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 "Electronic Ignition (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 0.050 in. 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° apart). The seventh slot is spaced 10° 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 (revolutions per minute).

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 revolutions 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 Ignition Control Module.

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.

RESULTS OF INCORRECT OPERATION
An open or ground in the Ignition Control (IC) or bypass Circuits will set a Diagnostic Trouble Code (DTC) 42 and cause the Engine to run on the Ignition Control Module (ICM) timing.

The Engine Control Module (ECM) uses information from the Intake Air Temperature (IAT) and Engine Coolant Sensors in addition to revolution per minute (rpm) to calculate spark advance as follows:
- Cold Engine = more spark advance.
- Engine under minimum load based on rpm and low amount of air flow = more spark advance.
- Hot Engine = less spark advance.
- Engine under heavy load based on rpm and high air flow = less spark advance.

HOW DIAGNOSTIC TROUBLE CODE (DTC) 42 IS DETERMINED
- When the system is running on the Ignition Control Module (ICM), that is, no voltage on the bypass line, the ICM Grounds (GND) the Ignition Control (IC) signal. The Engine Control Module (ECM) expects to see no voltage on the IC Line during this condition. If it sees a voltage, it sets Diagnostic trouble Code (DTC) 42 and will not go into the IC mode.
- When the revolutions per minute (rpm) for IC is reached (about 400 rpm) the ECM applies 5 volts to the Bypass Line and the IC should no longer be grounded in the ICM so the IC voltage should be varying.
- An open in the IC Circuit while running, will result in the Engine continuing to run, but in a back-up Ignition timing mode (Module timing). Calculated timing value and the Malfunction Indicator Lamp will not be "ON".
- If the IC Circuit is shorted to GND, the Malfunction Indicator Lamp will be turned "ON", and a DTC 42 will be set and the Engine will operate in module timing. Either condition may cause poor performance and reduced Fuel economy.
- If the bypass line is open or GND, the ICM will not switch to IC Mode so the IC voltage will be low and DTC 42 will be set.
- If the IC line is grounded, the ICM will switch to IC, but because the line is GND there will be no IC signal. A DTC 42 will be set.