Code 24, Vehicle Speed Sensor: Notes

The ECM applies and monitors 12 volts on the signal wire to VSS. The VSS alternately grounds and opens the signal circuit when drive wheels are turning. The pulsed signal is sent to the ECM. The ECM calculates vehicle speed based on pulse rate.

A Code 24 will be set under the following conditions:

• Code 21, 22, 33 or 34 does not exist.
• Speed sensor indicates less than 8 MPH.
• Engine speed is 1800-4400 RPM.
• P/N switch indicates gear is not in Park or Neutral.
• Coolant temperature is greater than 185°F (85°C).
• MAP signal indicates low pressure.
• Conditions listed exist for more than 3 seconds.
  NOTE: Test numbers refer to test numbers on diagnostic chart.

1. This test uses the scan tester to verify VSS operation.
2. The ECM supplies 12 volts to the signal wire . This test verifies signal wire circuit is not shorted to a voltage source. Test light should not come on when circuit is probed.
3. By probing signal wire with test light several times in a second, vehicle speed signal should be generated and displayed on scan tester.
4. This step checks for 12 volts being supplied to signal wire circuit by ECM. This test must be done with a voltmeter.
5. The Red/White/Black wire is the ignition feed, which supplies operating power to VSS.
6. The Black wire supplies ground path for VSS operation. If ground circuit is open, VSS cannot pulse circuit to ground.