Ignition System: Description and Operation
IGNITION CONTROLTwo different ignition Systems are used. One type of system is for the 4.0L 6-cylinder engine. The other is for the 4.7L V-8 engine.
The 4.0L 6-cylinder engine uses a one-piece coil rail containing three independent coils. Although cylinder firing order is the same as 4.0L engines of previous years, spark plug firing is not. The 3 coils dual- fire the spark plugs on cylinders 1-6, 2-5 and/or 3-4. When one cylinder is being fired (on compression stroke), the spark to the opposite cylinder is being wasted (on exhaust stroke). The one-piece coil bolts directly to the cylinder head. Rubber boots seal the secondary terminal ends of the coils to the top of all 6 spark plugs. One electrical connector (located at the rear end of the coil rail) is used for all three coils.
The 4.7L V-8 engine uses 8 dedicated and individually fired coil for each spark plug. Each coil is mounted directly to the top of each spark plug. A separate electrical connector is used for each coil.
Because of coil design, spark plug cables (secondary cables) are not used on either engine. A distributor is not used with either the 4.0L or 4.7L engines.
The ignition system is controlled by the Powertrain Control Module (PCM) on all engines.
The ignition system consists of:
- Spark Plugs
- Ignition Coil(s)
- Powertrain Control Module (PCM)
- Crankshaft Position Sensor
- Camshaft Position Sensor
- The MAP TPS, IAC and ECT also have an effect on the control of the ignition system.
AUTO SHUT DOWN RELAY
PCM Output
The 5-pin, 12-volt, Automatic Shutdown (ASD) relay is located in the Power Distribution Center (PDC). Refer to label on PDC cover for relay location.
The ASD relay supplies battery voltage (12+ volts) to the fuel injectors and ignition coil(s). With certain emissions packages it also supplies 12-volts to the oxygen sensor heating elements.
The ground circuit for the coil within the ASD relay is controlled by the Powertrain Control Module (PCM). The PCM operates the ASD relay by switching its ground circuit ON and OFF.
The ASD relay will be shut-down, meaning the 12-volt power supply to the ASD relay will be de-activated by the PCM if:
- the ignition key is left in the ON position. This is if the engine has not been running for approximately 1.8 seconds.
- there is a crankshaft position sensor signal to the PCM that is lower than pre-determined values.
ASD Sense - PCM Input
A 12 volt signal at this input indicates to the PCM that the ASD has been activated. The relay is used to connect the oxygen sensor heater element, ignition coil and fuel injectors to 12 volt + power supply.
This input is used only to sense that the ASD relay is energized. If the Powertrain Control Module (PCM) does not see 12 volts at this input when the ASD should be activated, it will set a Diagnostic Trouble Code (DTC).
CAMSHAFT POSITION SENSOR
Fig.2 CMP And Oil Pump Drive Shaft - 4.0L Engine:
Fig.3 CMP Location - 4.0L Engine:
4.0L
The Camshaft Position Sensor (CMP) on this engine is bolted to the top of the oil pump drive shaft assembly. The sensor and drive shaft assembly is located on the right side of the engine near the oil filter.
The CMP sensor contains a hall effect device called a sync signal generator to generate a fuel sync signal. This sync signal generator detects a rotating pulse ring (shutter) on the oil pump drive shaft. The pulse ring rotates 180 degrees 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 pulse ring (shutter) enters the sync signal generator, the following occurs: The interruption of magnetic field causes the voltage to switch high resulting in a sync signal of approximately 5 volts.
When the trailing edge of the pulse ring (shutter) leaves the sync signal generator, the following occurs: The change of the magnetic field causes the sync signal voltage to switch low to 0 volts.
4.7L
Fig.4 CMP Location - 4.7L Engine:
The Camshaft Position Sensor (CMP) on this engine is bolted to the front/top of the right cylinder head.
Fig.5 CMP Sensor And Tonewheel - 4.7L Engine:
The CMP sensor contains a hall effect device called a sync signal generator to generate a fuel sync signal. This sync signal generator detects notches located on a tonewheel. The tonewheel is located at the front of the camshaft for the right cylinder head. As the tonewheel rotates, the notches pass through the sync signal generator. The pattern of the notches (viewed counter-clockwise from front of engine) is: 1 notch, 2 notches, 3 notches, 3 notches, 2 notches 1 notch, 3 notches and 1 notch. The signal from the CMP sensor 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.
COIL RAIL
Fig.11 Ignition Coil Assembly - 4.0L 6-Cylinder Engine:
A one-piece coil rail assembly containing three individual coils is used on the 4.0L 6-cylinder engine. The coil rail must be replaced as one assembly. The bottom of the coil is equipped with 6 individual rubber boots to seal the 6 spark plugs to the coil. Inside each rubber boot is a spring. The spring is used for a mechanical contact between the coil and the top of the spark plug. These rubber boots and springs are a permanent part of the coil and are not serviced separately.
