Vehicle Speed Sensor: Operation

The VSS is a 3-circuit (3-wire), magnetic, hall-effect sensor.

The 3 circuits are:

• A 5-volt power supply from the Powertrain Control Module (PCM).
• A ground is provided for the sensor though a low-noise sensor return circuit in the PCM.
• An input to the PCM is used to determine vehicle speed and distance traveled.

The speed sensor generates 8 pulses per sensor revolution. These signals, in conjunction with a closed throttle signal from the throttle position sensor, indicate a closed throttle deceleration to the PCM. When the vehicle is stopped at idle, a closed throttle signal is received by the PCM (but a speed sensor signal is not received).

Under deceleration conditions, the PCM adjusts the Idle Air Control (IAC) motor to maintain a desired MAP value. Under idle conditions, the PCM adjusts the IAC motor to maintain a desired engine speed.