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 Engine Control Module (ECM) by the Vehicle Speed Sensor (VSS) which is a permanent magnet generator that is mounted in the transaxle. The permanent magnet generator produces a pulsing voltage whenever vehicle speed is over about 3 mph (5 km/h). 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 Tech 1 scan tool. Output of the generator can also be seen by using a digital voltmeter on the AC scale while rotating the generator.

The function of VSS buffer used in past model years has been incorporated into the ECM. The ECM then supplies the necessary signal for the instrument panel for operating the speedometer. the odometer, and for the cruise control module.

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

1. DTC 24 will set if vehicle speed is less than 2 mph when:
^ Engine speed is between 1500 and 3600 RPM.
^ TP sensor is less than 2% (Auto) or greater than 7% (Man).
^ Not in park or neutral.
^ All conditions met for 20 seconds.
^ No DTC 21,22, 33 or 34.
^ Engine vacuum between 68 - 80 kPa (Auto), or 38-58 kPa (Man).

These conditions are met during a road load operation.

The permanent magnet generator only produces a signal if drive wheels are turning greater than 3 mph (5 km/h).

2. Check PROM for correct application before replacing ECM.

Diagnostic Aids:

A Tech 1 scan tool should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph (5 km/h).

A problem in CKT 434 will not affect the VSS input or the readings on a scan.

Check CKT 400 and CKT 401 for proper connections. Be sure they are clean and tight and the harness is routed correctly. - Intermittent Malfunctionsin "Diagnosis By Symptom".

(A/T) A faulty or misadjusted Park/Neutral Position (PNP) switch can result in a false DTC 24. Use a Tech 1 scan tool and check for proper signal while in drive (3T40). Chart C-1A Park/Neutral Switch Diagnosisfor Park/Neutral Position (PNP) switch diagnosis check.

DTC 24 can be falsely set if engine is "brake-torqued" in gear for 20 seconds.