Ignition System: Description and Operation

DESCRIPTION
The Electronic Ignition (EI) system controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide improved engine performance, fuel economy, and control of exhaust emissions, the Engine Control Module (ECM) controls spark advance timing with the Ignition Control (IC) system.

OPERATION
The "EI" is a "waste spark" system in which 2 plugs are fired simultaneously from one coil. The spark plugs are paired to a coil, so that when both fire, one cylinder is on the compression stroke and the other is on exhaust. There is little resistance across the plug gap on the exhaust stroke, so the plug requires very little voltage to fire, thereby providing the other plug (compression) with maximum available voltage.

The system uses a magnetic Crankshaft Position (CKP) sensor which protrudes into the block, within approximately .050" of the crankshaft reluctor. The reluctor is a special wheel cast on the crankshaft that has seven machined slots, six of which are equally spaced 60 degrees apart). The seventh slot is spaced 10 degrees from one of the other slots. As the reluctor rotates with the crankshaft, the slots change the magnetic field of the sensor, causing an induced voltage pulse.

By counting the time between pulses, the Ignition Control Module (ICM) can recognize the pulse of the seventh slot (sync pulse). Based on this sync pulse, the module sends the reference signal to the ECM to calculate CKP (used to fire coils in correct sequence) and engine speed.

To properly control timing, the ECM relies on the following information:

^ Engine load (manifold absolute pressure or vacuum).
^ Atmospheric (barometric) pressure.
^ Engine temperature.
^ Intake air temperature.
^ CKP
^ Engine speed (revolution per minute).

COMPONENTS
The ignition control system consists of the "EI" assembly (coils, module, and CKP sensor), ECM and connecting wires. These circuits perform the following functions:

"EI" reference
The CKP sensor sends a signal to the ICM which generates a reference pulse which is sent to the ECM. The ECM uses this signal to calculate CKP and engine speed (also used to trigger the injector).

Reference ground (GND)
This wire is GNd through the module and makes sure the GNd circuit has no voltage drop between the ICM and the ECM, which if open, could affect performance.

Bypass
At about 400 revolution per minute, the ECM applies 5 volts to this circuit to switch spark timing control from the ICM to the ECM. An open or grounded bypass circuit will set a diagnostic trouble code 42 and the engine will run at base timing, plus a small amount of advance built into the ICM.

IGNITION CONTROL (IC)
The ECM uses this circuit (when bypass voltage is applied) to trigger the "EI" module. The ECM uses the CKP to base its calculation of the amount of spark advance needed under present engine conditions.