The spark reference signal (CKT 647) is used by the PCM to improve ignition timing accuracy during crank and at engine speeds of up to 1200 rpm. The spark reference allows the use of EST mode below 400 rpm, eliminating the need to utilize bypass mode during start-up, and also allows the PCM to calculate true crankshaft position in 1/6 the time that use of the fuel control reference signal would permit. During normal operation, the PCM uses the spark reference signal to control ignition timing until the engine speed exceeds 1200 rpm, at which time the fuel control reference signal (CKT 430) is used. When conditions for setting code 17 exist, the fuel control reference signal is used by the PCM to enable and control EST. This condition will cause bypass mode to be used for ignition timing below 400 rpm and EST ignition timing to be degraded below 1200 rpm. Code 17 does not turn on the SERVICE ENGINE SOON light.
Code 17 will be set if:
Engine is running.
The PCM detects 240 fuel control pulses (80 crankshaft revolutions) without detecting any spark reference pulses.
Engine rpm is below 1200.
Numbers refer to circled numbers on the diagnostic chart.
Verifies that CKT 647 is not shorted to ground or open in the ignition jumper harness.
If a window on the harmonic balancer is interfacing with the 18x hall effect switch, the ignition module will ground the spark reference signal. Starter may have to be bumped several times to obtain a voltage reading.
Voltage reading should be lower than that obtained with engine not running, indicating a pulsed reference signal.
An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation. Check for:
Poor Connections or Damaged Harness - Inspect PCM harness connectors for backed out terminals or broken down insulation in CKT 647.
Intermittent Test - If connections and harness check OK, try monitoring voltage on CKT 647 with DVM while moving the related wiring harness and connectors with the engine idling. This may help to isolate the location of the malfunction.
Monday, July 17th, 2006 AT 5:26 PM