Ignition System: Description and Operation
IGNITION SYSTEMNOTE: These engines use a fixed ignition timing system. Basic ignition timing is not adjustable. All spark advance is determined by the Powertrain Control Module (PCM).
Fig.1 Ignition Coil:
The distributorless ignition system used on these 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 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
The engine uses resistor type spark plugs. 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.
Fig.2 Checking Spark Plug Electrode Gap:
Spark plugs that have low mileage may be cleaned and reused if not otherwise defective. Adjust the gap between the electrodes to the dimensions specified in the chart by bending the ground electrode just above the attachment weld) with the appropriate tool.
Never apply any force between the electrode or damage to the center electrode assembly will result.
WARNING: The tapered seat plugs for this application are torque-critical! It is imperative that 17.6 Nm ± 2 (13 ± 2 ft. lbs.) is NOT exceeded!
Always tighten spark plugs to the specified torque. Over tightening can cause distortion and damage.Tighten spark plugs to 17.6 ± 2 Nm (13 ± 2 ft. lbs.) torque.
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 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 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 vanous 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
WARNING: THE DIRECT IGNITION SYSTEM GENERATES APPROXIMATELY 40,000 VOLTS. PERSONAL INJURY COULD RESULT FROM CONTACT WITH THIS SYSTEM.
Fig.3 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.
Fig.4 Power Distribution Center (PDC):
AUTOMATIC SHUTDOWN RELAY
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 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
Fig.5 Crankshaft Position Sensor:
The crankshaft position sensor mounts to the engine block below the generator and near the oil filter.
The PCM sends approximately 9 volts to the Hall-effect sensor. This voltage is required to operate the Hall-effect chip and the electronics inside the sensor. A ground for the sensor is provided through the sensor return circuit. The input to the PCM occurs on a 5 volt output reference circuit that operates as follows: The Hall-effect sensor contains a powerful magnet. As the magnetic field passes over the dense portion of the counterweight, the 5-volt signal is pulled to ground (0.3 volts) through a transistor in the sensor. When the magnetic field passes over the notches in the crankshaft counterweight, the magnetic field turns OFF the transistor in the sensor, causing the PCM to register the 5-volt signal. The PCM identifies crankshaft position by registering the change from 5 to 0 volts, as signaled from the Crankshaft Position sensor.
Fig.5 Timing Reference Notches - Typical:
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 Tow (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
Fig.8 Target Magnet - Typical:
The camshaft position sensor is mounted to the rear of the cylinder bead. The sensor also acts as a thrust plate to control camshaft endplay.
The PCM sends approximately 9 volts to the Hall- effect sensor. This voltage is required to operate the Hall-effect chip and the electronics inside the sensor. The input to the PCM occurs on a 5 volt output reference circuit. A ground for the sensor is provided through the sensor return circuit. The PCM identifies camshaft position by registering the change from 5 to 0 volts, as signaled from the Camshaft Position sensor.
Fig.9 Target Magnet Polarity:
A target magnet attaches to the rear of the camshaft 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.
Fig.7 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.
KNOCK SENSOR
The knock sensor threads into the cylinder block. The knock sensor is designed to detect engine vibration that is caused by detonation.
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.
The voltage signal produced by the knock sensor increases with the amplitude of vibration. The PCM receives as an input the knock sensor voltage signal. If the signal rises above a predetermined level, the PCM will store that value in memory and retard ignition timing to reduce engine knock. If the knock sensor voltage exceeds a preset value, the PCM retards ignition timing for all cylinders. It is not a selective cylinder retard.
The PCM ignores knock sensor input during engine idle conditions. Once the engine speed exceeds a specified value, knock retard is allowed.
Knock retard uses its own short term and long term memory program.
Long term memory stores previous detonation information in its battery-backed RAM. The maximum authority that long term memory has over timing retard can be calibrated.
Short term memory is allowed to retard timing up to a preset amount under all operating conditions (as long as rpm is above the minimum rpm) except WOT. The PCM, using short term memory, can respond quickly to retard timing when engine knock is detected. Short term memory is lost any time the ignition key is turned OFF.
NOTE: Over or under tightening affects knock sensor performance, possibly causing improper spark control.
IGNITION SWITCH
In the RUN position, the ignition switch connects power from the Power Distribution Center (PDC) to a fuse in the fuse block, back to a bus bar in the PDC. The bus bar feeds circuits for the Powertrain Control Module (PCM), Proportional purge solenoid, EGR solenoid, and ABS system. The bus bar in the PDC feeds the coil side of the radiator fan relay, A/C compressor clutch relay, and the fuel pump relay. It also feeds the Airbag Control Module (ACM)
LOCK KEY CYLINDER
The lock cylinder is inserted in the end of the housing opposite the ignition switch.
Fig.10 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 Unlock, 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.