Ignition System: Description and Operation

IGNITION SYSTEM

NOTE: The 2.0, 2.4, and 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 distributorless ignition system used on 2.0/2.4L 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, and camshaft position sensor.

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.

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.

SPARK PLUGS-2.0/2.4L
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.

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. An isolated plug displaying an abnormal condition indicates that a problem exists in the corresponding cylinder.

Setting Spark Plug Electrode Gap:

Spark plugs that have low mileage may be cleaned and reused if not otherwise defective, carbon or oil fouled. Refer to the Spark Plug Condition. After cleaning, file the center electrode flat with a small flat point file or jewelers file. Adjust the gap between the electrodes to the dimensions specified in the chart.

Special care should be used when installing spark plugs in the 2.0/2.4L cylinder head spark plug wells. Be sure the plugs do not drop into the wells, damage to the electrodes can occur.

Always tighten spark plugs to the specified torque. Over tightening can cause distortion resulting in a change in the spark plug gap. Overtightening can also damage the cylinder head. Tighten spark plugs to 28 N.m (20 ft. lbs.) torque.

SPARK PLUGS PLATINUM
The engine 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. 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 (2.5L), coil pack (2.0/2.4L), 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-2.0/2.4L

WARNING: THE DIRECT IGNITION SYSTEM GENERATES APPROXIMATELY 40,000 VOLTS. PERSONAL INJURY COULD RESULT FROM CONTACT WITH THIS SYSTEM.

Ignition Coil Pack:

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. 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.

IGNITION COIL-2.5L

Ignition Coil:

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 02 Heater Monitor test is being run. Refer to the On-Board Diagnostics. 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 and camshaft position sensor signals to determine engine speed and ignition timing (coil dwell) If the PCM does not receive crankshaft and camshaft position sensor signals when the ignition switch is in the Run position, it will de-energize the ASD relay.

CRANKSHAFT POSITION SENSOR-2.0/2.4L

Crankshaft Position Sensor:

The crankshaft position sensor mounts to the engine block behind the generator, just above the oil filter.

Timing References 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.

CRANKSHAFT POSITION SENSOR-2.5L

Crankshaft Position Sensor Location-Typical:

The crankshaft 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-2.0/2.4L

Target Magnet - Typical:

The camshaft position sensor is mounted to the rear of the cylinder head, The sensor also acts as a thrust plate to control camshaft endplay.

Target Magnet Polarity:

A target magnet attaches to the rear of the cam- shaft and indexes to the correct position. 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.

Camshaft Position Sensor:

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 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.

CAMSHAFT POSITION SENSOR-2.5L

Camshaft Position Sensor:

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.