Dimmer Switch: Description and Operation

The speedometer has include in its electronics an ambient light sensor. The sensor is mounted on the printed circuit board adjacent to the fluorescent display tube. There is a clear opening in the cluster lens and tunnels through the mask and dial so that passenger compartment light (ambient light) will fall on the sensor. The amount of light striking the sensor determines the brightness of the display. The same electronics in the speedometer also senses the voltage of the instrument panel dimming rheostat and combines this information with the ambient dimming information. When the headlamps and parking lamps are OFF, the rheostat has no effect and the display brightness will be controlled only by the ambient sensor.