Speed Sensor failures are often the main cause of taking the car to the repair shop. The Speed Sensor is a device that measures how fast the vehicle moves, and sends this signal to the ECU automotive computer.

The ECU uses this signal to calculate the exact time the automatic transmission should change gear. It also calculates fuel injection time, and ignition time.

The VSS Speed Sensor is also fundamental for the correct operation of the speedometer of the board or cluster.

This sensor is located in the gearbox on the output shaft, or also in the crankshaft of the car. There will always be two sensors for the computer to make a comparison of these signals.

Speed sensor faults and Vss sensor faults common What to do?

The automatic transmission shift sensor is related to the automatic transmission speed sensor. However, the symptoms of automatic transmission sensor failure are different.

The speed sensor works on a very simple principle: there is a sprocket that goes to either the output shaft of the transmission or the crankshaft of the engine.

Automatic transmission speed sensor

When the shaft rotates, the sprocket attached to it also rotates. This wheel comes with a free space of one or more of its teeth. And on the other side is the sensor which is an optical sensor, which detects the signals from each of those teeth of the wheel as they pass in front of it.

When it passes the white space of the wheel in front of the speed sensor, it detects that a complete turn has been completed, and this signal is sent to the ECU to calculate the speed.

Symptoms of Speed Sensor Faults

A Speed sensor can fail for many reasons, however the main causes are the following:

  • Contamination at the sensor tip or corrosion on the sprocket.
  • Broken wiring.
  • Disconnection of the Sensor to the ECU.
  • Bad Sensor installation or too far away from the sprocket.
  • Sensor with defective internal resistance causing a slow measurement.
  • Damage to the speed sensor coil.

However, if you detect any of these symptoms, take your car to a repair shop:

  • Speedometer that doesn’t work or works slowly: Cars with an electronic injection system use the signal from the speed sensor to display speed on the dashboard. Therefore a faulty sensor causes the speed not to be displayed or to be displayed slowly.
  • Check Engine Light Activated: If the ECU does not detect the speed sensor signal, it will activate the Check Engine’s dashboard fault indicator light or light, in this case the car should be taken for inspection with an Automotive Scanner.
  • Slow Change in Automatic or Mechanical Transmission Gears: When there are VSS speed sensor faults, the ECU cannot send the signal to the transmission computer, to indicate that the solenoids must make the gear changes. Therefore one of the causes that must be taken into account to detect faults in the speed sensor is this. It also affects cars with manual transmissions, since these modern systems use this signal to be more efficient in the adjustments of the gear changes.
  • Problems with Cruise Control: One cause or reason for suspicion that the VCC Speed Sensor is failing, is when the cruise control is not activated, or the speed is not increased or decreased when the cruise control is active. This is because when cruise control is active, it depends on the signal from the speed sensor to sustain the gear on long trips.

Technical Manual on the Operation of the VSS Speed Sensor

The Vehicle Speed Sensor (VSS) is a variable reluctance or Hall effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle.

If the vehicle is moving at a relatively low speed, the sensor produces a signal with a low frequency. As the speed of the vehicle increases, the sensor generates a signal with a higher frequency.

The PCM or ECU uses the frequency signal generated (Input Signals) by the Vehicle Speed Sensor VSS to control parameters such as fuel injection, ignition control, transmission change programming and clutch converter programming.

How Speed Sensors Work Today

Nowadays the speed sensors are mostly hall effect guaranteed the quality of the digital signal always (even at low speeds), the speed sensor signal is received by the PCM and is used by multiple systems incorporated into the vehicle through the CAN network, as is the case of stability control systems, ABS braking systems, control and learning in automatic transmissions, injection management and ignition time.