Circuit Description

Powertrain Control Module (PCM) creates vehicle speed output signal by rapidly grounding Vehicle Speed Sensor (VSS) output circuit via an internal switch called a driver. Driver operates at same rate as VSS signal input. Various modules recognize voltage being pulled to ground as an indication of vehicle speed. Driver supplies ground for module being controlled. When PCM commands a module ON, voltage of the control circuit should be low (near zero volts). When PCM commands control circuit to a module OFF, voltage potential of the circuit should be high (near battery voltage).