Code 24, Vehicle Speed Sensor Circuit: Notes

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 connects to Digital Ratio Adapter Controller (DRAC), which alternately grounds and opens circuit No. 437 when drive wheels are turning. The circuit is opened and closed approximately 2000 times per mile when drive wheels are turning. 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 after the following conditions are met for at least 10 seconds: circuit No. 437 is constant, engine speed is more than 2000 RPM, vehicle speed signal at ECM terminal A9 is less than 5 MPH.

1. This test monitors ECM voltage on circuit No. 437. With wheels turning, the pulsating action causes varying voltage. Voltage variation will be greater at low wheel speeds, and an average of 4-6 volts at about 20 MPH.
2. Less than one volt at ECM connector indicates circuit No. 437 wire is shorted to ground. Disconnect circuit No. 437 at DRAC. If voltage now reads more than 10 volts, the DRAC is faulty. If voltage remains less than 10 volts, then circuit No. 437 wire is grounded. If circuit No. 437 is not grounded, check for a faulty ECM connector or ECM.
3. A steady 8-12 volts reading at the ECM connector indicates that circuit No. 437 is open or DRAC is faulty.
4. Normal voltage indicates a possible intermittent condition.