indicator light) or acoustic (e.g. buzzer) control device.

The monitoring principle for the flashing lights on the towing vehicle (doubling of the flashing frequency in the event of light failure, the indicator light installed in the instrument panel flashes twice as fast as if the flashing light were intact) is adopted for the flashing lights on the trailer. This means that even if one indicator light on the coupled trailer fails, the indicator light in the instrument panel reacts with double the flashing frequency.

The AFC control unit requires a continuous power supply (terminal 30, battery plus) for its function.

AFC technology offers many advantages

  • Since the control unit is installed independently of the original flasher unit in the vehicle (which remains unaffected), AFC technology can be used in a wide range of vehicle types.
  • This results in an excellent suitability for vehicle-specific and universal electrical kits.
  • No installation work is required in the area of the instrument panel or the central electrical system (installation of the C2 indicator light or replacement of the flasher unit is not necessary).
  • The removal and installation of the steering wheel (airbag) and/or instrument cluster is no longer necessary.
  • The search for a suitable installation location for the C2 indicator lamp is no longer necessary.
    The appearance of the dashboard is not negatively affected by a C2 indicator lamp often built into a fairing or switch panel.
  • The wiring of the electrical kit often takes place only in the rear of the vehicle.
  • Usually considerable savings in installation and time, which more than compensates for the possible extra cost of an AFC electrical set compared to a conventional electrical set (i.e. with C2 indicator light).
  • Vehicle-specific electrical kits with AFC technology almost always offer easy installation.

Trailer recognition

Trailer identification is used to inform certain devices in the towing vehicle whether a trailer is coupled or not. These devices can be e.g. trailer module, parking aid or an AFC module, which must be able to adapt their function to operation with or without a trailer.

Trailer identification can be carried out via a microswitch built into the socket on the towing vehicle, which is actuated by the trailer plug when it is plugged into the socket and when it is unplugged from the socket.

Another possibility for trailer recognition is that an electronic system in the towing vehicle recognises the trailer when the trailer plug is plugged into the socket via now closed circuits on the trailer (e.g. brake light, indicator light). When the trailer plug is removed from the socket these circuits are interrupted again and it is recognised that no trailer is now coupled.

Latest trailers are equipped with LED lighting. These vehicles use more complex electronics to determine whether a trailer is connected or not. Our trailer modules, which have LED in their name, are equipped with such electronics.

Trailer module

A trailer module is a control unit which is used to operate the lighting of a trailer (caravan, rear cargo carrier) partially or completely.

The trailer module is usually installed with the electrical kit in the boot of the towing vehicle. The lighting functions of the towing vehicle control the trailer module. The continuous current supply of the trailer module then takes over the supply of the trailer lighting, i.e. the lighting functions of the towing vehicle are only additionally loaded with minimal control currents.

Vehicles with Check-Control often require an electrical kit with trailer module so that the additional lighting of the trailer does not impair the function of the Check-Control or damage electronic components due to overloading.

Flasher unit (flasher relay)

The flasher unit controls the flashing impulses on the flashing lights of the vehicle and, if applicable, the trailer. It also indicates the correct function and failure of the flashing lights on the vehicle and, if applicable, on its trailer via one or more indicator lights.

Nowadays, flasher units work almost exclusively electronically and are often combined with other components, e.g. integrated in the central electrical system, or form a unit with the warning light switch. They can also assume additional functions, such as feedback of the closing and opening process in the case of a remote-controlled central locking system. When installing a C2 indicator light for monitoring the trailer indicators, a flasher unit with C2 connection is usually installed instead of the original flasher unit.

C2 indicator light

Until the advent of AFC technology, it was almost exclusively used as the control device for the legally required monitoring of trailer indicators. An additional green indicator light (the C2 indicator light) is installed in the dashboard within the driver's field of vision. On some vehicle types, a slot for the C2 indicator lamp is provided in the instrument cluster.

The C2 indicator light is controlled by the flasher unit.

The C2 indicator light, together with the standard turn signal indicator light, indicates the correct function and failure of the turn signal lights on a combination. When driving without a trailer, the C2 indicator light can and may flash up to three times after the indicator has been switched on, after which it must go out.


or also lamp monitoring, lamp failure check, lamp test device

This device can also be integrated into an on-board computer. It indicates malfunctions or failures of lighting functions (e.g. low beam, tail light, brake light, etc.) visually and/or acoustically.

Whether a vehicle is equipped with Check-Control and, if so, which lighting functions are monitored in detail, can be found out quite easily by removing the individual bulbs. It is often helpful to take a look at the vehicle's operating instructions.

Vehicles with Check-Control almost always require a special electrical kit to ensure compatibility with the monitoring equipment. The use of an incorrect electrical kit often leads to the destruction of electronic components or to lighting failures or lighting defects.

C.C. Interface (Check Control Interface)

For some vehicle types there are models with and without check control (e.g. Audi A4). Depending on the version of the vehicle, an electrical kit is required which uses the Check-Control function of the vehicle or provides it itself.

In such cases, it is possible to use an electrical kit with a check-control interface, i.e. there is a basic electrical kit for one vehicle type (7 and 13 poles each). It is sufficient for models without check-control. A plug connection (the so-called check-control interface) is stored in the vehicle without further electrical connection. On models with check control, an extension set must be connected to this plug connection. The two electrical kits (basic and extension kit) thus together form the electrical kit for models with Check-Control.

Continuous plus

or also power supply, continuous power supply

Contact no. 9 of the 13-pin trailer plug connection (according to DIN/ISO 11446) or the Multicon trailer plug connection is designed for the connection of a continuous positive line, i.e. this contact is connected (if required) directly to the positive pole of the towing vehicle battery.

