Ignition System: Description and Operation
GENERAL DESCRIPTIONThe Electronic Ignition (EI) system controls fuel combustion by providing a spark to ignite the compressed air / fuel mixture at the correct time. To provide optimum engine performance, fuel economy, and control of exhaust emissions, the Powertrain Control Module (PCM) controls spark advance of the ignition system. EI has several advantages over a mechanical system:
^ No moving parts.
^ Less maintenance.
^ Remote mounting capability.
^ No mechanical load on the engine.
^ More coil cool down time between firing events.
^ Elimination of mechanical timing adjustments.
^ Increased available ignition coil saturation time.
OPERATION
The Electronic Ignition system does not use the conventional distributor and coil. This ignition system consists of three ignition coils, an Ignition Control Module (ICM), a Camshaft Position Sensor, two Hall-effect crankshaft position sensors, related connecting wires, and the ignition control and fuel metering portion of the PCM.
Conventional ignition coils have one end of the secondary winding connected to the engine ground. In this ignition system, neither end of the secondary winding is grounded. Instead, both ends of each coil secondary winding is attached to a spark plug. Each cylinder is paired with the cylinder that is opposite it (1 - 4, 2 - 5, 3 - 6).
These two plugs are on "companion" cylinders, i.e., on top dead center at the same time. When the coil discharges, both plugs fire at the same time to complete the series circuit. The cylinder on compression is said to be the "event" cylinder and the one on exhaust is the "waste" cylinder.
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. This method of ignition is called a "waste spark" ignition system.
Since the polarity of the ignition coil primary and secondary windings is fixed, one spark plug always fires with a forward current flow and its "companion" plug fires with a reverse current flow. This is different from a conventional ignition system that fires all the plugs with the same direction of current flow. Since it requires approximately 30% more voltage to fire a spark plug backwards, the ignition coil design is improved, with saturation time and primary current flow increased. This allows higher secondary voltage to be available from the ignition coils - greater than 40 kilovolts (40,000 volts) at any engine RPM. The voltage required by each spark plug is determined by the polarity and the cylinder pressure. The cylinder on compression requires more voltage to fire the spark plug than the one on exhaust.
It is possible for one spark plug to fire even though a plug wire from the same coil may be disconnected from its "companion" plug. the disconnected plug wire acts as one plate of a capacitor, with the engine being the other plate. These two "capacitor plates" are charged as a 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 still-connected spark plug.
Secondary voltage requirements are very high with an "open" spark plug or wire. The ignition coil has enough reserve energy to fire the still-connected plug at idle, but possibly not under high engine load. A more noticeable misfire may be evident under load; both spark plugs may then be misfiring.
COMPONENTS
The PCM is responsible for maintaining proper spark and fuel injection timing for all driving conditions.
To provide optimum driveability and emissions, the PCM monitors input signals from the following components in calculating Ignition Control (IC) spark timing:
^ Ignition control module (ICM).
^ Engine Coolant Temperature (ECT) sensor.
^ Intake Air Temperature (IAT) sensor.
^ Mass Air Flow (MAF) sensor.
^ PRNDL input from transaxle range switch.
^ Throttle Position (TP) sensor.
^ Vehicle Speed Sensor (VSS).
^ Desired torque.
MODES OF OPERATION
The ignition system uses the same four ignition module-to-PCM circuits as did previous Delco engine management systems using distributor-type ignition. Ignition Control (IC) spark timing is the PCM's method of controlling spark advance and ignition dwell when the ignition system is operating in the IC mode.
There are two "modes" of ignition system operation:
Module Mode
In the module mode, the ignition system operates independently of the PCM, with module mode spark advance always at 10° BTDC. The PCM has no control of the ignition system when in this mode. In fact, the PCM could be disconnected from the vehicle and the ignition system would still fire the spark plugs, as long as the other ignition system components were functioning. (This would provide spark but no fuel injector pulses, and a no-start.) The PCM switches to IC mode (PCM controlled spark advance) as soon as the engine begins cranking. Once the change is made to IC mode, it will stay in effect until either: the engine is turned "OFF," the engine quits running, or a PCM / IC fault is detected. If a PCM / IC fault is detected while the engine is running, the ignition system will switch to the module mode. The engine may quit running, but will restart and stay in the module mode with a noticeable driveability complaint.
