The towing vehicle has an automatic sensing system which detects when a trailer is coupled to the vehicle. This comprises a holding relay which is operated when the trailer brake light circuit (7) is connected in parallel to the brake light circuit (3) of the towing vehicle. The reduction in engine torque is controlled via the electronic engine control circuit with a reduction in the maximum throttle setting. Alternatively an adjustable stop can be positioned in the path of the throttle control. A holding relay (6) is switched by the connection of the trailer plug and provides a simple input signal for the engine management circuit.
