Powertrain Controls - ECM/PCM

DTC 25 Chart:

Ignition Control (IC) Circuit(s):

CIRCUIT DESCRIPTION
The spark reference (24X) signal (CKT 647) is used by the PCM to improve ignition timing accuracy during engine operation below 1200 RPM. The spark reference circuit enables the PCM to calculate spark timing in 1/6 the time that the fuel control (4X) signal circuit would allow. During normal engine operation the PCM uses the spark reference CKT 647 to control spark timing below 1200 RPM and the fuel control CKT 430 to control the spark timing above 1200 RPM. If the PCM detects fuel control pulses without detecting spark reference pulses, DTC 25 will set.

DTC 25 WILL SET WHEN
- DTCs 35 and 41 are not set.
- Engine speed is greater than 496 RPM.
- Cam pulses are detected by the PCM.
- Spark reference pulses are not detected by the PCM.
- Spark timing is not being controlled by IC module (module mode).
- All above conditions are present for 4 or more seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
- With a current DTC 25 set, the PCM will illuminate the Malfunction Indicator Lamp (MIL) (Service Engine Soon).
- The PCM will use the Electronic Ignition (EI) module to control spark timing below 400 RPM (cranking engine) and the fuel control signal to control spark above 400 RPM. This will result in degraded spark timing below 1200 RPM.

DTC 25 WILL CLEAR WHEN
The fault no longer exists.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Check to see if DTC 35 is set because a REF LO problem can cause other ignition problems.
2. Check ignition control module's ability to produce 24X reference pulses. If system is operating correctly, you will measure 200 Hertz or greater.
3. Checking to see if ICM is providing PCM the proper 24X reference signal.
4. Checking for open or short to ground on CKT 647.

DIAGNOSTIC AIDS
This diagnostic test, the PCM checks for 4X reference pulses being received without 24X reference pulses. When the engine is running and the PCM is receiving 4X reference pulses without 24X reference pulses for 4 seconds, DTC 25 is set.