Vehicle Speed Sensor: Description and Operation

VEHICLE SPEED SIGNAL-PCM INPUT

Fig.9 Vehicle Speed Sensor-Manual Transmission:

The vehicle speed sensor is located in the transmission extension housing.

Fig.10 Output Speed Sensor-Automatic Transaxle:

Automatic
The transaxle control module (TCM) supplies the vehicle speed signal to the PCM based on the output shaft speed. The PCM sends a 5 volt signal to the TCM. The TCM switches this signal to a ground, and then opens the circuit at a rate of 8000 pulses per mile. When the PCM counts 8000 pulses, the PCM assumes the vehicle has traveled one mile. The output speed sensor is located on the side of the transaxle.

The speed and distance signals, along with a closed throttle signal from the TPS, determine if a closed throttle deceleration or normal idle condition (vehicle stopped) exists. Under deceleration conditions, the PCM adjusts the idle air control motor to maintain a desired MAP value. Under idle conditions, the PCM adjusts the idle air control motor to maintain a desired engine speed.

Manual
The sensor input is used by the PCM to determine vehicle speed and distance traveled and is a Hall-effect sensor.

The Hall-effect 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.

Like all Hall-effect sensors, the electronics of the sensor needs a power source. This power source is provided by the PCM. The sensor switches a 5 volt signal sent from the PCM from a ground to an open circuit. It is the same 9 volt power supply that is used by the CKP and CMP sensors.

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).

The VSS used on 3 speed automatic and manual transaxle vehicles This sensor is mechanically driven by a pinion gear that is in mesh with the right axle drive shaft. When the PCM counts 4000 pulses, the PCM assumes the vehicle has traveled one mile.