Code 24, Vehicle Speed Sensor: Notes

The ECM supplies and monitors 12-volts signal on circuit No. 437. Circuit No. 437 connects Vehicle Speed Sensor (VSS) which alternately grounds circuit No. 437 when drive wheels are turning. This pulsating action takes place about 2000 times per mile, and the ECM calculates vehicle speed based on time between "pulses". SCAN tester reading should closely match speedometer reading with drive wheels turning.

Code 24 will be set if the following conditions exist:

• Circuit No. 437 voltage is constant.
• Engine speed is more than 200 RPM.
• Vehicle speed signal (voltage on terminal "A9") is less than 10 MPH.
• All conditions must be met for 10 seconds.

The above conditions are met during a road load deceleration.

1. This monitors ECM voltage on circuit No. 437. With wheels turning, pulsing action will result in varying voltage. Variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 MPH.
2. Voltage of less than one volt at ECM connector indicates that circuit No. 437 is shorted to ground. Disconnect circuit No. 437 at VSS. If voltage now reads greater than 10 volts, VSS is faulty. If voltage remains less than 10, circuit No. 437 is grounded. If circuit No. 437 is not grounded, check for faulty ECM connector or ECM.
3. Steady 8-12 volts at ECM connector indicates circuit No. 437 is open, or faulty VSS.
4. This is normal voltage that indicates possible intermittent condition.