How Do You Troubleshoot a Speedometer?

The techniques for troubleshooting a speedometer vary according to the age of the vehicle. For example, cars built before 1990 made use of a cable, while newer cars use speed sensors that determine how fast the vehicle is traveling. Older vehicles may require the cable to be adjusted, while newer vehicles may require replacement of the speed sensor.

For older vehicles, or those built before 1990, the first thing to check is the speedometer cable. Inspect the cable and see if there is any slack or wear on it. If there is, proceed to replace the cable.

Newer vehicles use a vehicle speed sensor. The vehicle speed sensor, also known as the VSS, will be mounted somewhere on the transmission. This sensor reads how fast certain gears in the transmission spin and sends the information back to the speedometer, which displays the speed of the vehicle. If these sensors are caked in oil or cracked, the speedometer will fail to work. Use the following instructions to troubleshoot a speed sensor on a newer vehicle.

  1. Inspect the sensor
  2. Locate the vehicle speed sensor on the transmission and see if it is disconnected, cracked or dirty.

  3. Run a diagnostics check
  4. While the sensor is connected, turn the car on and plug in a code scanner. Run the scan to see if any codes appear. If the vehicle speed sensor shows up, replace the unit.