With TBI

Code 24 Chart:

Wiring Diagram For VSS:

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 located in the transaxle. The generator produces a pulsing voltage whenever the vehicle speed is over 3.0 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 supplies 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:
a. Transmission is not in park or neutral.
b. Engine speed is between 1800 and 4400 rpm.
c. TPS is at 0% (closed throttle).
d. Low load condition (low MAP voltage, high manifold vacuum).
e. All conditions must be met for 3 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. Before replacing the ECM, the PROM should be checked for correct application.


DIAGNOSTIC AIDS:

1. The "Scan" tool should indicate a vehicle speed whenever the drive wheels are turning greater than 3.0 mph.
2. A problem in circuit 389 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.