Electronic Ignition

The electronic ignition system on the 3.0L Dual Overhead Cam (DOHC) engine provides spark energy to ignite the air/fuel mixture necessary for combustion. The Engine Control Module (ECM) controls spark under all engine-running conditions. The system components include: the ECM, two Electronic Ignition (EI) modules each which house three primary and three secondary coils, spark plug boots, spark plugs, Camshaft Position (CMP) sensor, knock sensor and associated wiring.

The spark dwell (On-time) and degrees of spark advance are dependent upon engine speed, intake airflow as indicated by the mass airflow sensor (MAF) and engine coolant temperature. The ECM can vary spark advance from 50° BTDC to 30° ATDC under all engine-running conditions when no spark knock is present.

IGNITION CONTROL - The primary function of the EI module is to charge and discharge each of the three primary coils individually based on ECM control. The ECM uses independent primary coil control circuits designated for each cylinder. The ECM uses a high control signal of near 5 volts to charge up the primary coil and a low control signal of near 0 volts to discharge the primary coil. If the primary coil is charged and the control signal is low, the secondary coil will fire through its secondary tower. The secondary voltage across the spark plug can reach a maximum of 40,000 volts.

In order to determine when to fire a cylinder, the ECM uses the Crankshaft Position (CKP) sensor signal in conjunction with the Camshaft Position (CMP) sensor signal to determine cylinder #1 Top Dead Center (TDC) compression. The crankshaft has 58 machined notches with a double space between two of the notches representing cylinder #1 TDC. The CMP sensor signal will change from a high 5 volt signal to a low 0 volt signal when cylinder #1 is 90 (before its top dead center compression stroke).

SPARK KNOCK CONTROL - The ECM uses the knock sensor to determine when spark knock exists and can retard timing up to a maximum of 35 (on any individual cylinder (12 (of instant spark retard plus 12( of dynamic spark adaptation plus 11( of APP tip in spark retard). (NOTE: Under most conditions, the ECM can control spark knock using only 12( of spark retard authority.) The knock sensor is a piezoelectric flat response (wide resonant frequency band) device that produces an AC voltage of different amplitude and frequency based on engine mechanical vibration. The amplitude and frequency are dependent on the level of knock the sensor detects. The more the knock the higher the amplitude.

The ECM learns an average noise level from the knock sensor (based on a calibrated average) and monitors the signal to verify that it stays above a minimum value. The knock sensor signal is only used during the TDC combustion event of the firing cylinder. When in a combustion event, the ECM filters the knock signal and compares it to the normal calibration noise level for that RPM. If the ECM has determined that knock is present during the combustion event, it will retard timing on the next firing cylinders until the knock is eliminated. The ECM will always try to work back to a zero compensation level or no spark retard.

The Scan tool displays the actual amount of spark advance degrees and actual amount of spark retard degrees as SPARK and SPARK RETARD CYL #1-6. If excessive spark knock is detected, the retarding of timing will cause a reduced power condition.