Ignition System: Description and Operation

PURPOSE:
In much the same manner as the distributor type ignition system, this system uses the EST signal from the ECM. The ECM controls the spark above 400 RPM and compensates for all driving conditions. The ECM relies on the following inputs to calculate the EST signal:

^ Calculated BARO
^ Coolant Temperature Sensor (CTS)
^ Crankshaft sensor
^ Engine speed
^ Knock Sensor
^ MAP sensor
^ MAT sensor
To diagnose any of the above components refer to COMPUTERIZED ENGINE CONTROLS.

CONSTRUCTION:
The Direct Ignition System (DIS) is a distributorless type ignition system which consists of a crankshaft sensor, the Electronic Spark Timing (EST) portion of the Electronic Control Module (ECM), three separate ignition coils, and an ignition module.

OPERATION:
The crankshaft sensor sends a signal through the ignition module to the ECM. The ECM uses this reference signal to calculate engine speed and crankshaft position.

Ignition Current Flow:

Three separate coils are mounted to the module assembly. Each coil provides the spark for two plugs simultaneously. Each coil can also be replaced separately. A "Waste Spark" method of distribution is used on this system. Each cylinder is paired with its opposing cylinder in firing order so that one cylinder on the compression stroke fires simultaneously with its opposing cylinder on the exhaust stroke. Requiring less voltage to fire the plug on the exhaust stroke, most of the available voltage is sent to the compression stroke cylinder. This process is reversed as the cylinders' roles are reversed.

The ignition module controls spark timing under 400 RPM and must receive a "sync-pulse" followed by a Crank Signal in that order to enable engine start up. The ignition module monitors the crankshaft sensor signals and based on these signals, sends a reference signal to the ECM to allow the correct spark and fuel injector control to be maintained during all driving conditions.