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 (4004 pulses per mile) for operating the speedometer, and odometer.

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

^ Engine is running.
^ No code 28 or 31.
^ When P/N switch is open (indicating drive range).
^ When high gear switch is open indicating transmission is in 3rd gear.
^ All conditions met for 40 seconds.
^ RPM is greater than 1000.

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

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

2. Before replacing the ECM check Mem-Cal for correct application.


DIAGNOSTIC AIDS:


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

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