Powertrain Controls - ECM/PCM

DTC 24 Vehicle Speed Sensor Circuit:

Vehicle Speed Sensor (VSS) Circuit:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) applies and monitors 12 volts on CKT 381. CKT 381 connects to the vehicle speed buffer which alternately grounds CKT 381 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the PCM will calculate vehicle speed based on the time between "pulses."
Tech 1 scan tool reading should closely match the speedometer reading with drive wheels turning.
To prevent misdiagnosis, the technician should identify the type of Vehicle Speed Sensor (VSS) used prior to using this chart. Disregard a Diagnostic Trouble Code (DTC) 24 set when drive wheels are not turning.

DIAGNOSTIC TROUBLE CODE (DTC) 24 WILL SET WHEN
Vehicle speed is less than 2 mph, engine speed greater than 1400 RPM, Throttle Position (TP) angle greater than 5%, load condition is less than 25 kPa of vacuum with all conditions met for 30 seconds.

ACTION TAKEN (PCM will default to)
The Malfunction Indicator Lamp (MIL) will become illuminated.

DTC 24 WILL CLEAR WHEN
A current DTC 24 will clear after each ignition key cycle or detects vehicle speed greater than 2 mph. A history DTC 24 will clear after 50 consecutive ignition key cycles without a current DTC 24 being stored.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Disregard DTC 24 that sets when drive wheels are not turning.
2. 8-12 volts, at the I/P connector, indicates CKT 381 is open between the I/P connector and the VSS, or there is a faulty vehicle speed sensor. A voltage of less than 1 volt at the I/P connector indicates that CKT 381 wire is shorted to ground. If, after disconnecting CKT 381 the voltage reads above 10 volts, the vehicle speed buffer is faulty. If voltage remains less than 8 volts, then CKT 381 wire is grounded. If CKT 381 is not grounded, there is a faulty connection at the PCM, or a faulty PCM.

DIAGNOSTIC AIDS
Tech 1 scan tool should indicate a vehicle speed whenever the drive wheels are turning.
A faulty or misadjusted Park/Neutral Position (PNP) switch can result in a false DTC 24. Use a Tech 1 scan tool and check for proper signal while in drive. Refer to Chart C-1A Park/Neutral Position Switch Diagnosis under Computers and Control Systems/System Diagnosis/Diagnostic Charts/C Charts for PNP switch diagnosis check. Chart C-1A Park/Neutral Switch Diagnosis
If all OK, refer to Intermittents under Computers and Control Systems/System Diagnosis/Diagnosis By Symptom. - Intermittent Malfunctions