Vehicle Speed Sensor: Description and Operation

Vehicle Speed Sensor:

ANALOG METER
The vehicle speed sensor, located in the speedometer head, used for needle type gauges is a magnetic reed switch that rotates in conjunction with the transmission output shaft. It signals the ECU in open/closed terms. The ECU interprets the speed at which the open/closed signals are conducted into vehicle speed.

Fig. 59 Vehicle Speed Sensor:

DIGITAL METER
The vehicle speed sensor used for digital readout type gauges, mounted to the firewall in the engine compartment, is a photo diode type. The rotor plate (shutter), which turns in conjunction with the transmission output speed via the speedometer cable, has several slits. Photo diodes and light emitting diodes (LED's) are used for the wave forming circuit.
When the rotor passes between the photo diodes and LED's the slits in the rotor plate cut the light being transmitted to the photo diode from the LED. This generates pulses which are changed into ON/OFF signals by the wave forming circuit. The ECU interprets these signals to calculate vehicle speed and distance traveled.