Powertrain Controls - ECM/PCM

Code Chart 24:

VSS Circuit:

CIRCUIT DESCRIPTION:
Vehicle speed information is provided to the Engine Control Module (ECM) by the Vehicle Speed Sensor (VSS), which is a Permanent Magnet (PM) generator, and it is mounted in the transaxle. 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 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 ECM. The ECM then supplies the necessary signal to the instrument panel (4000 pulses per mile) for operating the speedometer and the odometer.

TEST DESCRIPTION: Number(s) below refer to circled number(s) on the diagnostic chart.

1. DTC 24 will set if vehicle speed equals 0 mph when:

^ Engine speed is between about 1400 and 3600 RPM.
^ Low load condition (low MAP voltage, high manifold vacuum).
^ Transaxle not in park or neutral.
^ All above conditions are met for 4 seconds. These conditions are met during a road load deceleration.

Disregard a DTC 24 that sets when the drive wheels are not turning. This can be caused by a faulty PNP 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:
Scan tool should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph.

A problem in CKT 817 will not affect the VSS input or the readings on a scan tool.

Check CKTs 400 and 401 for proper connections to be sure they are clean and tight and the harness is routed correctly. - Intermittent Malfunctions A faulty or misadjusted Park/Neutral Position (PNP) switch can result in a false DTC 24. Use a scan tool and check for the proper signal while in a drive range. For the PNP switch check Chart C-1A Park/Neutral Switch Diagnosis