Powertrain Controls - ECM/PCM

Code 24 Diagnostic Chart:

Vehicle Speed Sensor (VSS) Wiring Schematic:

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 about 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 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, 4T60E transaxle (if equipped); and for the cruise control module.

DTC 24 WILL SET WHEN
^ Vehicle speed is less than 2 mph.
^ Engine speed is between 1600 and 3000 RPM (Man), 1100 and 5200 RPM (3T40), or 3000 and 4775 RPM (4T60E).
^ TP sensor is less than 2% (3T40) less than 30% (4T60E) or between 7 and 20% (manual).
^ Not in park or neutral (3T40 or 4T60E).
^ No DTC 21, 22, 33 or 34.
^ Engine vacuum between 68-78 kPa (3T40), 38-58 kPa (Man) or 0.3-40 kPa (4T60E).
^ All conditions met for 3 seconds (4T60E), 5 seconds (3T40) or 20 seconds (manual).

OR

^ VSS is greater than 18 mph and then immediately goes to 0 mph.

ACTION TAKEN (PCM WILL DEFAULT TO)
^ Malfunction Indicator Lamp (MIL) illuminates.
^ 4T60E transaxle (if equipped) will shift and remain in 3rd gear

DTC 24 WILL CLEAR WHEN
The next ignition cycle is performed.

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. At this point, the PCM is not sending vehicle speed data to the scan tool. If the scan tool is connected and functioning properly, 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. Be sure they are clean and tight and the harness is routed correctly.
^ Refer to "DIAGNOSIS BY SYMPTOM/INTERMITTENTS" Testing and Inspection
^ (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). For Park/Neutral Position (PNP) switch diagnosis check, refer to C-CHARTS/CHART C-1A" Chart C-1A Park/Neutral Switch Diagnosis.
^ DTC 24 can be falsely set if engine is "braketorqued" in gear for 20 seconds.