DTC 24, Vehicle Speed Signal Low (With 4L80-E A/T): Notes

The PCM/TCM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 connects to Vehicle Speed Sensor (VSS) buffer, which alternately grounds circuit No. 437 when receiving voltage pulses from VSS (drive wheels are turning). The pulsing action occurs approximately 2000 times per mile and the ECM calculates vehicle speed based on the time between pulses. Scan tester reading should closely match speedometer reading with drive wheels turning. DTC 24 sets when the following conditions are met for 5 seconds:

• Transmission is not in Park or Neutral.
• Circuit No. 437 voltage is constant.
• Engine speed is more than 1200 RPM.
• VSS signal indicates less than 2 MPH.
  NOTE: Test numbers refer to numbers on diagnostic chart.

1. This test monitors voltage on circuit No. 437. With wheels turning, the pulsating action causes a 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 PCM/TCM connector indicates circuit No. 437 is shorted to ground. Disconnect circuit No. 437 at VSS buffer. If voltage now reads more than 10 volts, the VSS buffer is faulty. If voltage remains less than 10 volts, then circuit No. 437 is grounded. If circuit No. 437 is not grounded, check for a faulty PCM/TCM connector or faulty PCM/TCM.
3. A steady 8-12 volt reading at the PCM/TCM connector indicates circuit No. 437 is open or VSS buffer is faulty.
4. This is normal voltage, which indicates a possible intermittent condition.