Powertrain Controls - ECM/PCM

Code 24 Chart:

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 PM generator produces a pulsing voltage whenever the vehicle speed is over 3 mph. The AC 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.

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:

^ Transmission is not in park or neutral.
^ Engine speed is between 1400 and 3600 RPM.
^ Low load condition (low MAP voltage) high manifold vacuum.
^ All conditions must be met for 4 seconds.

These conditions are met during a road load deceleration. Disregard a Code 24 that sets when the drive wheels are not turning. This can be caused by a faulty park/neutral switch circuit. The PM generator only produces a signal if the drive wheels are turning greater than 3 mph.

2. At this point, the ECM is not sending vehicle speed data to the Scan tool. If the Scan tool is connected and functioning properly, the ECM is at fault.

DIAGNOSTIC AIDS:
The "Scan" tool should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph. A problem in circuit 817 will not affect the VSS input or the readings on the "Scan" tool. 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. For P/N switch diagnosis, Chart C-1A Park/Neutral Switch Diagnosis 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, Testing and Inspection