Ignition System: Description and Operation
IGNITION SYSTEMThe 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 Sensor, Camshaft Position Sensor, Throttle Position Sensor and MAP Sensor.
Distributor and Camshaft Position Sensor:
DISTRIBUTOR
This engine is 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.
SPARK PLUGS
These engines use resistor type spark plugs.
Spark plug resistance values range from 6000 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 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
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.
CRANKSHAFT POSITION SENSOR
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.
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.
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.