Ignition System: Description and Operation
ONIGNITION SYSTEMTwo different ignition systems are used. One type of system is for the 2.5L cylinder, the 3.9L V-6 engine and the 5.9L V-8 engine. The other is for the 4.7L V-8 engine.
The ignition systems used on 2.5L 4-cylinder, 3.9L V-6 and 5.9L V-8 engines are basically identical using a conventional distributor and remotely mounted coil. The 4.7L V-8 engine does not use a distributor and has 8 separate coils.
The ignition system is controlled by the Powertrain Control Module (PCM) on all engines.
The ignition system consists of:
- Spark Plugs
- Ignition Coil(s)
- Secondary Ignition Cables 25L/3.9L/5.9L engines)
- Distributor (contains rotor and camshaft position sensor) 25L/3.9L/5.9L engines)
- Powertrain Control Module (PCM)
- Crankshaft Position and Camshaft Position Sensors
- The MAP, TPS, IAC and ECT also have an effect on the control of the ignition system.
DISTRIBUTOR
Distribution And Camshaft Position Sensor - Typical (3.9L/5.9L):
All 2.5L/3.9L/5.9L engines are equipped with a camshaft driven mechanical distributor containing a shaft driven distributor rotor. All distributors are equipped with an internal camshaft position (fuel sync) sensor.
The camshaft position sensor provides fuel injection synchronization and cylinder identification.
The distributor does not have built in centrifugal or vacuum assisted advance. Base ignition timing and all timing advance is controlled by the Power- train Control Module (PCM). Because ignition timing is controlled by the PCM, base ignition timing is not adjustable on any of these engines.
All 2.5L/3.9L/5.9L distributors contain an internal oil seal that prevents oil from entering the distributor housing. The seal is not serviceable.
SPARK PLUGS
All engines use resistor type spark plugs. 4.7L V-8 engines are equipped with "fired in suppressor seal" type spark plugs using a copper core ground electrode.
Because of the use of an aluminum cylinder head on the 4.7L engine, spark plug torque is very critical.
To prevent possible pre-ignition and/or mechanical engine damage, the correct type/heat range/number spark plug must be used.
Remove the spark plugs and examine them for burned electrodes and fouled, cracked or broken porcelain insulators. Keep plugs arranged in the order in which they were removed from the engine. A single plug displaying an abnormal condition indicates that a problem exists in the corresponding cylinder.
Spark plugs that have low mileage may be cleaned and reused if not otherwise defective, carbon or oil fouled. Also refer to Spark Plug Conditions.
CAUTION: Never use a motorized wire wheel brush to clean the spark plugs. Metallic deposits will remain on the spark plug insulator and will cause plug misfire.
SPARK PLUG CABLES
Spark plug cables are sometimes referred to as secondary ignition wires.
The spark plug cables transfer electrical current from the ignition coil(s) and/or distributor, to individual spark plugs at each cylinder. The resistive spark plug cables are of nonmetallic construction, The cables provide suppression of radio frequency emissions from the ignition system.
IGNITION COIL (EXCEPT 4.7L ENGINE)
A single ignition coil is used. The coil is not oil filled. The coil windings are embedded in an epoxy compound. This provides heat and vibration resistance that allows the coil to be mounted on the engine.
The Powertrain Control Module (PCM) opens and closes the ignition coil ground circuit for ignition coil operation.
Battery voltage is supplied to the ignition coil positive terminal from the ASD relay. If the PCM does not see a signal from the crankshaft and camshaft sensors (indicating the ignition key is ON but the engine is not running), it will shut down the ASD circuit.
Base ignition timing is not adjustable on any engine. By controlling the coil ground circuit, the PCM is able to set the base timing and adjust the ignition timing advance. This is done to meet changing engine operating conditions.
IGNITION COIL-4.7L ENGINE
Ignition Coil:
Ignition Coil Location:
The 4.7L V-8 engine uses 8 dedicated, and individually fired coil for each spark plug. Each coil is mounted directly to the top of each spark plug.
