Vehicle Speed Sensor: Description and Operation

TRANSMISSION CONTROL MODULE -- AUTO TRANS -- PCM INPUT
The Transmission Control Module (TCM) supplies 3 inputs to the PCM:
- Vehicle Speed (road speed)
- Torque Management
- Gear Position

Vehicle Speed
The TCM supplies vehicle speed and distance traveled inputs to the PCM. From these inputs and the throttle position sensor input, the PCM determines when a deceleration condition occurs.

Torque Management
The PCM receives a torque management input from the TCM. The PCM receives the input when the transmission shifts gears. In response, the PCM shuts off a number of fuel injectors when the transmission shifts gears.

Vehicle Speed And Distance
The 4 speed automatic Transmission Control Module (TCM) supplies the speed input to the PCM. The PCM determines acceleration rates. The speed control software in the PCM uses vehicle speed and acceleration to control to the set speed.

Vehicles with a 3 speed automatic or manual transmission have a vehicle speed sensor (VSS) mounted to an adapter near the transmission output shaft. The sensor is driven through the adapter by a speedometer pinion gear. The VSS pulse signal is monitored by the PCM to determine vehicle speed and to maintain speed control set speed. Refer to the appropriate Powertrain Management/Computer and Control System for diagnosis and testing of this component

Fig. 24 Vehicle Speed Sensor--Manual Transmission:

VEHICLE SPEED SENSOR - PCM INPUT (MANUAL TRANSMISSION)
The vehicle speed sensor is located in the transmission extension housing Fig. 24. The sensor input is used by the PCM to determine vehicle speed and distance traveled, manual transmission only. Refer to Transmission Control Module for automatic transmission.

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

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 idle conditions, the PCM adjusts the IAC motor to maintain a desired engine speed.

The vehicle speed sensor signal is also used to operate the following functions or systems:
- Speedometer
- Speed Control
- Daytime Running Lights (Canadian Vehicles Only)
- Open Throttle IAC position.