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 distributorless 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 (PDC):
AUTO SHUT DOWN RELAY
The relay is located in the Power Distribution Center (PDC). For the location of the relay within the PDC, refer to the PDC cover for location. Check electrical terminals for corrosion and repair as necessary.
The ASD sense circuit informs the PCM when the ASD relay energizes. A 12 volt signal at this input indicates to the PCM that the ASD has been activated. This input is used only to sense that the ASD relay is energized.
When energized, the ASD relay supplies battery voltage to the fuel injectors, ignition coils and the heating element in each oxygen sensor. If the PCM does not receive 12 volts from this input after grounding the ASD relay, it sets a Diagnostic Trouble Code (DTC).
When energized, the ASD relay provides power to operate the injectors, ignition coil, generator field, O2 sensor heaters (both upstream and downstream), and also provides a sense circuit to the PCM for diagnostic purposes. The PCM energizes the ASD any time there is a Crankshaft Position sensor signal that exceeds a predetermined value. The ASD relay can also be energized after the engine has been turned OFF to perform an O2 sensor heater test, if vehicle is equipped with OBD II diagnostics.
As mentioned earlier, the PCM energizes the ASD relay during an O2 sensor heater test. This test is performed only after the engine has been shut OFF. The PCM still operates internally to perform several checks, including monitoring the O2 sensor heaters.
CAMSHAFT POSITION SENSOR
Fig.2 Camshaft Position Sensor:
Fig.3 Camshaft Position Sensor Location:
The camshaft position sensor is mounted in the front of the head on 2.7L or belt cover on the 3.2/3.5L. It is a hall effect device.
Fig.4 Camshaft Position Sensor:
Fig.5 Camshaft Position Sensor:
The CMP sensor contains a hall effect device that provide cylinder identification to the Powertrain Control Module (PCM). The sensor generates pulses as groups of notches on the camshaft sprocket pass underneath it. The PCM keeps track of crankshaft rotation and identifies each cylinder by the pulses generated by the notches on the camshaft sprocket. Crankshaft pulses follow each group of camshaft pulses.
When metal aligns with the sensor, voltage goes low (less than 0.3 volts). When a notch aligns with the sensor, voltage spikes high (5.0 volts). As a group of notches pass under the sensor, the voltage switches from low (metal) to high (notch) then back to low. The number of notches determine the amount of pulses. If available, an oscilloscope or DRB III PEP Module can display the square wave patterns of each timing event.
IGNITION COIL
Fig.7 Ignition Coil - 2.7L:
Fig.8 Ignition Coil - 3.2/3.5L:
The engines are equipped with a coil on plug ignition system. Each cylinder has a dedicated coil that sits atop each plug. No secondary wires are required and connection from the coil to plug is made with a boot that is attached to the coil. The combination of the coil, boot, and fasteners is known as the ignition coil assembly. The Powertrain Control Module (PCM) determines which coil 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. For limp home capability, the engine can run on crankshaft sensor input only. Refer to Auto Shutdown (ASD) Relay-PCM Output for relay operation.
Fig.9 Coil Capacitor:
Fig.10 Coil Capacitor:
The coil's low primary resistance (0.4 - 0.6 ohm) allows the PCM to fully charge the coil for each firing. There is a coil capacitor added to each bank of cylinders for radio noise suppression.
IGNITION COIL CAPACITOR
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. For limp home capability, the engine can run on crankshaft sensor input only. Refer to Auto Shutdown (ASD) Relay-PCM Output for relay operation.
Fig.11 Coil Capacitor:
Fig.12 Coil Capacitor:
The coil's low primary resistance (0.4 - 0.6 ohm) allows the PCM to fully charge the coil for each firing. There is a coil capacitor added to each bank of cylinders for radio noise suppression.
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.
SPARK PLUG
The 2.7, 3.2, and 3.5L engines use platinum resistor spark plugs. They have resistance values of 6,000 to 20,000 ohms when checked with at least a 1000 volt tester. For spark plug identification and specifications, Refer to the Specifications.
Do not use an ohm meter to check the resistance of the spark plugs. This will give an inaccurate reading.
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.
Fig.17 Platinum Pads:
A thin platinum pad is welded to both or center electrode end(s) as shown. 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.