Ignition System: Description and Operation
IGNITION SYSTEMDESCRIPTION
Two 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.
OPERATION
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 seat 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.
Powertrain Control Module(PCM) Location:
POWERTRAIN CONTROL MODULE (PCM)
DESCRIPTION
The Powertrain Control Module (PCM) is located in the engine compartment behind the coolant overflow tank.
OPERATION
The ignition system is controlled by the PCM. The PCM opens and closes the ignition coil ground circuit to operate the ignition coil(s). This is done to adjust ignition timing, both initial (base) and advance, and for changing engine operating conditions.
The amount of electronic spark advance provided by the PCM is determined by five input factors: engine coolant temperature, engine rpm, intake manifold temperature, manifold absolute pressure and throttle position.
SPARK PLUGS
DESCRIPTION
Both the 4.0L 6-cylinder and the 4.7L V-8 engine use resistor type spark plugs. 4.7L V-8 engines are equipped with "fired in suppressor seal" type spark plugs using a copper core ground 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.
OPERATION
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. Replace spark plugs at the intervals, Lubrication and Maintenance.
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.
CAUTION: 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.
Ignition Coil Assembly:
IGNITION COIL - 4.0L ENGINE
DESCRIPTION
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.
Coil Location:
1. The coil is bolted directly to the cylinder head. One electrical connector (located at rear of coil) is used for all three coils.
OPERATION
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.
Ignition Coil:
Ignition Coil Location:
IGNITION COIL - 4.7L ENGINE
DESCRIPTION
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.
1mOPERATION
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.
AUTOMATIC SHUTDOWN RELAY
DESCRIPTION
The ASD relay is located in the Power Distribution Center (PDC). Refer to label on PDC cover for relay location.
OPERATION
As one of its functions, the ASD relay will supply battery voltage to the ignition coil(s). The ground circuit to operate the ASD relay is controlled by the Powertrain Control Module (PCM). The PCM regulates ASD relay operation by switching its ground circuit on-and-off,
CKP Sensor Location:
CRANKSHAFT POSITION SENSOR - 4.0L ENGINE
DESCRIPTION
The Crankshaft Position Sensor (CKP) is mounted to the transmission bellhousing at the left/rear side of the engine block.
OPERATION
Engine speed and crankshaft position are provided through the crankshaft position sensor. The sensor generates pulses that are the input sent to the powertrain control module (PCM). The PCM interprets the sensor input to determine the crankshaft position. The PCM then uses this position, along with other inputs, to determine injector sequence and ignition timing.
The sensor is a hall effect device combined with an internal magnet. It is also sensitive to steel within a certain distance from it.
CKP Sensor Operation:
On 4.0L 6-cylinder engines, the flywheel/drive plate has 3 sets of four notches at its outer edge.
The notches cause a pulse to be generated when they pass under the sensor. The pulses are the input to the PCM. For each engine revolution there are 3 sets of four pulses generated.
The trailing edge of the fourth notch, which causes the pulse, is four degrees before top dead center (TDC) of the corresponding piston.
The engine will not operate if the PCM does not receive a crankshaft position sensor input.
CKP Sensor Location:
CRANKSHAFT POSITION SENSOR - 4.7L ENGINE
DESCRIPTION
The Crankshaft Position Sensor (CKP) is mounted into the engine block above the starter motor.
OPERATION
Engine speed and crankshaft position are provided through the crankshaft position sensor. The sensor generates pulses that are the input sent to the powertrain control module (PCM). The PCM interprets the sensor input to determine the crankshaft position. The PCM then uses this position, along with other inputs, to determine injector sequence and ignition timing.
The sensor is a hall effect device combined with an internal magnet. It is also sensitive to steel within a certain distance from it.
CKP Sensor Operation And Tonewheel:
On the 4.7L V-8 engine, a tonewheel is bolted to the engine crankshaft. This tonewheel has sets of notches at its outer edge .
The notches cause a pulse to be generated when they pass under the sensor. The pulses are the input to the PCM.
CMP And Oil Pump Drive Shaft:
CMP Location:
CAMSHAFT POSITION SENSOR - 4.0L ENGINE
DESCRIPTION
The Camshaft Position Sensor (CMP) on the 4.0L 6-Cylinder 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.
OPERATION
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.
CMP Location:
CAMSHAFT POSITION SENSOR - 4.7L ENGINE
DESCRIPTION
The Camshaft Position Sensor (CMP) on the 4.7L V-8 engine is bolted to the front/top of the right cylinder head.
CMP Sensor And Tonewheel:
OPERATION
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.
IGNITION SWITCH AND KEY LOCK CYLINDER
DESCRIPTION
The electrical ignition switch is located on the steering column. It is used as the main on/off switching device for most electrical components. The mechanical key lock cylinder is used to engage/disengage the electrical ignition switch.
OPERATION
Vehicles equipped with an automatic transmission and a floor mounted shifter: a cable connects the interlock device within the steering column assembly, to the transmission floor shift lever. This interlock device is used to lock the transmission shifter in the PARK position when the key lock cylinder is in the LOCKED or ACCESSORY position. The interlock device within the steering column is not serviceable. If repair is necessary, the steering column assembly must be replaced. Refer to Steering for procedures. The shifter interlock cable can be adjusted or replaced. Refer to Transmissions for procedures.
Vehicles equipped with an automatic transmission and a steering column mounted shifter: an interlock device is located within the steering column. This interlock device is used to lock the transmission shifter in the PARK position when the key lock cylinder is in the LOCKED or ACCESSORY position. The interlock device within the steering column is not serviceable. If repair is necessary, the steering column assembly must be replaced.
Vehicles equipped with a manual transmission and a floor mounted shifter: a lever is located on the steering column behind the ignition key lock cylinder. The lever must be manually operated to allow rotation of the ignition key lock cylinder for key removal (turning switch to off position). The lever mechanism is not serviced separately. If repair is necessary, the steering column assembly must be replaced.