The electrical cable used for the continuous positive connection must have a cross-section of at least 2.5 mm and be fused with a fuse rating of max. 20A. Contact no. 13 is the earth connection associated with contact no. 9. Here too, an electrical cable with a cross-section of at least 2.5 mm2 must be used. In addition, the earth lines (contact no. 3, 11 and 13) on the trailer side must not be electrically connected to each other.

The continuous positive supply of the trailer plug connection is very often required, for example, when carrying a caravan, for example to operate a refrigerator or interior lighting. However, it also brings with it a certain risk:

When the engine of the towing vehicle is at a standstill, the switched-on consumers in the trailer (caravan) discharge the battery of the towing vehicle. Depending on the size and state of charge of the battery, the power requirement of the consumers and the duration of the discharging process, after a certain time the battery will be discharged to such an extent that the engine of the towing vehicle can no longer be started without outside help.

To avoid this, the continuous positive line can be routed via a voltage monitor.

It monitors the battery voltage and interrupts the continuous positive supply to the trailer when a certain value is reached. When the battery voltage increases, the voltage monitor automatically restores the continuous positive supply.


Parking aid

or also: Parktronic, APS (Acoustic Parking System) or PDC (Park Distance Control)

The parking aid is a vehicle device that monitors the area behind the vehicle when reversing and indicates obstacles to the driver either acoustically (warning tone) or visually. It is activated when reverse gear is engaged or, in the case of an automatic transmission, when reverse gear is engaged.

When operating with a trailer, the parking aid should be switched off. If not, it will give incorrect information when reversing, caused by the trailer, because it recognises the trailer as an obstacle. It does not detect the important area behind the trailer.

The parking aid can be switched off or on again when coupling or uncoupling the trailer, for example, by means of a microswitch built into the socket. More complex control units also take over this function automatically as soon as no trailer is detected.


Mechanical switch-off contact

A mechanical cut-off contact is a switching contact integrated into the socket which switches off the rear fog light on the towing vehicle when the trailer is operated (rear fog light cut-off).

The mechanical switch-off contact is actuated by a pressure pin from the trailer plug when plugged into the socket.



This is an encapsulated switch built into the socket, which is operated by a rocker switch from the trailer plug when plugged into the socket.

A distinction is made between normally open contacts, which close a contact when plugged in, normally closed contacts, which open a contact, and changeover contacts, which change over a contact.

Microswitches are used, for example, to switch off the rear fog light on the towing vehicle when towing a trailer, for trailer recognition or to switch a parking aid off and on again.

Rear fog light cut-off (NSL)

Rear fog light cut-off on the towing vehicle prevents possible glare to the driver by the towing vehicle's rear fog light reflected by the trailer or caravan.

When fitting rear load carriers (e.g. bicycle carriers) with additional rear light units, the rear fog light on the towing vehicle should be switched off. This prevents possible dazzling or irritation of following traffic due to too bright lighting.

The rear fog light on the towing vehicle must be switched off and on automatically by interrupting or establishing the electrical connection between the towing vehicle and the trailer.

There are several systems to realize this switching process:

  • The so-called mechanical shut-off contact in the socket.
  • Switching off by a microswitch built into the socket.

These two systems are actuated by the trailer plug when plugged into the socket and when unplugged from the socket.

  • The electronic shutdown by a shutdown relay or control unit.

It is connected between the rear fog light switch, rear fog light on the towing vehicle and the socket.

The switching process is either triggered by the current consumption of the rear fog light on the trailer or takes place in the case of electronic trailer recognition.

Vehicle-specific electrical kit

Electric kits that are specially designed for one or a few types of vehicles. They are characterized by the following features:

  1. Exactly dimensioned cable lengths
    The division of the wiring harnesses is designed to fit exactly in a vehicle-specific electrical kit, so that overall optimum wiring can be laid without any problems.
  2. Electrical connection in the vehicle (almost always to plug connections available in the vehicle)
    By using the existing plug connections, connection errors can be avoided and the installation time shortened.
  3. Optimally fitting and functioning components (electronics, socket, grommets, etc.)
    The components of a vehicle-specific electrical kit are designed exactly for the technical requirements of the vehicle in question and the associated towbar and can therefore always be professionally and easily installed.
  4. Complete installation material
    All parts required for the professional and smooth installation of the vehicle-specific electrical kit are included in the delivery.
  5. Detailed installation instructions
    The installation instructions describe the installation for the respective vehicle type, sometimes illustrations illustrate the installation. Any special features are described exactly and reference is made to the manufacturer's requirements.

Universal Electric Kit

This E-set can be used for many types of vehicles. It is characterized by the following features:

1. generously dimensioned cable lengths
In some cases, the wiring harness for the socket in a universal electrical kit is inappropriately divided, so that changes have to be made here during installation in order to either reach the individual connection points in or on the vehicle or to adapt the wiring to the vehicle.

2. use of cut-in connectors for the electrical connection in the vehicle
In the case of electrical connection with cut-in connectors, a line laid in the vehicle is determined (with the aid of circuit diagrams, current testers, etc.) and contacted with the aid of a metal part that cuts away the insulation of the line.

Cut-in connectors are sources of error:

  • if they are not processed conscientiously.
  • if the cable cross-section for which they are intended does not match the connected cable.

3. simple and universally applicable components (electronics, socket, grommets, etc.)
The components of a universal electrical kit are designed for standard installation. In case of special technical requirements on the vehicle, a specific solution should be preferred.

4. installation material required for the standard case
The installation material (the scope of delivery) of a universal electrical kit is also designed for standard installation. A vehicle-specific material requirement is not taken into account, so additional material procurement cannot be ruled out.

5. generally written installation instructions
The installation instructions also only describe the standard installation. Special installation requirements or circumstances are not described. The installation of a universal electrical kit requires certain knowledge and experience.