Electronic Ignition (EI) System Description

Electronic Ignition (EI) System Description

Electronic Ignition (EI) System Operation

The electronic ignition (EI) system produces and controls the high energy secondary spark. This spark ignites the compressed air/fuel mixture at precisely the correct time, providing optimal performance, fuel economy, and control of exhaust emissions. The powertrain control module (PCM) primarily collects information from the crankshaft position (CKP) and camshaft position (CMP) sensors to control the sequence, dwell, and timing of the spark.

Crankshaft Position (CKP) Sensor

The crankshaft position (CKP) sensor is a permanent magnet generator known as a variable reluctance sensor. The CKP sensor produces an AC voltage of varying amplitude and frequency. The frequency depends on the velocity of the crankshaft. The AC output depends on the crankshaft position and the battery voltage. The CKP sensor works in conjunction with a 58-tooth reluctor wheel attached to the crankshaft. As each reluctor wheel tooth rotates past the CKP sensor, the resulting change in the magnetic field creates an ON/OFF pulse 58 times per crankshaft revolution. The PCM processes the pulses to create a pattern that enables the PCM to determine the crankshaft position. The PCM can synchronize the ignition timing, the fuel injector timing, and the spark knock control based on the CKP sensor and the camshaft position (CMP) sensor inputs. The CKP sensor is also used to detect misfire and for tachometer display. The PCM learns the variations between all 58 teeth under different speed and load conditions to correctly detect misfires. The CKP sensor circuits consist of a signal circuit and a low reference circuit. The two wires are twisted together to prevent electromagnetic interference on the CKP sensor circuits.

Crankshaft Reluctor Wheel

The crankshaft reluctor wheel is part of the crankshaft. The reluctor wheel consists of 58 teeth and a reference gap. Each tooth on the reluctor wheel is spaced 6 degrees apart with a 12-degree space for the reference gap. The pulse from the reference gap is known as the sync pulse. The sync pulse is used to synchronize the coil firing sequence with the crankshaft position, while the other teeth provide cylinder location during a revolution.

Camshaft Position (CMP) Sensor

The camshaft position (CMP) sensor is triggered by a notched reluctor wheel built onto the exhaust camshaft sprocket. The CMP sensor provides four signal pulses every camshaft revolution. Each notch, or feature of the reluctor wheel is of a different size which is used to identify the compression stroke of each cylinder and to enable sequential fuel injection. The CMP sensor is connected to the PCM by the following circuits:

* A 5-volt circuit
* A low reference circuit
* A signal circuit

Ignition Coil/Module

Each ignition coil/module has the following circuits:

* An ignition voltage circuit
* A ground
* An ignition control (IC) circuit
* A low reference circuit

The PCM controls the individual coils by transmitting timing pulses on the IC circuit of each ignition coil/module to enable a spark event.

The spark plugs are connected to each coil by a short boot. The boot contains a spring that conducts the spark energy from the coil to the spark plug. The spark plug electrode is tipped with platinum for long wear and higher efficiency.

Powertrain Control Module (PCM)

The PCM controls all ignition system functions, and constantly corrects the spark timing. The PCM monitors information from various sensor inputs that include the following:

* The throttle position (TP) sensor
* The engine coolant temperature (ECT) sensor
* The mass air flow (MAF) sensor
* The intake air temperature (IAT) sensor
* The vehicle speed sensor (VSS)
* The engine knock sensor (KS)

Modes of Operation

During normal operation the PCM controls all ignition functions. If either the CKP or CMP sensor signal is lost, the engine will continue to run because the PCM will default to a limp home mode using the remaining sensor input. As mentioned above, each coil is internally protected against damage from excessive voltage. If one or more coils were to fail in this manner, a misfiring condition would result. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.