Powertrain Controls - ECM/PCM

Code 24 Chart:

Code 24 Wiring Diagram:

CIRCUIT DESCRIPTION:

The Electronic Control Module (ECM) applies and monitors 12 volts on CKT 437. CKT 437 connects to the Vehicle Speed Sensor (VSS) buffer which alternately grounds CKT 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between "pulses". A "Scan" tool reading should closely match with speedometer reading with drive wheels turning.

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:

- CKT 437 voltage is constant
- Engine speed between 1400 and 3600 rpm
- Less than 2% throttle opening
- Low load condition (low air flow)
- Not in park or neutral
- All conditions must be met for 4 seconds
- These conditions are met during a road load deceleration.

2. A voltage of less than 1 volt at the 15-way I/P connector indicates that the CKT 437 wire may be shorted to ground. Disconnect CKT 437 at the VSS buffer. If voltage now reads above 10 volts, the VSS buffer is faulty. If voltage remains less than 10 volts, then CKT 437 wire is grounded or open. If 437 is not grounded or open, check for a faulty ECM connector or ECM.

DIAGNOSTIC AIDS:

- If "Scan" displays vehicle speed, check park/neutral switch CHART C-1A on vehicle with auto trans. If switch is OK check for intermittent connections. An open or short to ground in CKT 437 will result in a Code 24. If the customer also complained about a loss of mph on the Instrument Panel (I/P), check the Permanent Magnet (PM) generator circuit.

- Refer to TESTING PROCEDURES/DIAGNOSIS BY SYMPTOM/INTERMITTENTS.