Ignition System: Description and Operation
IGNITION SYSTEMNOTE: The 2.5L engines use a fixed ignition timing system. Basic ignition timing is not adjustable. All spark advance is determined by the Powertrain Control Module (PCM).
The 2.5L engine uses a distributor, crankshaft sensor and ignition coil. Basic ignition timing is not adjustable. The system's main components are the distributor, distributor pickup, camshaft signal, crankshaft signal and ignition coil.
OPERATION
The Crankshaft Position Sensor (CKP) and Camshaft Position Sensor (CMP) are hall effect devices. The Camshaft Position Sensor and Crankshaft Position Sensor generate pulses that are inputs to the PCM. The PCM determines crankshaft position from these sensors. The PCM calculates injector sequence and ignition timing from crankshaft position. For a description of both sensors, refer to Camshaft Position Sensor and Crankshaft Position Sensor.
PLATINUM SPARK PLUGS
The 2.5L engines utilize platinum spark plugs. Refer to the maintenance schedule.
All engines use resistor spark plugs. They have resistance values ranging from 6,000 to 20,000 ohms when checked with at least a 1000 volt spark plug tester.
Do not use an ohm meter to check the resistance of the spark plugs. This will give an inaccurate reading.
Platinum Pads:
The spark plugs are double platinum and have a recommended service life of 100,000 miles for normal driving conditions per schedule A. The spark plugs have a recommended service life of 75,000 miles for severe driving conditions per schedule B. A thin platinum pad is welded to both electrode ends as show in. Extreme care must be used to prevent spark plug cross threading, mis-gaping and ceramic insulator damage during plug removal and installation.
CAUTION: Cleaning of the platinum plug may damage the platinum tip.
SPARK PLUG CABLES
Spark Plug cables are sometimes referred to as secondary ignition wires. They transfer electrical current from the distributor to individual spark plugs at each cylinder. The resistor type, nonmetallic spark plug cables provide suppression of radio frequency emissions from the ignition system.
Check the spark plug cable connections for good contact at the coil and distributor cap towers and at the spark plugs. Terminals should be fully seated. The nipples and spark plug covers should be in good condition. Nipples should fit tightly on the coil and distributor cap towers and spark plug cover should fit tight around spark plug insulators. Loose cable connections can cause ignition malfunctions by permitting water to enter the towers, corroding, and increasing resistance. To maintain proper sealing at the terminal connections, the connections should not be broken unless testing indicates high resistance, an open circuit or other damage.
Clean high tension cables with a cloth moistened with a non-flammable solvent and wipe dry. Check for brittle or cracked insulation. Plastic clips in various locations protect the cables from damage. When the cables are replaced the clips must be used to prevent damage to the cables.
IGNITION COIL
Ignition Coil-2.5L Engine:
The ignition coil is located inside the distributor. The distributor is mounted to the right end of the engine block behind the thermostat housing.
The 2.5L engine uses an epoxy type coil. The coils are not oil filled. The windings are embedded in a heat and vibration resistant epoxy compound.
On a 2.5L, the PCM controls ignition timing by turning on and off the transistor in the distributor. By switching the ground path for the coil on and off, the PCM adjusts ignition timing to meet changing engine operating conditions.
The PCM operates the ignition coil through the Auto Shutdown (ASD) relay. When the relay is energized by the PCM, battery voltage is connected to the ignition coil positive terminal. The PCM will de-energize the ASD relay if it does not receive an input from the distributor pick-up. Refer to Auto Shutdown (ASD) Relay and Fuel Pump Relay.
AUTOMATIC SHUTDOWN RELAY
Power Distribution Center (PDC):
The ASD relay is located in the PDC. The inside top of the PDC cover has a label showing relay and fuse identification.
The Automatic Shutdown (ASD) relay supplies battery voltage to the fuel injectors, generator field, electronic ignition coil and the heating elements in the oxygen sensors.
