Electronic Ignition

The electronic ignition system on single overhead cam (SOHC) and dual overhead cam (DOHC) engines provides spark energy to ignite the air/fuel mixture necessary for combustion. The Powertrain Control Module (PCM) controls spark under all engine running conditions. The system components include: the PCM, electronic ignition (FT) module/coil pack, spark plugs, spark plug wires and knock sensor.

The spark dwell (On-time) and degrees of spark advance are dependent upon engine speed, Manifold Absolute Pressure (MAP), and Engine Coolant Temperature (ECT). The PCM can vary spark advance from 39° BTDC to 3° 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 the coil packs based on PCM control. The PCM has two control circuits, one for the 2/3 coil and the other for the 1/4 coil. The PCM uses a high control signal of near 5 volts to charge up the coil and a low control signal of near 0 volts to discharge the coil. If the coil is charged and the control signal is low, the coil will fire through its secondary towers. The secondary voltage can reach a maximum of 40,000 volts.

The secondary current always travels in the same direction and in a series type circuit. For example, when the PCM fires the 1/4 coil, the current will flow out of the #1 coil tower, to the #1 spark plug wire, to the #1 spark plug, through the block, up through the #4 spark plug, through the #4 spark plug wire and back to the #4 coil tower. If one of the wires/plugs were to open, the other mating cylinder would still fire out of its coil tower because the circuit would be completed through the EI module bolts.

In order to determine when to fire a cylinder, the PCM uses the Crankshaft Position (CKP) sensor. The crankshaft has 7 machined notches, 2 of which are close together representing a double pulse. The PCM uses this double pulse to identify cylinder #4 Top Dead Center (TDC). However, the PCM still has to identify whether cylinder #4 is on TDC compression or TDC exhaust. This is accomplished by the use of Compression Sense Ignition.

COMPRESSION SENSE IGNITION - Both the SOHC and DOHC engines utilize Compression Sense Ignition, which eliminates the need for a camshaft position sensor. The EI module has sensing circuitry that detects when cylinder #4 has fired on its compression stroke and relays this information to the PCM. The PCM can then correctly synchronize the fuel injectors for sequential fuel injection.

The EI module uses capacitive pickup plates located under the 1/4 coil to determine when cylinder #4 has fired on compression. These plates are used to differentiate the polarity and voltage amplitude difference between the 1/4 secondary ignition circuits. Since each coil tower is of opposite polarity and the waste spark (2 - 4 kV) generally fires before the compression spark (10 - 25 kV), the module can determine cylinder #4 compression. When the EI module detects a positive to negative polarity sequence and a high negative voltage spike, it will pull the PCM 5 volt cam signal circuit to ground. The PCM knows that cylinder #4 had just fired on its compression stroke when this transition occurs.

The EI module, however, cannot always detect when cylinder #4 has fired on compression. These occurrences include:
^ During deceleration
^ Very low engine load conditions when engine is running
^ If a secondary ignition problem occurs on cylinder #1 or #4

Too few cam pulses (cam signal circuit not being pulled to ground) are a result of decreased cylinder #4 secondary resistance or increased cylinder #1 secondary resistance. Too many cam pulses (cam signal circuit being pulled to ground too often) are a result of decreased cylinder #1 secondary resistance or increased cylinder #4 secondary resistance.

SPARK KNOCK CONTROL The PCM uses the knock sensor to determine when spark knock exists and can retard timing up to a maximum of 19°. The knock sensor is a piezoelectric flat response (wide resonant band) device that produces an AC voltage of different amplitude and frequency based on engine mechanical vibration. The amplitude and frequency are dependent upon the level of knock the sensor detects.

The PCM learns a minimum noise level at idle from the knock sensor and uses stored normal noise level calibration values for the rest of the RPM band. The knock sensor signal is only used during the TDC combustion event of the firing cylinder. When in a combustion event, the PCM filters the knock signal and compares it to the normal calibration noise level for that RPM. If the PCM 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 PCM will always try to work back to a zero compensation level or no spark retard.

If knock is present, the PCM will increment three counters that can be read on the Scan tool. The LOW, MID and HIGH SPARK MODIFIERS represent three different RPM bands the PCM uses to store knock retard degrees. The Scan tool also displays the actual amount of spark retard degrees as SPARK RETARD CYL #1 - 4. If excessive spark knock is detected, the retarding of timing will cause a reduced power condition.