Direct Ignition System (DIS) / EST

DIS Circuit:

PURPOSE
Monitors and translates the signals from the crank angle sensors.
Allows the ECM to control ignition coil firing.

LOCATION
Under the DIS coil pack.

OPERATION
Distributorless ignition system (DIS) uses a "waste spark" method of spark distribution. Each cylijnder is paired with its opposing cylinder in the firing order. One cylinder fires on compression stroke and the other on the exhaust stroke.

Since the cylinder on the exhaust stroke requires very little available voltgae to fire the plug, moxst of the voltage is used to fire the compression stroke plug. The process reverses when the cylinders reverse roles.

Electronic Spark Timing (EST) is ECM controlled. The ECM monitors information from various sensors, computes the desirable spark timing and signals the DIS module to induce spark in the ignition coils To perform this function, the ECM must know crankshaft position, engine rpm, engine load (manifold pressure or vacuum), atmospheric (barometric) pressure and engine temperature.

The ECM utilizes information from the MAP sensor, coolant and intake air temperature sensors and inductive pickup sensor to calculate spark advance as follows:

- Engine load (MAP sensor)
- Atmospheric pressure (MAP sensor)
- Manifold air temperature sensor (MAT sensor)
- Crankshaft position (Inductive pickup coil)
- Engine speed (Inductive pickup coil)


DIS/EST SYSTEM CIRCUITS

RPM Reference Input (ECM Terminal B-5)
- This camshaft 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 camshaft position, engine speed and injector duration.

Reference Ground (ECM Terminal B-3)
- This circuit is grounded through the module and insures that the ground circuit has no voltage drop between the module and the ECM which would affect performance.

Bypass Control (ECM Terminal D-5)
- At about 400 rpm, the ECM applies 5V to this circuit to switch spark timing control from DIS module to the ECM. An open or grounded bypass circuit will result in the engine operating in a back-up timing mode (module timing) at a calculated value. This may cause poor performance and economy.

EST Output (ECM Terminal D-4)
- The DIS module sends a reference signal to the ECM when the engine is cranking. While the rpm is under 400, the