The PCM controls the ASD relay by switching the ground path for the solenoid side of the relay on and off. The PCM turns the ground path off when the ignition switch is in the Off position unless the O2 Heater Monitor test is being run. Refer to the On-Board Diagnostics in the Emission Control. When the ignition switch is in On or Start, the PCM momentarily turns on the ASD relay. While the relay is on the PCM monitors the Crankshaft Position Sensor and Camshaft Position Sensor signals to determine engine speed and ignition timing (coil dwell). If the PCM does not receive Crankshaft Position Sensor and Camshaft Position Sensor signals when the ignition switch is in the Run position, it will de-energize the ASD relay.
CRANKSHAFT POSITION SENSOR
Crankshaft Position Sensor Location-Typical:
The Crankshaft Position Sensor is located on the rear of the transmission housing, above the differential housing. The sensor connector has a Christmas tree attached to the heater tube bracket. The bottom of the sensor is positioned next to the drive plate.
Crankshaft Position Sensor-Adjustable:
Timing Slots:
The Crankshaft Position Sensor detects slots cut into the transmission driveplate extension. There are 3 sets of slots. Each set contains 4 slots, for a total of 12 slots. Basic timing is set by the position of the last slot in each group. Once the Powertrain Control Module (PCM) senses the last slot, it determines crankshaft position (which piston will next be at TDC) from the Camshaft Position Sensor input. The 4 pulses generated by the Crankshaft Position Sensor represent the 69°, 49°, 29°, and 9° BTDC marks. It may take the PCM one engine revolution to determine crankshaft position.
The PCM uses crankshaft position reference to determine injector sequence, ignition timing and the presence of misfire. Once the PCM determines crankshaft position, it begins energizing the injectors in sequence.
CAMSHAFT POSITION SENSOR
Camshaft Position Sensor-2.5L Engine:
The 2.5L engine is equipped with a camshaft driven mechanical distributor, containing a shaft driven distributor rotor. The distributor is also equipped with an internal camshaft position (fuel sync) sensor. This sensor provides fuel injection synchronization and cylinder identification to the PCM.
The PCM determines fuel injection synchronization and cylinder identification from inputs provided by the camshaft position sensor and crankshaft position sensor. From the two inputs, the PCM determines crankshaft position.
The camshaft position sensor contains a hall effect device called a sync signal generator. This sync signal generator detects a rotating pulse ring (shutter) on the distributor shaft. The pulse ring rotates 180 through the sync signal generator. Its signal 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.
When the leading edge of the shutter enters the sync signal generator, the interruption of magnetic field causes the voltage to switch high. This causes a sync signal of approximately 5 volts.
When the trailing edge of the shutter leaves the sync signal generator, the change of magnetic field causes the sync signal voltage to switch low to 0 volts.
Since the shutter rotates at half crankshaft speed, it may take 1 engine revolution during cranking for the PCM to determine the position of piston number 6.
KNOCK SENSOR
The knock sensor threads into the cylinder block.
When the knock sensor detects a knock in one of the cylinders, it sends an input signal to the PCM. In response, the PCM retards ignition timing for all cylinders by a scheduled amount.
Knock sensors contain a piezoelectric material which constantly vibrates and sends an input voltage (signal) to the PCM while the engine operates. As the intensity of the crystal's vibration increases, the knock sensor output voltage also increases.
NOTE: Over or under tightening affects knock sensor performance, possibly causing improper spark control.
LOCK KEY CYLINDER
The lock cylinder is inserted in the end of the housing opposite the ignition switch.
Ignition Lock Cylinder Detents:
The ignition key rotates the cylinder to 5 different detents:
- Accessory/Off (Lock)
- Unlock
- On/Run
- Start
IGNITION INTERLOCK
All vehicles equipped with automatic transaxles have an interlock system. The system prevents shifting the vehicle out of Park unless the ignition lock cylinder is in the OFF, RUN or START position. In addition, the operator cannot rotate the key to the lock position unless the shifter is in the Park position. On vehicles equipped with floor shift refer to the Transaxle for Automatic Transmission Shifter/Ignition Interlock.