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 if:

• Codes 21, 22, 33 or 34 ARE NOT present.
• Speed sensor indicates less than 5 MPH.
• P/N switch indicates NOT IN Park or Neutral.
• Coolant temperature greater than 185°F (85°C).
• MAP signal less than 14.5 in. Hg (1.6L) or 6.8 in. Hg (2.0L).
• Conditions listed exist for more than 3 seconds.

Test Number 1)  This test uses the Scan tester to verify VSS operation.

Test Number 2)  The ECM supplies 12 volts to circuit No. 437, but this test should not light a test light. This test verifies circuit No. 437 is not shorted to a voltage source.

Test Number 3)  By probing circuit No. 437 with a test light several times a second, vehicle speed signal should be generated and displayed on the Scan tester.

Test Number 4)  This test must be done using a voltmeter and will check for 12 volts being supplied to circuit No. 437 by the ECM.

Test Number 5)  Circuit 439 is the ignition feed supplying operating power to VSS.

Test Number 6)  Circuit No. 450 supplies ground path for VSS operation. If circuit No. 450 is open, VSS cannot pulse circuit No. 437 to ground.