Ignition System: Description and Operation

SYSTEM OPERATION
The Electronic Ignition (EI) system consists of two separate ignition coils, an electronic Ignition Control Module (ICM) and a secondary conductor housing mounted to an aluminum cover plate. A Crankshaft Position (CKP) sensor, related connecting wires and the Ignition Control (IC) portion of the PCM make up the remainder of the system.

A distributorless ignition system, such as this one, uses a waste spark method of spark distribution. Each cylinder is paired with the cylinder that is opposite it (1 and 4 or 2 and 3). The spark occurs simultaneously in the cylinder coming up on the compression stroke and in the cylinder coming up on the exhaust stroke.

The cylinder on the exhaust stroke requires very little of the available energy to fire the spark plug. The remaining energy will be used as required by the cylinder on the compression stroke. The same process is repeated when the cylinders reverse roles.

It is possible in a no load condition for one plug to fire even through the spark plug lead from the same coil is disconnected from the other spark plug. The disconnected spark plug lead acts as one plate of a capacitor, with the engine being the other plate. These two capacitor plates are charged as a current surge (spark) jumps across the gap of the connected spark plug. The plates are then discharged as the secondary energy is dissipated in an oscillating current across the gap of the spark plug still connected. Because of the direction of current flow in the primary winding and thus, in the secondary winding, one plug fires from the center electrode to the side electrode while the other fires from side electrode to center electrode.

This system utilizes the IC signal from the PCM to control spark timing. To properly control ignition timing, the PCM also relies on the following information:

^ Engine load (manifold pressure or vacuum).
^ Engine coolant temperature.
^ Intake air temperature.
^ Crankshaft position.
^ Engine speed (RPM).
^ Spark knock indication.

Crankshaft Position (CKP) Sensor. This system uses a magnetic CKP sensor, mounted remotely from the ICM, which protrudes into the block within approximately 0.050 inch of the crankshaft reluctor. The reluctor is a special wheel cast into the crankshaft with seven slots machined into it, six of which are equally spaced (60° apart). A seventh slot, is spaced 10° from one of the other slots and serves to generate a sync-pulse. As the reluctor rotates as part of the crankshaft, the slots change the magnetic field of the sensor, creating an induced voltage pulse.

Based on the CKP sensor pulses, the ICM sends 7X reference signals to the PCM which is used to indicate crankshaft position and engine speed. The ICM continues to send these reference pulse to the PCM at a rate of seven per 360° of crankshaft rotation. This signal is called the 7X reference because it occurs 7 times per crankshaft revolution. The 7X reference signal is necessary for the PCM to determine when to activate the fuel injectors.

By comparing the time between pulses, the PCM can recognize the pulse representing the seventh slot (sync pulse) which starts the calculation of ignition coil sequencing. The second crank pulse following the sync pulse signals the PCM to fire the # 2-3 ignition coil and the fifth crank pulse signals the PCM to fire the # 1-4 ignition coil.

The Camshaft Position Sensor (CMP) is used to correlate crankshaft to camshaft position so that the PCM can determine which cylinder is ready to be fueled by the injector. The CMP sensor is also used to determine which cylinder is misfiring when misfire is present. If the PCM receives an intermittent signal from the CMP, then the CMP Resync Counter will increment. When the PCM cannot use the information from the CMP sensor, a DTC is set and the PCM will fuel the engine using the Alternating Synchronous Double Fire (ASDF) method. This sensor has no effect on the EI system.

A fault in the CMP sensor circuits will cause a DTC P0341 or P0342.