The speedo cable of a motor vehicle is an important component that alerts the driver to the speed at which the vehicle is travelling. It is part of an electromagnetic system that relates the rate at which the wheels of the car are spinning to the speedometer on the vehicle’s dashboard. The shaft that is responsible for rotating the wheels of the vehicle is attached to the speedometer by the speedo cable – a long, flexible cable that is made up of a series of twisted wires.
When one end of this cable rotates it causes the other end to do likewise. The rotation of the cable causes a magnet that is situated within the speedometer, in a metal cup known as the speed cup, to rotate at a similar speed. The rotation of the magnet causes electric currents to flow inside the speed cup causing eddy currents which create motion. The motion that is created within the cup causes the speedometer pointer to move and the faster the cable turns the greater are the eddy currents that are generated and the more the speedometer is pushed up the dial.
The obvious sign of a faulty speedo cable is that the speedometer ceases to function. All connections should be checked and, if necessary, the faulty speedo cable replaced.