Powertrain Controls - ECM/PCM

Code 24 Chart:

Wiring Diagram For VSS, and Park/Neutral Relay.:

CIRCUIT DESCRIPTION:

The ECM applies and monitors 12.0 volts on circuit 437. Circuit 437 connects to the Vehicle Speed Sensor (VSS), which alternately grounds circuit 437 when the 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. A "Scan" tool reading should match with the speedometer reading, with the drive wheels turning. Disregard a Code 24 if the drive wheels are not turning. To prevent misdiagnosis, the technician should identify the type of vehicle speed sensor used prior to using the Code 24 chart.


TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.

1. Code 24 will set if the vehicle speed is less than 8 km/h (5 mph) when:
a. Engine speed is between 1200 and 4400 rpm.
b. TPS is less than 3% throttle opening (closed throttle).
c. Low load condition (low MAP voltage, high manifold vacuum).
d. All conditions must be met for 5 seconds. These conditions are met during a road load deceleration.

2. A voltage between 8.0-12.0 volts, at the Vehicle Speed Sensor (VSS) buffer connector, indicates there is a faulty vehicle speed sensor. A voltage of less than 1.0 volt indicates that the wire in circuit 437 is open or shorted to ground. If after disconnecting circuit 437 at the VSS buffer, the voltage reads above 10.0 volts, the vehicle speed sensor is faulty. If the voltage remains below 8.0 volts, then circuit 437 wire is grounded. If circuit 437 is not grounded, there is a faulty connection or open at the ECM, or a faulty ECM.


DIAGNOSTIC AIDS:

1. A "Scan" tool should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph.
2. A faulty or mis-adjusted PARK/NEUTRAL switch can result in a false Code 24. Use a "Scan" tool and check for a proper signal while in drive. See CHART C-1A for P/N switch diagnosis.
3. Check circuit 400 and circuit 401 for proper connections to be sure they are clean and tight, and the harness is routed correctly.
4. If a Code 24 is intermittent, see DIAGNOSIS BY SYMPTOM.