Ignition System: Description and Operation
IGNITION CONTROLThe ignition system is controlled by the Powertrain Control Module (PCM) on all engines.
The ignition system consists of:
- Spark Plugs
- Ignition Coil
- Secondary Ignition Cables
- Distributor (contains rotor and camshaft position sensor)
- Powertrain Control Module (PCM)
Also to be considered part of the ignition system are certain inputs from the Crankshaft Position, Camshaft Position, Throttle Position and MAP Sensors.
AUTOMATIC 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
Fig. 11 Camshaft Position Sensor - Typical:
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. 17 Distributor And Camshaft Position Sensor:
DISTRIBUTOR
All 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 Powertrain Control Module (PCM). Because ignition timing is controlled by the PCM, base ignition timing is not adjustable.
The distributor is held to the engine in the conventional method using a holddown clamp and bolt.
Although the distributor can be rotated, it will have no effect on ignition timing.
All distributors contain an internal oil seal that prevents oil from entering the distributor housing. The seal is not serviceable.
IGNITION COIL
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.
SPARK PLUG
The 5.9L V-8 engines use resistor type spark plugs.
Spark plug resistance values range from 6,000 to 20,000 ohms (when checked with at least a 1000 volt spark plug tester). Do not use an ohmmeter to check the resistance values of the spark plugs. Inaccurate readings will result.
To prevent possible pre-ignition and/or mechanical engine damage, the correct type/heat range/number spark plug must be used.
Always use the recommended torque when tightening spark plugs. Incorrect torque can distort the spark plug and change plug gap. It can also pull the plug threads and do possible damage to both the spark plug and the cylinder head.
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 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.