Fig.12 Coil Location - 4.0L Engine:
1. The coil is bolted directly to the cylinder head. One electrical connector (located at rear of coil) is used for all three coils.
Although cylinder firing order is the same as 4.0L Jeep engines of previous years, spark plug firing is not. The 3 coils dual-fire the spark plugs on cylinders 1-6, 2-5 and/or 3-4. When one cylinder is being fired (on compression stroke), the spark to the opposite cylinder is being wasted (on exhaust stroke).
Battery voltage is supplied to the three ignition coils from the ASD relay. The Powertrain Control Module (PCM) opens and closes the ignition coil ground circuit for ignition coil operation.
Base ignition timing is not adjustable. By controlling the coil ground circuit, the PCM is able to set the base timing and adjust the ignition timing advance. This is done to meet changing engine operating conditions.
The ignition coil is not oil filled. The windings are embedded in an epoxy compound. This provides heat and vibration resistance that allows the ignition coil to be mounted on the engine.
Because of coil design, spark plug cables (secondary cables) are not used. The cables are integral within the coil rail.
Fig.16 Ignition Coil - 4.7L Engine:
Fig.17 Ignition Coil Location - 4.7L Engine:
IGNITION COIL
The 4.7L V-8 engine uses 8 dedicated, and individually fired coil for each spark plug. Each coil is mounted directly to the top of each spark plug.
Battery voltage is supplied to the 8 ignition coils from the ASD relay The Powertrain Control Module (PCM) opens and closes each ignition coil ground circuit at a determined time for ignition coil operation.
Base ignition timing is not adjustable. By controlling the coil ground circuit, the PCM is able to set the base timing and adjust the ignition timing advance. This is done to meet changing engine operating conditions.
The ignition coil is not oil filled. The windings are embedded in an epoxy compound. This provides heat and vibration resistance that allows the ignition coil to be mounted on the engine.
Because of coil design, spark plug cables (secondary cables) are not used.
Fig.20 Coil Capacitor (Left Side Shown):
IGNITION COIL CAPACITOR
Two coil capacitors are used One of them is located near the center of, and on the left side of the intake manifold. The other capacitor is located near the center of, and on the right side of the intake manifold.
The 2 coil capacitors are used to prevent high-voltage spikes from interfering with the operation of certain powertrain sensors. They are also used to help prevent radio interference.
KNOCK SENSOR
4.7L High-Output Engine
The 2 knock sensors are bolted into the cylinder block under the intake manifold.
Two knock sensors are used on this V-8 engine if equipped with the high-output package; one for each cylinder bank. When the knock sensor detects a knock in one of the cylinders on the corresponding bank, it sends an input signal to the Powertrain Control Module (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 the knock sensor voltage signal as an input. 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 at Wide Open Throttle (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 the sensor mounting bolts will affect knock sensor performance, possibly causing improper spark control. Always use the specified torque when installing the knock sensors.
SPARK PLUG
Both the 4.0L 6-cylinder and the 4.7L V-8 engine use resistor type spark plugs. Standard 4.7L V-8 engines are equipped with "fired in suppressor seal" type spark plugs using a copper core ground electrode. High-Output (H.O.) 4.7L V-8 engines are equipped with unique plugs using a platinum rivet located on the tip of the center electrode.
Because of the use of an aluminum cylinder head on the 4.7L engine, spark plug torque is very critical.
To prevent possible pre-ignition and/or mechanical engine damage, the correct type/heat range/number spark plug must be used. Do not substitute any other spark plug on the 4.7L H.O. engine. Serious engine damage may occur.
Plugs on both engines 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 ohmmeter to check the resistance values of the spark plugs. Inaccurate readings will result.
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. A single plug displaying an abnormal condition indicates that a problem exists in the corresponding cylinder.
EXCEPT 4.7L H.O. ENGINE : Spark plugs that have low mileage may be cleaned and reused if not otherwise defective, carbon or oil fouled. Also refer to Spark Plug Conditions. 4.7L H.O. ENGINE : Never clean spark plugs on the 4.7L H.O. engine. Damage to the platinum rivet will result.
CAUTION: EXCEPT 4.7L H.O. ENGINE : Never use a motorized wire wheel brush to clean the spark plugs. Metallic deposits will remain on the spark plug insulator and will cause plug misfire.
Fig.24 Plug Gap - 4.7L HO:
H.O. Gap Adjustment: If equipped with the 4.7L H.O. engine, do not use a wire-type gapping tool as damage to the platinum rivet on the center electrode may occur. Use a tapered-type gauge.