Road tractor semi-trailer combination

A road tractor semi-trailer combination comprising:

• a road tractor having a fifth wheel;
• a semi-trailer having a kingpin which can be coupled to the fifth wheel;
• a marker in the middle of a bottom of the kingpin that includes a magnet or a coil of electro-conductive wire;
• a sensor unit connected to the fifth wheel for detecting the position of the marker relative to the fifth wheel, which sensor unit comprises a sensor that includes a magnet or a coil of electro-conductive wire by which the position of marker can be detected;
• a displacement means for displacing the sensor from a resting position to an operating position, wherein in the resting position the sensor is at a first distance from the marker and is incapable of detecting the position of the marker during the coupling and/or immediately after the coupling of the semi-trailer to the road tractor, and wherein in the operating position the sensor is located at a second distance from the marker and the sensor is capable of detecting the position of the marker;
• a spring connected to the fifth wheel and pushing the sensor unit in operating position against the bottom of the kingpin.