The back-up speed sensor is built into the combination meter assembly and is operated by the speedometer cable. The sensor consists of an electrical reed switch and a multiple pole permanent magnet assembly. As the speedometer cable turns, the permanent magnet rotates past the reed switch. The magnetic flux lines between the poles of the magnet cause the contacts to open and close as they pass. The sensor outputs four pulses for every one revolution of the speedometer cable.
The sensor can also be a photocoupler type which uses a photo transistor and light-emitting diode (LED). The LED is aimed at the phototransistor and separated by a slotted wheel. The slotted wheel is driven by the speedometer cable. As the slotted wheel rotates between the LED and photo diode, it generates 20 light pulses for each rotation. This signal is converted within the phototransistor to four pulses sent to the ECU.
Was this article helpful?