What is the colour of the control cable in trailers with a two-circuit pneumatic braking system?
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Correct answer
B. Yellow.
Explanation
In the photo, there is a red two-axle trailer with a yellow grille at the back, designed for an agricultural tractor. The control line of a two-circuit air brake system is yellow - answer B.
In the two-line connection of the brakes of a tractor and a trailer, each line has a different function and a different coupling head colour. The yellow control line carries the signal from the control valve in the tractor: when you press the brake pedal, the pressure in it rises and the trailer brakes harder the harder you press. Red (answer A) is the supply line, which fills the trailer's air tank. Black (answer C) is found in older single-line systems. When connecting, make sure that the coupling heads are clean and properly latched, and then check while stationary whether the trailer really brakes. A leak only becomes apparent at the first braking, and then it may be too late.
Legal basis: § 49 para. 2 of the Regulation on the technical conditions of vehicles and the scope of their necessary equipment; colour coding of coupling heads according to ISO 1728