IC Mode
In the IC mode, the ignition spark timing and ignition dwell time is fully controlled by the PCM. IC spark advance and ignition dwell is calculated by the PCM using the following inputs:
^ Engine speed - (spark reference or fuel control reference)
^ Crankshaft position - (spark reference or fuel control reference)
^ Engine Coolant Temp - (ECT) sensor
^ Throttle Position - (TP) sensor
^ Knock Signal - Knock sensor
^ Park/Neutral Position - PRNDL input
^ Vehicle Speed - Vehicle Speed Sensor (VSS)
^ PCM and ignition system supply voltage.
PCM TO IGNITION CONTROL MODULE CIRCUITS
Fuel control reference PCM input (CKT 430)
From the ignition control module, the PCM uses this signal to calculate engine RPM and crankshaft position. The PCM compares pulses on this circuit to any that are on ground CKT 453, ignoring any pulses that appear on both. The PCM also uses the pulses on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, no fuel injection pulses will occur and the engine will not run.
Spark reference (CKT 647)
The spark reference signal is used to accurately control spark timing at low RPM and allow IC operation during crank. Below 1200 RPM, the PCM is monitoring CKT 647 and using it as the reference for ignition timing advance. When engine speed exceeds 1200 RPM, the PCM begins using CKT 430, fuel control reference to control spark timing. If the spark control reference circuit is not received by the PCM while the engine is running, a DTC P0321 will be set and fuel control reference will be used to control spark advance under 1200 RPM, and module mode will be in effect at under 400 RPM. The engine will continue to run and start normally.
Reference low (CKT 453)
This is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the ignition module. Although this circuit is electrically connected to the PCM, it is not connected to ground at the PCM. The PCM compares voltage pulses on the reference input CKT 430 to those on this circuit, ignoring pulses that appear on both. If the circuit is open, or connected to ground at the PCM, it may cause poor engine performance and possibly a Malfunction Indicator Lamp (MIL) (Service Engine Soon) with no DTC.
Bypass signal (CKT 424)
The PCM either allows the ignition control module to keep the spark advance at "module mode" 100 BTDC, or the PCM signals the ignition module that the PCM is going to control the spark advance (IC mode). The ignition control module determines correct operating mode based on the level of voltage that the PCM sends to the ignition control module on the bypass circuit. The PCM provides 5 volts to the ignition control module if the PCM is going to control spark timing (IC mode). If the PCM does not turn "ON" the 5 volts, or if the ignition control module doesn't receive it, the module will keep control of spark timing (module mode). An open or grounded bypass control CKT 424 will set DTC P1361 and the ignition system will stay at module mode advance.
Ignition Control (IC) output (CKT 423)
The IC output circuitry of the PCM sends out timing pulses to the ignition control module on this circuit. When in the "module mode," the ignition control module grounds these pulses. When in the IC mode, these pulses are the ignition timing pulses used by the ignition control module to energize one of the ignition coils. Proper sequencing of the 3 ignition coils, i.e.; which coil to "fire," is always the job of the ignition control module. If CKT 423 is grounded when the engine is started, DTC P1361 will set and the ignition system will stay in the module mode. If CKT 423 becomes open or grounded during IC mode operation, DTC P1350 or P1361 may set and the engine will quit running but will restart. Upon restart, following an ignition cycle, DTC P1361 will be set, and the ignition system will operate in "module mode."
Knock Sensor (KS) (CKT 496)
The KS system is comprised of a knock sensor and the PCM. The PCM monitors the KS signal (CKT 496) to determine when engine detonation occurs. When the knock sensor detects detonation, the PCM retards the timing (IC) to reduce detonation. Retarded timing can also be a result of excessive valve lifter, pushrod or other mechanical engine or transmission noise.
Cam signal (CKT 630)
The PCM uses this signal to determine the position of the # 1 piston during its power stroke This signal is used by the PCM to calculate true Sequential Fuel Injection (SFI) mode of operation. A loss of this signal will set DTC P0342 an extra cam pulse will set DTC P0341. If the cam signal is lost while the engine is running the fuel injection system will shift to a calculated sequential fuel injection mode based on the last cam pulse, and the engine will continue to run. The engine can be re-started and will run in the calculated sequential mode as long as the fault is present with a 1 in 6 chance of being correct.