Vehicle Speed Sensor - VSS Sensor

The Speed Sensor - VSS Sensor, is one of the most important components of the vehicle. It is associated with several systems and its electronic synchronization allows the vehicle to move smoothly.

What is the VSS Sensor?

vss sensor

This device is responsible for telling the automotive head unit how fast the vehicle is moving.  The VSS senses the rotation or revolutions of a mechanical component of the vehicle and sends the signal to the vehicle's internal computer. Thanks to the vehicle's speed sensor, the ECU is able to control other functions related to the vehicle's travel.

How does the VSS sensor work?

Some speed sensors have a magnetized end, which captures the rotary motion by generating pulses. In other cases, they base their operation on the HALL effect. In turn, a signal proportional to the rotational speed is generated. This signal is received by the ECU and its frequency is variable according to the vehicle's speed.

The function of the Speed Sensor

With time and technological advancement, the function of this sensor has developed. Before, it was only a component that indicated when to make a shift in the automatic transmission. Now, the VSS sensor indicates at what speed the vehicle is running, and goes on to control other indicators such as the speedometer, the odometer, and the power converter.

When this speed sensor sends the signal, the computer can synchronize the operation of other vehicle systems. For example, ignition timing, fuel injection, ABS, and traction control.

VSS sensor function features

  • The speed sensor sends signals to the engine depending on the speed that which the driver actuates the accelerator, i.e. if the driver drives slowly, the sensor will send low frequencies to the engine, and if he drives fast, the sensor will send high frequencies.
  • The sensor also fulfills the function called cut-off, which is responsible for the car not consuming fuel while the driver does not press the accelerator.
  • The sensor is composed of a magnet, which moves as the engine accelerates, generating a magnetic field that forms the current in the coil.

Types of Speed Sensor

There are at least two types. One difference between them is the type of signal they emit, they can send voltage signals by alternating current or an analog signal. So, we briefly detail their classification:

Magnetic sensor.

In this model, the function is performed by a magnet at one end. The magnetic impulse generates a signal of low intensity or voltage.

Optical sensor

This type emits an LED light that projects on a specific point. There must also be a phototransistor that emits it to the ECU as pulses.

Speed Sensor Failures

One of the most common failures is positional. If the space between the sensor and the rotating part or component is too wide, it will not be able to capture the movement or exert the magnetism to send the signal. This distance can be affected over time, either by dirt or wear on the end of the VSS sensor. This causes erroneous readings. Since the wave amplitude or frequency is very wide.

In the case of magnetized sensors, there are cases of damaged coil, deteriorated cables, and sulfated connections.

How to test the condition of the VSS sensor

You should use the oscilloscope since the signal can branch to several systems. You will want to check your vehicle's schematic to try to understand where and in which way the sensor signal is traveling. This will help you confirm if it is reaching the module.

Also, you will need to test the wiring for continuity and resistance, for which you can use a multimeter.

When a fault occurs in the speed sensor, one of the following ODBII codes may be present:

  • P0500: There is a fault in the speed sensor.
  • P0501: The speed is out of range.
  • P0502: The input signal is too low.
  • P0503: Voltage is intermittent.

When the VSS Sensor is diagnosed in this way, it exhibits the following symptoms:

  • Check engine light on.
  • The brake light is on in the panel.
  • The speedometer runs erratically, up and down.
  • Erratic or common jerking.
  • Abrupt speed changes in gear.
  • ABS failure.

How to clean the Speed Sensor?

The VSS sensor is checked for dirt. The device and its wiring can be cleaned. However, it must be handled with extreme care. Follow these steps to clean the VSS sensor:

  1. Locate the sensor and make sure if it is an open or covered one. If you see the reference ring on the back of the brake rotor, then it is an open sensor. Disguised or hidden sensors are located inside the differential housing. These do not require regular cleaning or maintenance because they are hidden from dust and dirt.
  2. Unbolt the sensor and carefully separate it from the bracket. Be careful with the wires around the housing so as not to damage them.
  3. Spray a cloth with a degreaser and carefully clean the sensor, especially the upper part, because it is the most sensitive.
  4. With a brush and degreaser, remove the dirt from the reference ring. Clean between the holes of the ring until they are free of dirt.
  5. Put the sensor back in place.

Where is the VSS sensor located?

The location of the vehicle speed sensor varies by manufacturer. Generally, it is housed in the transmission or on the rear axle. In other cases, it is in the cluster speedometer cable. You can find out the exact location of the Speed Sensor by acquiring the car's electrical diagrams.

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