Electronic Ignition System Description
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Electronic Ignition System Description
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Electronic Ignition (EI) System Operation
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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 engine control module (ECM) primarily collects information from the crankshaft position and camshaft position sensors to control the sequence, dwell, and timing of the spark.
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Crankshaft Position Sensor
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The crankshaft position sensor circuits consist of an engine control module (ECM) supplied 5 V reference circuit, low reference circuit, and an output signal circuit. The crankshaft position sensor is an internally magnetic biased digital output integrated circuit sensing device. The sensor detects magnetic flux changes of the teeth and slots of a 58-tooth reluctor wheel on the crankshaft. Each tooth on the reluctor wheel is spaced at 60-tooth spacing, with 2 missing teeth for the reference gap. The crankshaft position sensor produces an ON/OFF DC voltage of varying frequency, with 58 output pulses per crankshaft revolution. The frequency of the crankshaft position sensor output depends on the velocity of the crankshaft. The crankshaft position sensor sends a digital signal, which represents an image of the crankshaft reluctor wheel, to the ECM as each tooth on the wheel rotates past the crankshaft position sensor. The ECM uses each crankshaft position signal pulse to determine crankshaft speed and decodes the crankshaft reluctor wheel reference gap to identify crankshaft position. This information is then used to determine the optimum ignition and injection points of the engine. The ECM also uses output from the crankshaft position sensor to determine the camshaft position relative to the crankshaft, and to detect when a cylinder is misfiring.
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Crankshaft Reluctor Wheel
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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 poles provide cylinder location during a revolution.
The camshaft position sensor is triggered by a notched reluctor wheel built onto the intake camshaft sprocket. The camshaft position 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 camshaft position sensor is connected to the engine control module (ECM) by the following circuits:
The knock sensor system enables the engine control module (ECM) to control the ignition timing for the best possible performance while protecting the engine from potentially damaging levels of detonation. The knock sensor produces an alternating current signal that varies with the level of vibration with the engine running. The ECM adjusts the spark timing according to the amplitude and frequency in the knock sensor signal. The ECM receives the knock sensor signals via 2 isolated signal circuits. The ECM learns a minimum knock sensor noise level for the entire speed range. The ECM monitors a normal knock sensor signal.
The ECM generates a 20 kHz signal on the outgoing circuit and monitors that this is sent back and registered on the return circuit. The ECM processes the return signal to check the internal knock sensor processor.
Each ignition coil/module has the following circuits:
The engine control module (ECM) 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 coated with platinum for long wear and higher efficiency.
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Engine Control Module (ECM)
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The engine control module (ECM) controls all ignition system functions, and constantly corrects the spark timing. The ECM monitors information from various sensor inputs that include the following (if applicable):
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Throttle position (TP) sensor
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Engine coolant temperature (ECT) sensor
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Mass air flow (MAF) sensor
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Intake air temperature (IAT) sensor
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The vehicle speed sensor (VSS)
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Engine knock sensor (KS)
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Manifold absolute pressure (MAP) sensor
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During normal operation the engine control module (ECM) controls all ignition functions. If either the crankshaft position or camshaft position sensor signal is lost, the engine will continue to run because the ECM will default to a limp home mode using the remaining sensor input. 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.
