2500 4x2 Pickup V8-318 5.2L Magnum (1994)
Vehicle Speed Sensor: Description and Operation
Vehicle Speed Sensor (VSS) Typical
PURPOSE
The Vehicle Speed Sensor (VSS) is used to inform Powertrain Control Module (PCM) of vehicle deceleration and engine idle. The PCM uses this
information to control Idle Air Control (IAC) motor position during these conditions.
PCM uses sensor input with input from Throttle Position Sensor (TPS) and VSS to differentiate between closed throttle deceleration or closed
throttle idle.
During closed throttle deceleration, the PCM actuates IAC motor to maintain desired Manifold Absolute Pressure (MAP) sensor value.
During idle, PCM actuates the IAC motor to maintain desired engine idle speed.
OPERATION
The VSS has a set of contacts inside that are opened and closed by a rotating 8 pole permanent magnet. As the magnet rotates, the magnetic field
opens and closes the contacts 8 times per revolution, or 8000 times per mile.
The PCM sends a 8 volt signal to the VSS. If the sensors contacts are open, the voltage stays at 8 volts. If the contacts are closed, the voltage will
drop to 0 volts.
