How Vehicle Speed Sensors Improve Performance

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Purpose

Vehicle speed sensors (VSS) are used in late model vehicles to monitors vehicle speed. Depending on make and model, vehicle's computer uses the signal from the vehicle speed sensor to control a number of functions, including:

• Emission control functions
• Power steering assist
• Electronic shock control
• Idle air control motor operation
• Canister purge
• Torque converter clutch lockup
• Gear change (shifting) for automatic transmissions

In some applications, the vehicle speed sensor replaces the mechanical speedometer cable as a means of providing input to an electronic speedometer or electronic cruise control system.

Location

Depending on the sensor type, the vehicle speed sensor may be located on the transmission, differential, transaxle (permanent magnet-type) or speedometer head (photo transistor-type).

Operation

There are two primary types of vehicle speed sensors in use today; the photo transistor sensor and the reluctance sensor. While the specific implementation of these sensors varies between vehicle makes and models, operation of each type of sensor is essentially the same, as described below.

Photo Transistor Sensors

The photo transistor-type sensor is comprised of a light emitting diode (LED), a photo transistor and a mechanically-controlled shutter. As the mechanical speedometer cable rotates, it actuates a shutter positioned in front of the LED. As the shutter operates, it alternately blocks and opens the path between the LED and the photo transistor, resulting in a "blinking" effect at the photo transistor. The photo transistor generates a "pulsing" output signal that is proportional to the vehicle's speed. The vehicle's computer uses the signal to calculate the exact vehicle speed.

Reluctance Sensors

Reluctance sensors monitor the speed of rotating parts by using a magnet to sense the position of gaps (notches) in a rotating "reluctor" ring. The passing of the notches across the magnet causes a change in the magnetic field, resulting in an alternating current (AC) output (a series of high- and low voltage pulses) from the sensor. The frequency of the AC signal increases as vehicle speed increases.

Some vehicle applications use an "AC-to-DC converter" between the sensor and the vehicle's computer to convert the alternating output of the sensor to a digital voltage for use by an electronic speedometer and/or engine control.

Drivability Symptoms

The drivability symptoms associated with a malfunctioning or defective vehicle speed sensor depend on the specific control functions that utilize the speed signal input. Typical examples include:

• Torque converter clutch control: Premature lockup or no lockup
  
• Variable assist power steering: no change in steering "feel" with increase in speed
  
• Electronic instrumentation (speedometer, tachometer, etc.): Unstable, inaccurate or fluctuating indications
  
• Stalling, stumbling or rough idle
  
• Degradation of cruise-control and/or automatic transmission operation.
  

Testing

The procedures for testing vehicle speed sensors vary depending on vehicle make and model, and on the specific application of the sensor. Testing most vehicle speed sensors requires the use of dedicated test equipment such as a scan tool, signal generator or simulator tool.

Refer to the vehicle's service manual for test equipment requirements and test procedures.

Repair/Replacement

The vehicle speed sensor is a sealed unit. If you determine it is defective, it must be replaced. No repair or adjustment is possible.

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