K 1500 Truck 4WD V8-393 6.5L DSL (1994)
Vehicle Speed Sensor Signal Buffer: Description and Operation
DESCRIPTION
The Speedometer/Odometer is electronic and does not require a speedometer cable. The speedometer receives a vehicle speed signal from the
Vehicle Speed Sensor Buffer. The Vehicle Speed Sensor Buffer receives an AC voltage signal from the transmission mounted Vehicle Speed
Sensor (VSS).
The VSS generates a signal that indicates the speed of the vehicle. The Signal is processed by the solid-state Vehicle Speed Sensor Buffer to
supply inputs to the Powertrain Control Module (PCM), the Cruise Control Module and the Speedometer.
OPERATION
The VSS is mounted in the transmission. A toothed rotor rotates near a coil, producing voltage pulses in the coil.
The rotor that is attached to the transmission turns four times faster than a standard speedometer cable. The coil near the rotor generates 40 pulses
per revolution.
The frequency of the AC voltage coming from this coil depends on the vehicle speed. As the speed increases, so does the number of voltage pulses
per second.
The Vehicle Speed Sensor Buffer takes the voltage pulses from the sensor and uses them to close three solid-state output switches. Each output
terminal is switched to ground at a rate that is proportional to the speed of the vehicle. The Speedometer is switched by the primary rate of the
VSS. The PCM and the Cruise Control use a lower frequency. Their input switches are operated by a circuit that divides the sensor frequency by
two.
Different Vehicle Speed Sensor Buffers are used to match the vehicle final drive ratio to the components.
