Powertrain Controls - ECM/PCM

Code Chart 24:

VSS Circuit:

CIRCUIT DESCRIPTION:
Vehicle speed information is provided to the ECM by the vehicle speed sensor (VSS) which is a permanent magnet (PM) generator that is mounted in the transmission. The PM generator produces a pulsing voltage whenever vehicle speed is over about 3 mph, (5 kPh). The AC voltage level and the number of pulses increases with vehicle speed. The ECM then converts the pulsing voltage to mph, which is used for calculations and the mph can be displayed with a "Scan" tool.
The function of VSS buffer used in past model years has been incorporated into the PROM portion of the ECM. The ECM supplies the necessary signals to the instrument panel (4004 pulses per mile) for operating the speedometer and cruise control.

DTC 24 will set if:

^ Engine is running.
^ No DTC 31.
^ No DTC 28 (3 speed) or DTC 29 (4 speed).
^ When P/N switch is open (indicating drive range).
^ When high gear switch is open indicating transmission is in 3rd gear (3T40) or 4th gear (4T60).
^ VSS signal indicates a vehicle speed less than 3 mph.
^ All conditions met for 20 seconds.

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

1. The PM generator only produces a signal if drive wheels are turning greater than 3 mph (5 kPh).

2. Check PROM for correct application before replacing ECM.

DIAGNOSTIC AIDS:

^ "Scan" should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph (5 kPh).
^ Check circuits 400 and 401 for proper connections. Be sure they are clean and tight and the harness is routed correctly. - Intermittent Malfunctions