Vehicle Speed Sensor: Description and Operation

1986-87 Models
The speed sensor consists of a permanent magnet generator that is directly driven by the transmission. The output of the permanent magnet generator is a signal with a repetition rate of 4000 pulses per mile. The signal frequency is proportional to vehicle speed and cycles at a rate of 1.1 Hz./mph. The voltage generated will vary between .5 and 40 volts.
The speed sensor produces a weak voltage signal and must be used for several speed inputs, therefore, a buffer amplifier is positioned between the speed sensor and the ECM to amplify and distribute the speed signal, and invert this speed signal into a format which can be understood by the ECM.