Powertrain Controls - ECM/PCM

Code 24 Chart:

Wiring Diagram For VSS:

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.


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

1. Code 24 will set if:
a. Voltage in circuit 437 is constant.
b. Engine speed is between 1400 and 3600 rpm.
c. TPS is less than 2% throttle opening.
d. Low load condition.
e. Not in PARK or NEUTRAL.
f. All conditions must be met for 4 seconds. These conditions are met during a road load deceleration.

2. A voltage of less than 1.0 volt, at the 15 way I/P connector, indicates that the circuit 437 wire may be shorted to ground. Disconnect circuit 437 at the vehicle speed sensor buffer. If voltage remains less than 10.0 volts, then circuit 437 is grounded or open. If circuit 437 is not grounded or open, check for a faulty ECM connector or ECM.


DIAGNOSTIC AIDS:

1. If the "Scan" tool displays vehicle speed, check PARK/NEUTRAL switch, see CHART C-1A, on vehicles with automatic transmission. If the switch is OK, check for an intermittent connections.
2. An open or short to ground in circuit 437 will result in a Code 24.
3. If a Code 24 is intermittent, see DIAGNOSIS BY SYMPTOM.