The ignition system is designed to ignite the compressed air/fuel mixture in an internal combustion engine by a high voltage spark from an ignition coil. The ignition system also provides engine timing information to the powertrain control module (PCM) for proper vehicle operation and misfire detection.
Integrated Electronic Ignition (EI) System
The integrated EI system consists of a crankshaft position (CKP) sensor, coil pack(s), connecting wiring, and PCM. The coil-on-plug (COP) integrated EI system uses a separate coil per spark plug, and each coil is mounted directly onto the plug. The COP integrated EI system eliminates the need for spark plug wires, but does require input from the camshaft position (CMP) sensor. Operation of the components are as follows:
Note: Electronic ignition engine timing is entirely controlled by the PCM. Electronic ignition engine timing is NOT adjustable. Do not attempt to check base timing. You will receive false readings.The CKP sensor is used to indicate the crankshaft position and speed by sensing a missing tooth on a pulse wheel mounted to the crankshaft. The CMP sensor is used by the COP integrated EI system to identify top dead center of compression of cylinder 1 to synchronize the firing of the individual coils.
- The PCM uses the CKP signal to calculate a spark target and then fires the coil pack(s) to that target shown. The PCM uses the CMP sensor, not COP integrated EI systems to identify top dead center of compression of cylinder 1 to synchronize the firing of the individual coils.
- The coils and coil packs receive their signal from the PCM to fire at a calculated spark target. Each coil within the pack fires 2 spark plugs at the same time. The plugs are paired so that as one fires during the compression stroke the other fires during the exhaust stroke. The next time the coil is fired the situation is reversed. The COP system fires only one spark plug per coil and only on the compression stroke.
The PCM acts as an electronic switch to ground in the coil primary circuit. When the switch is closed, battery positive voltage (B+) applied to the coil primary circuit builds a magnetic field around the primary coil. When the switch opens, the power is interrupted and the primary field collapses inducing the high voltage in the secondary coil windings and the spark plug is fired. A kickback voltage spike occurs when the primary field collapses. The PCM uses this voltage spike to generate an ignition diagnostic monitor (IDM) signal. IDM communicates information by pulse width modulation in the PCM.
- The PCM processes the CKP signal and uses it to drive the tachometer as the clean tach out (CTO) signal.
Figure 54: Integrated Electronic Ignition (EI) System (Refer to the On-Board Diagnostics Monitor System Overview for Icon Definitions.)
Figure 55: Six Cylinder Integrated Electronic Ignition (EI) Waveforms. 4, 8, and 10-Cylinder are Similar.
Crankshaft Position (CKP) Sensor
The CKP sensor is a magnetic transducer mounted on the engine block adjacent to a pulse wheel located on the crankshaft. By monitoring the crankshaft mounted pulse wheel, the CKP is the primary sensor for ignition information to the PCM. The pulse wheel has a total of 35 teeth spaced 10 degrees apart with one empty space for a missing tooth. The 6.8L 10-cylinder pulse wheel has 39 teeth spaced 9 degrees apart and one 9 degree empty space for a missing tooth. By monitoring the pulse wheel, the CKP sensor signal indicates crankshaft position and speed information to the PCM. By monitoring the missing tooth, the CKP sensor is also able to identify piston travel in order to synchronize the ignition system and provide a way of tracking the angular position of the crankshaft relative to a fixed reference for the CKP sensor configuration. The PCM also uses the CKP signal to determine if a misfire has occurred by measuring rapid decelerations between teeth.
Figure 56: Typical Crankshaft Position (CKP) Sensor (Actual Sensor May Vary)
Camshaft Position (CMP) Sensor
The CMP sensor used by COP integrated EI system is a magnetic transducer mounted on the engine front cover adjacent to the camshaft. By monitoring a target on the camshaft sprocket, the CMP sensor identifies cylinder one to the PCM. The COP integrated EI system uses this information to synchronize the firing of the individual coils.
Figure 57: Camshaft Position (CMP) Sensor
Coil packs come in 4-tower, Series 5 4-tower, 6-tower horizontal connector and Series 5 6-tower models. Two adjacent coil towers share a common coil and are called a matched pair. For 6-tower coil pack (6 cylinder) applications the matched pairs are 1 and 5, 2 and 6, and 3 and 4. For 4-tower coil pack (4 cylinder) applications the matched pairs are 1 and 4, and 2 and 3.
When the coil is fired by the PCM, spark is delivered through the matched pair towers to their respective spark plugs. The spark plugs are fired simultaneously and are paired so that as one fires on the compression stroke, the other spark plug fires on the exhaust stroke. The next time the coil is fired the situation is reversed. The next pair of spark plugs fire according to the engine firing order.
Figure 58: Horizontal Connector 6-Tower Coil Pack
Figure 59: Series 5 6-Tower Coil Pack
Figure 60: Four-Tower Coil Packs
Coil On Plug (COP)
The COP ignition operates similar to standard coil pack ignition except each plug has one coil per plug. COP has 3 different modes of operation: engine crank, engine running, and CMP Failure Mode Effects Management (FMEM).
Engine Crank/Engine Running
During engine crank the PCM will fire 2 spark plugs simultaneously. Of the 2 plugs simultaneously fired one will be under compression the other will be on the exhaust stroke. Both plugs will fire until camshaft position is identified by a successful camshaft position (CMP) sensor signal. Once camshaft position is identified only the cylinder under compression will be fired.
During CMP FMEM the COP ignition works the same as during engine crank. This allows the engine to operate without the PCM knowing if cylinder one is under compression or exhaust.