Ignition System: Description and Operation
DIRECT IGNITION SYSTEM (DIS)
The 2.5L Direct Ignition System consists of:
1. ECM
2. Two ignition coils
3. Crankshaft sensor
4. Ignition module
Components of the Direct Ignition System (DIS) are a coil pack, ignition module, crankshaft reluctor ring, magnetic sensor and the ECM (Electronic Control Module). The coil pack consists of two separate, interchangeable, ignition coils. Two coils are used because each coil fires for only two cylinders.
The ignition module is located under the coil pack and is connected to the ECM by a 6-pin connector. The ignition module controls the primary circuit to the coils, turning them on and off. It also controls spark timing below 400 rpm and if the ECM bypass circuit becomes open or grounded.
The magnetic pickup sensor inserts through the engine block, just above the pan rail, near the crankshaft reluctor ring. Notches in the crankshaft reluctor ring trigger the magnetic pickup sensor to provide timing information to the ECM. The magnetic pickup sensor provides a cam signal to identify correct firing sequence, and crank signals to trigger each coil at the proper time.
This system uses Electronic Spark Timing (EST) and control wires from the ECM, as with distributor systems. The ECM controls timing using crankshaft position, engine rpm, engine temperature, and manifold absolute pressure (MAP) sensing.
SYSTEM OPERATION
The direct ignition system (DIS) does not use the conventional distributor and coil. 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:
- Air flow
- Engine temperature
- Manifold air temperature
- Crankshaft position
- Engine speed (RPM)
"DIS" MODULE
The "DIS" module monitors the crank sensor signals and based on these signals and based on these signals sends a reference signal to the ECM so that correct spark and fuel injector control can be maintained during all driving conditions. During cranking, the "DIS" module monitors the "sync-pulse" to begin the ignition firing sequence and below 400 rpm the module controls spark advance by triggering each of the two coils at a pre-determined interval based on engine speed only. Above 400 rpm the ECM controls the spark timing (EST) and compensates for all driving conditions. The "DIS" module must receive a "sync-pulse" then a crank signal in that order to enable the engine to start. The "DIS" module is not repairable. When a module is replaced, the remaining "DIS" components must be transferred to the new module.