Circuit Description

The powertrain control module (PCM) creates the engine speed output signal by pulsing the circuit to ground at a predetermined hertz rate. The PCM pulses the circuit at the same rate as the engine speed signal input. The PCM monitors the voltage on the engine speed output circuit. If the PCM determines the voltage is out of the normal operating range, a DTC sets.

The PCM supplies a voltage to the engine speed output circuit. The PCM creates the engine speed output signal by rapidly grounding this circuit via an internal switch called a driver. The other components on this circuit recognize the voltage being pulled to ground as an indication of engine speed. The PCM driver supplies the ground for the component being controlled. Each driver has a fault line which the PCM monitors. When the PCM commands a component ON, the voltage of the control circuit should be LOW (near 0 volts). When the PCM commands the control circuit to a component OFF, the voltage potential of the circuit should be HIGH (near battery voltage). If the fault detection circuit senses a voltage other than what is expected, the fault line status changes, causing DTC P0654 to set.