Powertrain Controls - ECM/PCM

DTC 24 Vehicle Speed Sensor Circuit Diagnosis:

Vehicle Speed Sensor Wiring 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 on 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/km/h 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.

DTC 24 Will Set When:
^ Vehicle speed is less than 5 mph.
^ Engine speed is between 1600 and 4400 RPM.
^ Low load condition (low MAP voltage, high manifold vacuum).
^ Transaxle not in park or neutral.
^ All above conditions are met for 4 seconds or more.

Action Taken (ECM will default to): Malfunction indicator Lamp (MIL) illuminates.

DTC Chart Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.
1. 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, check PROM for correct application before replacing ECM.

Diagnostic Aids:
Scan tool should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph.

A problem in CKT 389 will not affect the VSS input or the readings on a scan tool.

Check CKTs 1232 and 1233 for proper connections to be sure they are clean and tight and the harness is routed correctly. Refer to "Intermittents" in "Diagnosis By Symptom." - 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. Refer to CHART C-1A for the PNP switch check. Chart C-1A Park/Neutral Switch Diagnosis