An electronic control unit (ECU) in the driver's cab receives a speed signal from an electronic tachograph / speedometer or from a manual transmission sensor, depending on the vehicle.
The ECU monitors this speed signal until the vehicle reaches the calibrated speed. The ECU sends a signal to the fuel control solenoid valve installed in the vehicle's fuel line, thereby controlling the flow of fuel to the injection pump. Once the calibrated speed has been reached, the valve closes, restricting the flow of fuel, which is vented through an integral, adjustable needle valve controlled return tube. This needle valve is adjusted to supply only enough fuel to the engine to keep the vehicle at a limited speed.