Speedometer

The electric speedometer does not have a speedometer cable. The source of speed information is a small permanent magnet alternating current generator called a Vehicle Speed Sensor (VSS).

The VSS is mounted on the transmission. The shaft of the VSS fits into rotor teeth on the transmission output shaft in the same manner as the speedometer cable of a mechanical speedometer system. The transmission output shaft rotates and turns the rotor teeth near a magnetic pick-up within the VSS. This generates an alternating current in the coil with a frequency that is proportional to the speed of the vehicle.

The alternating current from the VSS is sent to the Powertrain Control Module (PCM). The PCM modifies the current to a series of direct current pulses at a frequency proportional to the speed of the vehicle. These pulses form a vehicle speed signal that is transmiffed to the speedometer.

At the speedometer, a solid state circuit uses the speed signal in order to regulate the distribution of the current from the fuse block to 2 electromagnetic coils. The magnetic coils are at right angles to each other, near the base of the speedometer pointer pivot. At the base of the speedometer pointer pivot is a magnet that is acted upon by the magnetic fields of the 2 coils. The relative strength of the 2 magnetic fields determines the position of the magnet and accordingly, the position of the speedometer pointer. Pulses from the solid state circuit also are used in order to drive a small electric stepper motor. The stepper motor turns the numbered wheels of the odometer and the trip odometer. The numbered wheels are geared so that when any one wheel completes a revolution, the next wheel to the left is turned one-tenth of a revolution. Every tenth of a revolution advances the displayed number by one unit. The trip odometer can be reset to zero at any time. This allows the driver to record the distance traveled from any selected starting point.