DTC 24

Code 24 Chart:

Wiring Diagram For VSS Circuit:

CIRCUIT DESCRIPTION:

Vehicle speed information is provided to the ECM by the Vehicle Speed Sensor (VSS). It is a Permanent Magnet (PM) generator and is mounted in the transmission. The generator produces a pulsing voltage whenever the vehicle speed is over 3 mph. The voltage level and number of pulses increase as the vehicle's speed increases. The ECM converts the pulse voltage into miles per hour. This data is used for calculations and can be displayed on the "Scan" tool. The function of the VSS buffer has been incorporated into the ECM. The ECM amplifies the signal for the instrument panel, 4000 pulses per mile, for operating the odometer and speedometer. If the vehicle is equipped with cruise control, the ECM also provides a signal (2000 pulses per mile) to the cruise control module.



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

1. A Code 24 will set if vehicle speed is zero mph when:
a. Transmission is not in park or neutral.
b. Engine speed is between 1400 and 3600 RPM.
c. TPS is less than 2%.
d. Low load condition (low air flow).
e. All conditions must be met for 5 seconds. These conditions are met during a road load deceleration.
f. The PM generator only produces a signal if the drive wheels are turning greater than 3 mph.

2. Circuits 400, 401, and 993 are OK if the speedometer works properly. Code 24 is being caused by a faulty ECM, PROM, or an incorrect PROM.


DIAGNOSTIC AIDS:

1. The "Scan" tool should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph.
2. A problem in circuit 437 or 993 will not affect the VSS input or the readings on the "Scan" tool.
3. On automatic transmissions, a faulty or misadjusted 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. See CHART C-1A for P/N switch diagnosis.
4. Carefully check circuits 400 and 401 for proper connections to be sure they are clean and tight. Check for proper harness routing. If a Code 24 is intermittent, see DIAGNOSIS BY SYMPTOM.