Powertrain Controls - ECM/PCM

Code 24 Chart:

Code 24 Wiring Diagram:

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 and vehicle speed over 3 mph. Disregard a Code 24 if the drive wheels are not turning.


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

1. Code 24 will set if:
^ Engine speed is between 1200 and 4400 rpm.
^ TPS is less than 3% throttle opening.
^ Low load condition (low MAP voltage, high manifold vacuum).
^ All conditions must be met for 5 seconds. These conditions are met during a road load deceleration.

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


DIAGNOSTIC AIDS:

^ 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 or overdrive. Chart C-1A Park/Neutral Switch Diagnosisfor P/N switch diagnosis.
^ Check circuits 400 and 401 for proper connections to be sure they are clean and tight, and the harness is routed correctly.
^ If a Code 24 is intermittent, Testing and Inspection