WARNING
To avoid personal injury and/or vehicle damage, refer to the service precautions at the beginning of this section.
NOTE: For information on understanding electricity and troubleshooting electrical circuits, please refer to chassis electrical.
Coil on Plug (Cop) System
The coil over plug system was developed so that spark and spark timing could be better controlled on an individual cylinder basis. Each cylinder has an ignition coil mounted directly above the spark plug on the cylinder head cover. A short suppresser/connector replaces the spark plug wire and links the coil to the plug. There are different methods used for primary triggering. Some manufacturers use a combination coil/module, which means each coil has its own control circuit that is activated by the PCM. Others use remote mounted modules to trigger the coils.
Each individual coil is allowed to saturate while all other cylinders fire. For a V-8 engine, this allows a period of seven firing events for coil saturation, compared to three events for the same V-8 engine with a waste spark system. The coil over plug system also benefits from a minimum amount of energy lost, due to the resistance of spark plug wires.
Distributor System
If a distributor is not keyed for installation with only one orientation, it could have been removed and installed improperly and then rewired. The new wiring arrangement would maintain the correct firing order, but could change the relative placement of the plug towers in relation to the engine. For this reason it is imperative that you label all wires before disconnecting any of them. Also, before removal, compare the current wiring with the accompanying illustrations. If the current wiring does not match, make notes of the current plug wire locations and orientation of the distributor cap.
Magnetic Sensor / Pick-Up Coil
The magnetic sensor in electronic ignition system is made up of a small coil of wire wrapped around an iron core, a permanent magnet and a toothed wheel called a reluctor. These sensors can be found mounted in a distributor, or at the front, middle, or rear of the crankshaft or camshaft, and are two-wire sensors.
The permanent magnet produces a magnetic field that passes thru the center of the pick-up coil. As the reluctor turns, the small teeth enter the magnetic field. Because the metal is a better conductor for the field than the air between the magnet and reluctor, the field strength begins to increase and reaches its maximum when the reluctor teeth are closest to the sensor. An increase in magnetic field induces a positive voltage to the module. As the teeth leave the magnetic field, the decrease in pole strength induces a negative voltage into the module. This alternating positive and negative voltage causes a small AC current. This alternating current after passing through an analog/digital converter is used by the module or engine controller to trigger the primary circuit.
Hall-Effect Device
Another device that can be used to create a triggering signal is a hall-effect device. A hall-effect device can be thought of as a solid-state On/Off switch. The hall-effect switch is a three-wire device that must receive a power and ground. The hall-effect switch is used in conjunction with an interrupter ring with a series of slots or openings cut into it. Depending on the application, these slots are spaced around the ring in a specific configuration. As the ring rotates, the slots pass between the hall-effect switch, and alternately turns the voltage off and on. When a slot aligns with the hall-effect switch, the controller sees voltage on the signal line. When the area between slots passes the hall-effect switch, the signal is pulled low. This results in a voltage of 0V–0.1V at the controller.
The rotation of the interrupter ring causes the signal to toggle, which causes a continual series of digital pulses on the signal line. This digital pulse is the timing signal that is used by the ignition module or engine computer to open and close the primary circuit. The controller processes these pulses as the RPM signal.
Photo-Optical
Another device used to create a triggering signal is the photo optical sensor. Inside the distributor, there are pick-ups called the Reference pick-up and the Sync pick-up. Each pick-up has a Light Emitting Diode (LED) and a phototransistor. A slotted disc rotates between the pick-ups. The pair of LED’s and phototransistors generates crankshaft position and RPM signals (high and low-resolution signals). The LED’s are powered by a 9- or 12-volt source (depending on manufacturer). Each phototransistor is used to turn a 5-volt signal from the engine controller on and off.
If we look at the optical distributor used in the Chrysler 3.0L engine as an example, there are two areas of slots cut into the disc. The outer diameter of the disc, which generates the high-resolution signal, contains 350 slots. Each of these slots represents 1 degree of crankshaft rotation. An area of approximately 3/8" with no slots represents the remaining 10 degrees. The inner portion of the disc is the low-resolution signal and contains six 60-degree slots. Each of these slots represents the piston’s top dead center position for each cylinder. The controller uses the high-resolution signal to regulate spark timing up to 1200 RPM. This ensures timing accuracy, since crankshaft speed fluctuations are most likely to occur because of the firing pulses during cranking and idle. The low-resolution signal is used for injector firing, as well as ignition timing above 1200 RPM.
As the slots pass between the LED’s and the phototransistors, the transistors are toggled on and off. This occurs as the light beams from the LED’s are alternately interrupted. When the light beam from the LED strikes the phototransistor, the transistor turns on. This causes the 5-volt signal to be pulled low (0V–0.1V). When the rotating disc blocks the light beam, the transistor turns off. This causes the 5-volt signal to remain high.
Ignition Coil
The heart of the automotive ignition system is the ignition coil. The ignition coil is a step-up transformer, since it boosts battery voltage to the high voltage that is necessary for proper combustion.
The ignition coil consists of a primary winding and secondary winding wrapped around a soft iron core. The primary winding is made up of several hundred turns of heavy wire, while the secondary winding consists of thousands of turns of fine wire. The iron core is used to conduct magnetic lines of force efficiently.
When current flows through the primary winding, a magnetic field is created. The more time current is permitted to flow, the stronger the magnetic field becomes. When the current is turned off, the magnetic field collapses causing a high voltage to be induced in the secondary winding through the process of induction.
A few hundred volts will be generated in the primary winding because of the collapsing magnetic field across the heavy primary wire. However, as the magnetic lines of force cut across the thousands of turns of fine wire in the secondary, a far greater voltage is produced. The production of primary voltage is called self-induction, since the primary winding essentially magnifies its own initial voltage when the magnetic field collapses.
Related Symptoms
Faulty ignition system components along with loose connections, bad grounds, high resistance or opens in the circuit, may cause the following symptoms: