The vehicles covered by this information are equipped with a Computer Controlled Coil (C 3 I) ignition system. On later models, this system may also be called the Electronic Ignition (EI) system. The C 3 I (or EI) system is distributorless, and consists of 3 ignition coils, an Ignition Control Module (ICM), dual crank sensor, camshaft sensor, connecting wires, and the Electronic Spark Timing (EST) portion of the Electronic Control Module (ECM).
When the term Electronic Control Module (ECM) is used in this repair guide, it refers to the engine control computer, regardless of whether the term Powertrain Control Module (PCM) or Electronic Control Module (ECM) is used.
The ECM uses the EST circuit to control spark advance and ignition dwell, when the ignition system is operating in the EST mode. There are 2 modes of ignition system operation. These modes are as follows:
- Module mode - the ignition system operates independently of the ECM/PCM, with module mode spark advance always at 10 degrees BTDC. The ECM has no control of the ignition system when in this mode.
- EST mode - the ignition spark timing and ignition dwell time is fully controlled by the ECM. EST spark advance and ignition dwell is calculated by the ECM.
To control spark knock, and to use maximum spark advance to improve driveability and fuel economy, an Electronic Spark Control (ESC) system is used. This system consists of a knock sensor and the ECM. The ECM monitors the ESC signal to determine when engine detonation occurs.
The C 3 I ignition system uses a waste spark distribution method. Each cylinder is paired with the cylinder opposite it (1 and 4, 2 and 5, 3 and 6). The ends of each coil's secondary circuit is attached to a spark plug. These 2 plugs are on companion cylinders, cylinders that are at Top Dead Center (TDC) at the same time. The one that is on compression is said to be the "event" cylinder and the one on the exhaust stroke, the "waste" cylinder. When the coil discharges, both plugs fire at the same time to complete the series circuit.
Since the polarity of the primary and secondary windings are fixed, one plug always fires in a forward direction and the other in reverse. This is different than a conventional system firing all plugs the same direction each time. Because of the demand for additional energy; the coil design, saturation time and primary current flow are also different. This redesign of the system allows higher energy to be available from the distributorless coils, greater than 40 kilovolts at all rpm ranges.
During cranking, when the engine speed is beneath 400 rpm, the C 3 I module monitors the dual crank sensor sync signal. The sync signal is used to determine the correct pair of cylinders to be sparked first. Once the sync signal has been processed by the ignition module, it sends a fuel control reference pulse to the ECM.
During the cranking period, the ECM will also receive a cam pulse signal and will operate the injectors sequentially, based on true camshaft position only.
The sync signal is used only by the ignition module. It is used for spark synchronization at start-up only.
When the engine speed is below 400 rpm (during cranking), the C 3 I module controls spark timing. Once the engine speed exceeds 400 rpm (engine running), spark timing is controlled by the EST signal from the ECM. To control EST, the ECM uses the following inputs:
- Crankshaft position
- Engine speed (rpm)
- Coolant Temperature Sender
- Mass Air Flow
- Throttle Position Sensor
- Park/Neutral Switch
- Vehicle Speed Sensor
- ESC signal
The C 3 I ignition module provides proper ignition coil sequencing during both the module and the EST modes.
The ESC system is designed to retard spark timing up to 10 degrees to reduce spark knock in the engine. When the knock sensor detects spark knocking in the engine, it sends an A/C voltage signal to the ECM, which increases with the severity of the knock. The ECM then adjusts the EST to reduce spark knock.
See Figure 1
The C 3 I module monitors the sync-pulse and the crank signal. During cranking, the C 3 I module monitors the sync-pulse to begin the ignition firing sequence. During this time, each of the 3 coils are fired at a pre-determined interval based on engine speed only. Above 400 rpm, the C 3 I module is only use as a reference signal.
See Figure 1
The ignition coil assemblies are mounted on the C 3 I module. Each coil distributes the spark for 2 plugs simultaneously.Electronic Spark Control (ESC) System
The ESC system incorporates a knock sensor and the ECM. The knock sensor detects engine detonation. When engine detonation occurs, the ECM receives the ESC signal and retards EST to reduce detonation.Electronic Spark Timing (EST) System
The EST system includes the following circuits:
- Reference circuit - provides the ECM with rpm and crankshaft position information from the C 3 I module. The C 3 I module receives this signal from the crank sensor Hall-effect switch.
- Bypass signal - above 400 rpm, the ECM applies 5 volts to this circuit to switch spark timing control from the C 3 I module to the ECM.
- EST signal - reference signal is sent to the ECM via the C 3 I module during cranking. Under 400 rpm, the C 3 I module controls the ignition timing. Above 400 rpm, the ECM applies 5 volts to the bypass line to switch the timing to the ECM control.
Electronic Control Module (ECM) or Powertrain Control Module (PCM)
The ECM/PCM is responsible for maintaining proper spark and fuel injection timing for all driving conditions.Dual Crank Sensor
See Figures 2 and 3
The dual crank sensor is mounted in a pedestal on the front of the engine near the harmonic balancer. The sensor consists of 2 Hall-effect switches, which depend on 2 metal interrupter rings mounted on the balancer to activate them. Windows in the interrupters activate the Hall-effect switches as they provide a path for the magnetic field between the switches' transducers and magnets.
See Figure 4
The camshaft sensor sends signal to the ECM/PCM, which is used as a sync-pulse to trigger the injectors in the proper sequence.