Battery voltage is supplied to the 8 ignition coils from the ASD relay. The Powertrain Control Module (PCM) opens and closes each ignition coil ground circuit at a determined time for ignition coil operation.
Base ignition timing is not adjustable. By controlling the coil ground circuit, the PCM is able to set the base timing and adjust the ignition timing advance. This is done to meet changing engine operating conditions.
The ignition coil is not oil filled. The windings are embedded in an epoxy compound. This provides heat and vibration resistance that allows the ignition coil to be mounted on the engine.
Because of coil design, spark plug cables (secondary cables) are not used.
CRANKSHAFT POSITION SENSOR-2.5L ENGINE
The Crankshaft Position (CKP) sensor is located near the outer edge of the flywheel (or starter ringear).
Engine speed and crankshaft position are provided through the CKP sensor. The sensor generates pulses that are the input sent to the Powertrain Control Module (PCM). The PCM interprets the sensor input to determine the crankshaft position. The PCM then uses this position, along with other inputs, to determine injector sequence and ignition timing.
The sensor is a hall effect device combined with an internal magnet. It is also sensitive to steel within a certain distance from it.
CKP Sensor Operation:
The flywheel/drive plate has groups of four notches at its outer edge. On 2.5L 4-cylinder engines there are two sets of notches.
The notches cause a pulse to be generated when they pass under the sensor. The pulses are the input to the PCM. For each engine revolution there are two groups of four pulses generated on 2.5L 4-cylinder engines.
The trailing edge of the fourth notch, which causes the pulse, is four degrees before top dead center (TDC) of the corresponding piston.
The engine will not operate if the PCM does not receive a CKP sensor input.
CRANKSHAFT POSITION SENSOR-3.9L V-6 ENGINE
The Crankshaft Position (CKP) sensor is located near the outer edge of the flywheel (starter ringear).
Engine speed and crankshaft position are provided through the CKP sensor. The sensor generates pulses that are the input sent to the Powertrain Control Module (PCM). The PCM interprets the sensor input to determine the crankshaft position. The PCM then uses this position, along with other inputs, to determine injector sequence and ignition timing.
The sensor is a hall effect device combined with an internal magnet. It is also sensitive to steel within a certain distance from it.
CKP Sensor Operation:
The flywheel/drive plate has groups of notches at its outer edge. On 3.9L V-6 engines, there are three sets of double notches and three sets of single notches.
The notches cause a pulse to be generated when they pass under the sensor. The pulses are the input to the PCM.
The engine will not operate if the PCM does not receive a CKP sensor input.
CRANKSHAFT POSITION SENSOR-5.2/5.9L V-8 ENGINE
The Crankshaft Position (CKP) sensor is located near the outer edge of the flywheel (starter ringear).
Engine speed and crankshaft position are provided through the CKP sensor. The sensor generates pulses that are the input sent to the Powertrain Control Module (PCM). The PCM interprets the sensor input to determine the crankshaft position. The PCM then uses this position, along with other inputs, to determine injector sequence and ignition timing.
The sensor is a hall effect device combined with an internal magnet. It is also sensitive to steel within a certain distance from it.
CKP Sensor Operation:
On 5.2/5.9L V-8 engines, the flywheel/drive plate has 8 single notches, spaced every 45 degrees, at its outer edge.
The notches cause a pulse to be generated when they pass under the sensor. The pulses are the input to the PCM. For each engine revolution, there are 8 pulses generated on V-8 engines.
The engine will not operate if the PCM does not receive a CKP sensor input.
CKP Sensor Location:
CRANKSHAFT POSITION SENSOR-4.7L ENGINE
The Crankshaft Position Sensor (CKP) is mounted into the right-rear side of the engine block.
Engine speed and crankshaft position are provided through the crankshaft position sensor. The sensor generates pulses that are the input sent to the powertrain control module (PCM). The PCM interprets the sensor input to determine the crankshaft position. The PCM then uses this position, along with other inputs, to determine injector sequence and ignition timing.
The sensor is a hall effect device combined with an internal magnet. It is also sensitive to steel within a certain distance from it.
