Ignition System: Description and Operation
IGNITION SYSTEMNOTE: All 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 distributor less ignition system used on these engines is referred to as the Direct Ignition System (DIS). The system's three main components are the coils, crankshaft position sensor, and camshaft position sensor. The coil on plug ignition system utilizes an ignition coil for every cylinder, it is mounted directly over the each spark plug.
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 engine position from these sensors. The PCM calculates injector sequence and ignition timing from crankshaft & camshaft position. For a description of both sensors, refer to Camshaft Position Sensor and Crankshaft Position Sensor.
Fig.1 Power Distribution Center:
AUTO SHUT DOWN RELAY
The ASD relay is located in the PDC. The inside top of the PDC cover has label showing relay and fuse identification.
The Automatic Shutdown (ASD) relay supplies battery voltage to the fuel injectors, electronic ignition coil and the heating elements in the oxygen sensors.
A buss bar in the Power Distribution Center (PDC) supplies voltage to the solenoid side and contact side of the relay. The fuse also protects the power circuit for the fuel pump relay and pump. The fuse is located in the PDG.
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. When the ignition switch is in ON or Start, 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.
CAMSHAFT POSITION SENSOR
Fig.3 Camshaft:
The camshaft position sensor attaches to the rear of the cylinder head. 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 PCM sends approximately 8 volts to the hall affect 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.
Fig.4 Target Magnet - Typical:
Fig.5 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. The sensor output switch switches from high (5.0 volts) to low (0.5 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.
The sensor also acts as a thrust plate to control camshaft endplay.
Fig.8 Ignition Coil Pack:
IGNITION COIL
The coil pack consists of 2 coils molded together. The coil pack is mounted on the valve cover.
WARNING: THE DIRECT IGNITION SYSTEM GENERATES APPROXIMATELY 40,000 VOLTS. PERSONAL INJURY COULD RESULT FROM CONTACT WITH THIS SYSTEM.
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.
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.
SPARK PLUG
This engine uses resistor spark plugs. For spark plug identification and specifications, refer to Specifications.
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.12 Checking Spark Plug Electrode Gap:
Spark plugs that have low mileage may be cleaned and reused if not otherwise defective. Refer to the Spark Plug Condition. After cleaning, file the center electrode flat with a small point file or jewelers file. 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.
Always tighten spark plugs to the specified torque. Over tightening can cause distortion and damage. Tighten spark plugs to 28 Nm (20 ft. lbs.) torque.
SPARK PLUG CABLE
Spark plug cables are sometimes referred to as secondary ignition wires. The wires 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 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. Spark plug boot should completely cover the spark plug hole in the cylinder head cover. Install the boot until the terminal snaps over the spark plug. A snap must be felt to ensure the spark plug cable terminal engaged the spark plug.
Loose cable connections will corrode, increase resistance and permit water to enter the coil towers. These conditions can cause ignition malfunction. 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, and should be rotated about 300 below the horizontal.