Ignition System: Description and Operation

The Direct Ignition System (DIS), Figs. 1 through 6, is a distributorless type ignition system which consists of the following: two separate ignition coils on 4 cylinder models or 3 separate ignition coils on 6 cylinder models; a ``DIS'' ignition module; a Crankshaft Sensor; and the Electronic Spark Timing (EST) portion of the ECM (Electronic Control Module).
A ``Waste Spark'' method of distribution is used on this distributorless ignition 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.
In much the same manner as the distributor type ignition system, these systems use the EST signal from the ECM. The DIS module controls spark timing under 400 RPM and the ECM controls spark timing over 400 RPM.
The DIS 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. At the point of ignition firing, the DIS module monitors the ``sync-pulse,'' and below 400 RPM the module controls spark advance by triggering each of the two coils at a predetermined interval based on engine speed only. The ECM controls the spark above 400 RPM and compensates for all driving conditions. The DIS module must receive a ``sync-pulse'' followed by a Crank Signal in that order to enable engine start up.
The EST system utilizes the same EST to ECM circuits that distributor type ignition systems use. These system circuits include the DIS reference circuit, reference ground, bypass and the EST circuit.
In the DIS reference circuit, 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 pulse to calculate crankshaft position, engine speed and injector pulse width.
In the reference ground circuit, the 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.
In the bypass circuit, the ECM applies approximately 5 volts at about 400 RPM 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 backup ignition timing mode (module timing) at a calculated timing value. This may cause poor performance and reduce fuel economy.
In the EST circuit, 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 engine speed exceeds 400 RPM, the DIS module controls ignition timing. When engine speed exceeds 400 RPM, the ECM applies 5 volts to the bypass line to switch the timing to the ECM control (EST). An open or ground in the EST circuit will result in the engine continuing to run but in a backup ignition timing mode at a calculated timing value and the ``Service Engine Soon'' lamp will not light. If the EST fault is present the next time the engine is restarted, a code 42 will be set and the engine will operate in module timing. This may cause poor performance and reduce fuel economy.