Code 24 - Vehicle Speed Sensor (VSS)

ECM supplies and monitors 12-volt signal on circuit No. 437. Vehicle Speed Sensor (VSS), located in the I.P. cluster, will sense the speedometer rotating element and furnish this information to the buffer as a "pulsed" signal (2000 per mile). The buffer assembly will switch circuit No. 437 to ground for each pulse received. The ECM uses the time between pulses to determine vehicle speed.

NOTE: Disregard a Code 24 set, when drive wheels are not turning.

NOTE: The following step numbers refer to the numbers in the accompanying flow chart(s).

This test monitors the ECM voltage on circuit No. 437. With the wheels turning, the pulsing action will result in a varying voltage. The variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 MPH.
A voltage of less than one volt at the ECM connector indicates that the circuit No. 437 wire is shorted to ground. Disconnect circuit No. 437 at the Vehicle Speed Sensor. If voltage now reads above 10 volts, the Vehicle Speed Sensor is faulty. If voltage remains less than 10 volts, circuit No. 437 wire is grounded. If circuit No. 437 is not grounded, check for a faulty ECM connector or ECM.
A steady 8-12 volts at the ECM connector indicates circuit No. 437 is open or a faulty Vehicle Speed Sensor.
This indicates a possible intermittent condition.
If "Scan" displays vehicle speed, check park/neutral switch (CHART C1A) or vehicle with automatic transmission. If switch is okay, check for intermittents.

Fig 1: Code 24, Schematic: Vehicle Speed Sensor (VSS)

Fig 2: Flow Chart, Code 24: Vehicle Speed Sensor (VSS)

NOTE: To prevent misdiagnosis, the technician should review electrical section and identify the type of vehicle speed sensor used prior to using this chart. Disregard Code 24 if set when drive wheels are not turning.