Powertrain Controls - ECM/PCM

Code 24 Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION:

Vehicle speed information is provided to the ECM by the vehicle speed sensor which uses a permanent magnet (PM) generator and it is mounted in 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 can be displayed with a "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 on CKT 817, to the Central Control Module (CCM) for operating the speedometer and the odometer. The VSS signal is also sent to the cruise control module. The VSS signal is sent to the radio control head, so that the volume of the radio will increase as the vehicle speed increases.


NOTE: Because of all the possible color code combinations used on electrical wiring diagrams, always refer to ECM CONNECTOR IDENTIFICATION under ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS for correct color code identification of circuit.


TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 24 will set if vehicle speed is less than 3 mph when:

^ Engine speed is between 900 and 3000 rpm.
^ TPS is less than 2%.
^ MAP is less than 20 kPa (low load condition).
^ All conditions met for 4 seconds.

These conditions are met during a road load deceleration.

^ The PM generator only produces a signal if drive wheels are turning greater than 3 mph.

2. Code 24 is being caused by a faulty ECM, faulty Mem-Cal or an incorrect Mem-Cal.

3. If the vehicle is equipped with selective ride, remove the selective ride control module connector and retest. If the Scan tool now displays vehicle speed, the selective ride control module is faulty. Refer to CHASSIS ELECTRICAL DIAGRAMS/ELECTRICAL AND ELECTRONIC DIAGRAMS/RIDE CONTROL. If the Scan tool does not display vehicle speed, the fault is in the wiring or ECM.

DIAGNOSTIC AIDS:

^ "Scan" should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph.


^ Check CKT 400 and 401 for proper connections to be sure there clean and tight and the harness is routed correctly. Refer to TESTING PROCEDURES/DIAGNOSIS BY SYMPTOM/INTERMITTENTS.

^ A fault in circuit 400 and 401 may cause a Code 23 in the selective service ride control (SRC) module and the selective ride control light in the driver information center (DIC) may be "ON."