Ignition System: Description and Operation
IGNITION SYSTEMNOTE: The 2.4 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 distributorless ignition system used on 2.4L engines is referred to as the Direct Ignition System (DIS). Basic ignition timing is not adjustable.
The system's three main components are the coil pack, Crankshaft Position Sensor (CKP), and Camshaft Position Sensor (CMP).
OPERATION
The Crankshaft Position Sensor and Camshaft Position Sensor 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.
RESISTOR SPARK PLUGS
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.
Refer to the Specifications for gap and type of spark plug.
SPARK PLUG CABLES
Spark plug cables are sometimes referred to as secondary ignition wires. They transfer electrical current from the coil pack 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.
ELECTRONIC IGNITION COIL
Ignition Coil Pack - 2.4L Engine:
The coil pack consists of 2 coils molded together. The coil pack is mounted on the valve cover. High tension leads route to each cylinder from the coil.
WARNING: THE DIRECT IGNITION SYSTEM GENERATES APPROXIMATELY 40,000 VOLTS. PERSONAL INJURY COULD RESULT FROM CONTACT WITH THIS SYSTEM.
The coil fires two spark plugs every power stroke. One plug is the cylinder under compression, the other cylinder fires on the exhaust stroke. Coil number one fires cylinders 1 and 4. Coil number two fires cylinders 2 and 3. The PCM determines which of the coils to charge and fire at the correct time.
The Auto Shutdown (ASD) relay provides battery voltage to the ignition coil. The PCM provides a ground contact (circuit) for energizing the coil. When the PCM breaks the contact, the energy in the coil primary transfers to the secondary causing the spark. The PCM will de-energize the ASD relay if it does not receive the Crankshaft Position Sensor and Camshaft Position Sensor inputs. Refer to Auto Shutdown (ASD) Relay-PCM Output, for relay operation.
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:
The Crankshaft Position Sensor mounts to the engine block behind the generator, just above the oil filter.
Timing Reference Notches:
The PCM determines what cylinder to fire from the Crankshaft Position Sensor input and the Camshaft Position Sensor input. The second crankshaft counterweight has machined into it two sets of four timing reference notches including a 60 degree signature notch. From the Crankshaft Position Sensor input the PCM determines engine speed and crankshaft angle (position).
The notches generate pulses from high to low in the Crankshaft Position Sensor output voltage. When a metal portion of the counterweight aligns with the Crankshaft Position Sensor, the sensor output voltage goes low (less than 0.5 volts). When a notch aligns with the sensor, voltage goes high (5.0 volts). As a group of notches pass under the sensor, the output voltage switches from low (metal) to high (notch) then back to low.
CAMSHAFT POSITION SENSOR
Target Magnet - Typical:
The Camshaft Position Sensor attaches to the rear of the cylinder head. A target magnet attaches to the rear of the camshaft and indexes to the correct position. The sensor also acts as a thrust plate to control camshaft endplay.
Camshaft Position Sensor - 2.4L DOHC:
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.
Target Magnet Polarity:
The target magnet has four different poles arranged in an asymmetrical pattern. As the target magnet rotates, the Camshaft Position Sensor senses the change in polarity. The sensor input switches from high (5 volts) to low (0.30 volts) as the target magnet rotates. When the north pole of the target magnet passes under the sensor; the output switches high. The sensor output switches low when the south pole of the target magnet passes underneath.
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.