When the speed sensor on a 1999 Chevrolet Silverado begins to fail, the first noticeable sign may be a fluctuating speedometer. The speedometer’s needle may jump from one speed to another, or fall to zero and then suddenly return to the correct speed. The vehicle speed sensor, or VSS, is the sensor responsible for relaying the vehicle’s speed to the engine computer. In addition to using data from the VSS to calculate the vehicle’s speed, it also uses this data to make other computational decisions, including when to shift the transmission. The VSS is located on the tailshaft of the transmission or on transfer case in four wheel drive models. The sensor itself is a magnetic inductive sensor that monitors the magnetic field around a notched, reluctor wheel affixed to the tailshaft. The notches in this wheel interfere with the sensor’s magnetic field and the engine computer calculates the frequency of this interference and converts it into a speed value.
Turn off the engine, but leave the ignition on. Connect a scan tool to the Silverado’s scan tool terminal.
Navigate to the live data menu and select “VSS.” Test drive the vehicle in a safe, controlled area such as an empty parking lot where there is no possibility of hitting other cars or objects.
Look down at the scan tool’s display and note the speed value for the VSS. It should reflect the vehicle’s current speed. If the speed displayed is “0” or different from the speedometer reading, it’s likely there is a problem with the VSS or the VSS harness connector.
Note the voltage value for the VSS on the scan tool. This value should be listed below or above the VSS speed value. It should read between 1.5 and 12 volts. If the value is outside this range, the VSS is either not receiving the proper voltage or is faulty.
Raise the vehicle with an automotive jack and support it with jack stands. Locate the VSS on the tailshaft of the transmission of transfer case for vehicles equipped with four wheel drive. Disconnect the VSS’s electrical connector.
Turn the ignition on and probe the light blue wire on the connector with a digital multimeter set to DC volts. The multimeter should show between 1.5 and 12 volts. Connect a test lamp to a positive voltage source and probe the black, ground wire on the VSS connector. The test lamp should illuminate if the ground properly connected. If the ground is OK and you find voltage at the connector, then the VSS is faulty and needs to be replaced.
