Ignition System: Description and Operation
IGNITION CONTROLTwo different ignition Systems are used. One type of system is for the 2 5L 4-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 (2.5L/3.9L/5.9L engines)
- Distributor (contains rotor and camshaft position sensor) (2.5L/3.9L/5.9L engines)
- Powertrain Control Module (PCM)
- Crankshaft Position and Camshaft Position Sensors
- The MAP TPS, JAC and ECT also have an effect on the control of the ignition system.
AUTO SHUT DOWN RELAY
PCM Output
The 5-pin, 12-volt, Automatic Shutdown (ASD) relay is located in the Power Distribution Center (PDC). Refer to label on PDC cover for relay location.
The ASD relay supplies battery voltage (12+ volts) to the fuel injectors and ignition coil(s). With certain emissions packages it also supplies 12-volts to the oxygen sensor heating elements.
The ground circuit for the coil within the ASD relay is controlled by the Powertrain Control Module (PCM). The PCM operates the ASD relay by switching its ground circuit on and off.
The ASD relay will be shut-down, meaning the 12-volt power supply to the ASD relay will be de-activated by the PCM if:
- the ignition key is left in the ON position. This is if the engine has not been running for approximately 1.8 seconds.
- there is a crankshaft position sensor signal to the PCM that is lower than pre-determined values.
ASD Sense - PCM Input
A 12 volt signal at this input indicates to the PCM that the ASD has been activated. The relay is used to connect the oxygen sensor heater element, ignition coil and fuel injectors to 12 volt + power supply.
This input is used only to sense that the ASD relay is energized. If the Powertrain Control Module (PCM) does not see 12 volts at this input when the ASD should be activated, it will set a Diagnostic Trouble Code (DTC).
CAMSHAFT POSITION SENSOR
Except 4.7L
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.
Fig.4 CMP Location - 4.7L Engine:
4.7L
The Camshaft Position Sensor (CMP) on the 4.7L V-8 engine is bolted to the front/top of the right cylinder head.
Fig.5 CMP Sensor And Tonewheel - 4.7L Engine:
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 cylinder
Fig.8 Distributor And Camshaft Position Sensor - Typical (3.9/5.9L Shown):
DISTRIBUTOR
All this engine 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 Powertrain Control Module (PCM). Because ignition timing is controlled by the PCM, base ignition timing is not adjustable on any of these engines.
All this distributors contain an internal oil seal that prevents oil from entering the distributor housing. The seal is not serviceable.
IGNITION COIL
Except 4.7L
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.
Fig.22 Ignition Coil - 4.7L Engine:
Fig.23 Ignition Coil Location - 4.7L Engine:
4.7L
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.
SPARK PLUG
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 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. Replace spark plugs at the intervals recommended in Lubrication and Maintenance.
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 CABLE
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.