Code 24, Vehicle Speed Sensor Circuit: Notes

The ECM applies and monitors 12 volts on circuit No. 437 to the Vehicle Speed Sensor (VSS). The circuit is opened and closed at approximately 2000 times per mile when the drive wheels are turning. The ECM calculates vehicle speed based on the time between pulses. SCAN tester reading should closely match speedometer reading with drive wheels turning. Code 24 will set if: circuit No. 437 is constant, engine speed is more than 200 RPM, vehicle speed signal at terminal ECM A9 is less than 10 MPH (16 KM/H), all conditions are met for at least 10 seconds.

NOTE: Test numbers refer to test numbers on diagnostic chart.

Test Number 1) This test monitors the ECM voltage on circuit No. 437. With wheels turning, the pulsating action causes varying voltage. The variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 MPH (32 KM/H).

Test Number 2) Less than one volt at ECM connector indicates circuit No. 437 wire is shorted to ground. Disconnect VSS connector. If voltage now reads more than 10 volts, the VSS is faulty. If voltage remains less than 10 volts, then wire is grounded. If circuit No. 437 is not grounded, check for a faulty ECM connector or ECM.

Test Number 3) A steady 8-12 volt reading at the ECM connector indicates the circuit No. 437 is open or VSS is faulty.

Test Number 4) Normal voltage indicates a possible intermittent condition.