Speedometer

The Speedometer applies voltage to CKT 389. Each time the Vehicle Speed Sensor Buffer (VIN D), or the PCM (VIN L) grounds CKT 389 the Speedometer sees one "pulse." The Vehicle Speed Sensor Buffer grounds CKT 389 to produce a square wave at the rate of 4000 pulses per mile, The Speedometer converts the frequency of the square wave to vehicle speed. A high frequency square wave indicates a high vehicle speed, while a low frequency square wave indicates a low vehicle speed.