Code 24, Vehicle Speed Sensor: Notes

The ECM applies and monitors 12 volts on circuit No. 437 to VSS. The VSS alternately grounds and opens circuit No. 437 when drive wheels are turning. The pulsing action is supplied to ECM. The ECM calculates vehicle speed based on time between pulses.

A Code 24 will be set under the following conditions:

• Code 21, 22, 33 or 34 is not present.
• Speed sensor indicates less than 5 MPH.
• Engine speed is 1500-4400 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 circuit No. 437. This test verifies circuit No. 437 is not shorted to a voltage source. Test light should not come on when circuit No. 437 is probed.
  3. By probing circuit No. 437 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 circuit No. 437 by ECM. Test must be done using a voltmeter.
  5. Circuit No. 439 is the ignition feed, which supplies operating power to VSS.
  6. Circuit No. 450 supplies ground path for VSS operation. If circuit No. 450 is open, VSS cannot pulse circuit No. 437 to ground.