Powertrain Controls - ECM/PCM

DTC 24 - Vehicle Speed Sensor (VSS) Circuit Diagnosis:

Vehicle Speed Sensor Wiring Circuit:

Circuit Description:
Vehicle speed information is provided to the Powertrain Control Module (PCM) 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 3 mph (5 km/h). The AC voltage level and the number of pulses increase with vehicle speed. The PCM 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 PCM. The PCM then supplies the necessary signal for the instrument panel for operating the speedometer the odometer, and for the cruise control module.

DTC 24 Will Set When:
^ Vehicle speed is less than 2 mph.
^ Engine speed is between 1700 and 3600 RPM.
^ TP sensor is less than 2%.
^ Not in park or neutral.
^ All conditions met for 5 seconds.
^ DTC 21, 22, 33 or 34 is not set.
^ Engine vacuum is between 72-80 kPa.

Action Taken (PCM will default to): The Malfunction Indicator Lamp (MIL) will illuminate, and the cooling fan will enable.

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

1. The permanent magnet generator only produces a signal if drive wheels are turning greater than 3 mph (5 km/h). This step checks for a hard or intermittent condition.
2. Check EPROM for correct application before replacing PCM.

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).

Check CKT 400 and CKT 401 for proper connections. Make sure they are clean, 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 Tech 1 scan tool and check for proper signal while in drive (3T40). Refer to CHART C-1A for Park/Neutral Position (PNP) switch diagnosis check. Chart C-1A Park/Neutral Switch Diagnosis

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