Powertrain Controls - ECM/PCM

DTC 24 - Vehicle Speed Sensor (VSS) Circuit:

Vehicle Speed Sensor Wiring Circuit:

Circuit Description:

Vehicle speed information is provided to the ECM by the vehicle speed sensor, a Permanent Magnet (PM) generator mounted in the transmission. The PM generator produces a pulsing voltage whenever vehicle speed is over about 3 mph. The AC voltage level and the number of pulses increases with vehicle speed. The ECM converts the pulsing voltage to mph, which can be monitored with a Tech 1.

The function of the VSS buffer used in past model years has been incorporated into the ECM. The ECM supplies the necessary signal to the instrument panel (4004 pulses per mile) for operating the speedometer.

DTC 24 will set if vehicle speed signal equals less than 3 mph if:
^ Engine speed greater than 1,000 RPM.
^ No DTC 28 or 31.
^ PNP input is not grounded (indicating drive range).
^ High gear switch is open indicating transmission is in 3rd gear.
^ All conditions met for 20 seconds.

Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. The PM generator only produces a signal if drive wheels are turning greater then 3 mph.
2. Before replacing the ECM, check the PROM for correct application.

Diagnostic Aids:

Tech 1 should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph.

Check CKTs 400 and 401 for proper connections to be sure they are clean and tight and the harness is routed correctly. - Intermittent Malfunctions