Powertrain Controls - ECM/PCM

DTC 24 Vehicle Speed Sensor (VSS) Circuit:

Vehicle Speed Sensor (VSS) Schematic:

CIRCUIT DESCRIPTION
Vehicle speed 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 AC voltage signal whenever vehicle speed is over 3 mph (5 km/h). The AC voltage level and the number of pulses increases with vehicle speed. The PCM then converts the pulsing voltage into mph.

The PCM then supplies the necessary signal to the instrument panel to operate the speedometer, odometer, and the cruise control module.

DIAGNOSTIC TROUBLE CODE (DTC) 24 WILL SET WHEN
^ Vehicle speed is less than 2 mph (3 km/h).
^ Engine speed is between 1700 and 3600 RPM.
^ Throttle Position (TP) sensor is less than 1%.
^ Not in park or neutral (auto).
^ DTC 21, 22, 33 or 34 is not set.
^ Manifold Absolute Pressure (MAP) is between 72-80 kPa (21 - 23 in. HG).
^ All above conditions are met for 5 seconds.

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 vehicle speed whenever the drive wheels are turning greater than 3mph (5 km/h).
^ (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 a proper signal while in drive (3T40).

^ DTC 24 can be falsely set if engine is "brake-torqued" in gear for 20 seconds.
^ Output of the VSS generator can be monitored using a digital voltmeter while rotating the drive wheels.
^ Refer to "Intermittents," in Diagnosis by Symptom. - Intermittent Malfunctions