Powertrain Controls - ECM/PCM

Code 24 Chart:

Code 24 Wiring Diagram:

CIRCUIT DESCRIPTION:


Vehicle speed information is provided to the ECM by the Vehicle Speed Sensor (VSS) which is a Permanent Magnet (PM) generator that is mounted in the transmission. The PM generator produces a pulsing voltage whenever vehicle speed is over about 3 mph, (5 kph). 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 for the instrument panel (4000 pulses per mile) for operating the speedometer, the odometer, and for the cruise control module.


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

1. Code 24 will set if vehicle speed is less than 2 mph when:

^ Engine speed is between 1500 and 3600 rpm.
^ TPS is greater than 2%.
^ Not in park or neutral.
^ All conditions met for 5 seconds.
^ No Code 21, 22, 33 or 34.
^ MAP between 12-30 kPa.

These conditions are met during a road load operation. Disregard Code 24 that sets when drive wheels are not turning.

The PM generator only produces a signal if drive wheels are turning greater than 3 mph (5 kph).

2. Check MEM-CAL for correct application before replacing ECM.


DIAGNOSTIC AIDS:


^ "Scan" should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph, (5 kph).

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

^ Check CKT 400 and 401 for proper connections. Be sure they are clean and tight and the harness is routed correctly. - Intermittent Malfunctions

^ (A/T) A faulty or mis-adjusted park/neutral switch can result in a false Code 24. Use a "Scan" and check for proper signal while in drive (3T40). Chart C-1A Park/Neutral Switch Diagnosis

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