CKP Sensor Operation And Tonewheel:
On the 4.7L V-8 engine, a tonewheel is bolted to the engine crankshaft. This tonewheel has sets of notches at its outer edge.
The notches cause a pulse to be generated when they pass under the sensor. The pulses are the input to the PCM.
CAMSHAFT POSITION SENSOR
The Camshaft Position (CMP) sensor is located in the distributor.
The sensor contains a hall effect device called a sync signal generator to generate a fuel sync signal. This sync signal generator detects a rotating pulse ring (shutter) on the distributor shaft. The pulse ring rotates 180 degrees through the sync signal generator. Its signal is used in conjunction with the Crankshaft Position (CKP) sensor to differentiate between fuel injection and spark events. It is also used to synchronize the fuel injectors with their respective cylinders.
When the leading edge of the pulse ring (shutter) enters the sync signal generator, the following occurs:
The interruption of magnetic field causes the voltage to switch high resulting in a sync signal of approximately 5 volts.
When the trailing edge of the pulse ring (shutter) leaves the sync signal generator, the following occurs:
The change of the magnetic field causes the sync signal voltage to switch low to 0 volts.
CMP Location:
CAMSHAFT POSITION SENSOR-4.7L ENGINE
The Camshaft Position Sensor (CMP) on the 4.7L V-8 engine is bolted to the front/top of the right cylinder head.
CMP Sensor And Tonewheel:
The CMP sensor contains a hall effect device called a sync signal generator to generate a fuel sync signal. This sync signal generator detects notches located on a tonewheel. The tonewheel is located at the front of the camshaft for the right cylinder head. As the tonewheel rotates, the notches pass through the sync signal generator. The pattern of the notches (viewed counter-clockwise from front of engine) is: 1 notch, 2 notches, 3 notches, 3 notches, 2 notches 1 notch, 3 notches and 1 notch. The signal from the CMP sensor is used in conjunction with the crankshaft position sensor to differentiate between fuel injection and spark events. It is also used to synchronize the fuel injectors with their respective cylinders.
IGNITION SWITCH AND KEY LOCK CYLINDER
The electrical ignition switch is located on the steering column. It is used as the main�ON/OFF switching device for most electrical components. The mechanical key lock cylinder is used to engage/disengage the electrical ignition switch.
Vehicles equipped with an automatic transmission and a floor mounted shifter: a cable is used to connect the interlock device in the steering column assembly, to the transmission floor shift lever. This interlock device is used to lock the transmission shifter in the PARK position when the key lock cylinder is rotated to the LOCKED or ACCESSORY position. The interlock device within the steering column is not serviceable. If repair is necessary, the steering column assembly must be replaced.
If the ignition key is difficult to rotate to or from the LOCK or ACCESSORY position, it may not be the fault of the key cylinder or the steering column components. The brake transmission shift interlock cable may be out of adjustment.
Vehicles equipped with an automatic transmission and a steering column mounted shifter:an interlock device is located within the steering column. This interlock device is used to lock the transmission shifter in the PARK position when the key lock cylinder is in the LOCKED or ACCESSORY position. If it is difficult to rotate the key to or from the LOCK or ACCESSORY position, the interlock device within the steering column may be defective. This device is not serviceable. If repair is necessary, the steering column assembly must be replaced.
Vehicles equipped with a manual transmission, a floor mounted shifter, and a LEVER below the ignition key: A lever is located on the steering column behind the ignition key lock cylinder. The lever must be manually operated to allow rotation of the ignition key lock cylinder to the LOCK or ACCESSORY position. If it is difficult to rotate the key to the LOCK or ACCESSORY position, the lever mechanism may be defective. This mechanism is not serviceable. If repair is necessary, the steering column assembly must be replaced.
Vehicles equipped with a manual transmission, a floor mounted shifter, and NO LEVER below the ignition key: The ignition key cylinder must be depressed to allow it to be rotated into the LOCK or ACCESSORY position. If it is difficult to rotate the key to the LOCK or ACCESSORY position, the lock mechanism within the steering column may be defective. This mechanism is not serviceable. If repair is necessary, the steering column assembly must be replaced.