Ignition System: Description and Operation

PURPOSE
The Direct Ignition System (DIS) provides a spark discharge at the spark plugs on the compression and exhaust stroke of cylinder pairs (1-4, or 3-6, or 2-5).

OPERATION
The DIS does not use the conventional distributor and coil. This ignition system consists of three separate ignition coils a "DIS" ignition module and crankshaft sensor as well as the related connecting wires and the Electronic Spark Timing (EST) portion of the Electronic Control Module (ECM). 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 (1-4, or 3-6, or 2-5). 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 though 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 will fire from the center electrode to the side electrode while the other will fire from side electrode to center electrode. These systems utilize the EST signal from the ECM, as do distributor type ignition systems equipped with EST, to control spark timing.

Under 400 RPM, the "DIS" module controls spark timing (module timing mode) and over 400 RPM, the ECM controls spark timing (EST mode). To properly control ignition timing, the ECM relies on the following information:
^ Airflow
^ Engine temperature
^ Manifold air temperature
^ Crankshaft position
^ Engine speed (RPM)

CIRCUIT DESCRIPTION
This system uses the same EST to ECM circuits that distributor type systems use. Following is a brief description for each of the EST circuits.

"DIS" Reference circuit 430.
The crankshaft sensor generates a signal to the ignition module which results in a reference pulse being sent to the ECM. The ECM uses this signal to calculate crankshaft position and engine speed and injector pulse width.

Reference Ground circuit 453.
This wire is grounded through the module and insures that the ground circuit has no voltage drop between the ignition module and the ECM, which could affect performance.

Bypass circuit 424.
At about 400 RPM, the ECM applies 5 volts to this circuit to switch spark timing control from the "DIS" module to the ECM. An open or grounded bypass circuit will set a Code 42 and result in the engine operating in a back-up ignition timing mode (module timing) at a calculated timing value. This may cause poor performance and reduced fuel economy.

EST circuit 423.
The "DIS" module sends a reference signal to the ECM when the engine is cranking. While the engine is under 400 RPM, the "DIS" module controls the ignition timing. When the engine speed exceeds 400 RPM, the ECM applies 5 volts to the bypass line to switch the timing to ECM control (EST). An open in the EST circuit while running will result in the engine continuing to run, but in a backup ignition timing mode (module timing) at a calculated timing value and the "Service Engine Soon" light will not be "ON." If the EST fault is still present the next time the engine is restarted, a Code 42 will be set, and the engine will operate in module timing. If the EST circuit is shorted to ground, the 'Service Engine Soon" light will be turned "ON," a Code 42 will be set and the engine will operate in module timing. Either condition may cause poor performance and reduced fuel economy