Powertrain Controls - ECM/PCM
Diagnostic Trouble Code 24:
Vehicle Speed Sensor Circuit:
CIRCUIT DESCRIPTION
The Engine Control Module (ECM) applies and monitors 12 volts on CKT 817. CKT 817 connects to the Vehicle Speed Sensor (VSS) buffer module which alternately grounds CKT 817 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between "pulses."
Scan tool reading should closely match with speedometer reading with drive wheels turning.
TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. DTC 24 will set if vehicle speed is greater than or equal to 5 mph when:
^ Engine speed is between 1200 and 4400 RPM.
^ TP sensor is less than 3% (closed throttle).
^ Low load condition (low Manifold Absolute Pressure (MAP) voltage, high manifold vacuum).
^ All conditions met for 5 seconds.
These conditions are met during a road load deceleration.
2. 8-12 volts, at the Vehicle Speed Sensor (VSS) buffer module connector, indicates there is a faulty vehicle speed sensor. A voltage of less than 1 volt indicates that CKT 817 wire is open or shorted to ground. If after disconnecting CKT 817 at the VSS buffer module, the voltage reads above 10 volts, the vehicle speed sensor is faulty.
If voltage remains less than 8 volts, then CKT 817 wire is grounded. If CKT 817 is not grounded, there is a faulty connection or open at the ECM or a faulty ECM.
DIAGNOSTIC AIDS
The Tech 1 scanner should indicate a vehicle speed whenever the drive wheels are turning.
A faulty Park Neutral Position (PNP) circuit can result in a false DTC 24. Use a Tech 1 scanner and check for proper signal while in drive. Refer to Chart C-1A Park/Neutral Position Switch Diagnosis for PNP relay diagnosis check. Chart C-1A Park/Neutral Switch Diagnosis
Check CKT 400 and CKT 401 for proper connections to be sure they are clean and tight, and the harness is routed correctly.
Refer to Intermittents in Diagnosis By Symptom. Testing and Inspection