There are two principal ways in which the speedometer system of a modern motor vehicle sends information about the rate at which the wheels are turning and translates this information into a form that can be displayed on an analogue or digital speedometer. One of these is by way of a speedometer cable, which is a series of wires that generally connect the shaft that drives the wheels to the speedometer. In more modern vehicles the connection is electronic and is achieved through the speedo transducer.
The speedo transducer is an electronic pulse generator that is located on the side of the vehicle’s transmission. It sends the signals to the car’s computer electronically rather than mechanically. These signals are translated into the speed readings displayed by the speedometer.
If the speedo transducer is not working properly it may be due to a loose or dirty connection. Another possibility is that the transducer’s wires have been burnt by the vehicle’s exhaust or have been harmed by rubbing against the transmission.
If the problem is due to a faulty speedo transducer the part will need to be replaced and replacement transducers for all makes and models are readily accessible, not particularly expensive and easy to fit.