Download Chapter 47: Ignition Systems Overview

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► Ignition systems have a primary (low-voltage) circuit to connect and disconnect the ignition coil and a
secondary (high-voltage) circuit to send voltage from the battery to the spark plug.
► Common components of an ignition system are the spark plugs, high tension leads ignition coil, and
ignition-coil triggering device.
► Principles of an ignition system involve a circuit activating the ignition coil, which converts the battery’s
low voltage to high voltage. The voltage then travels to the spark plug in each cylinder, igniting the air–
fuel mixture and creating enough pressure to push down the pistons.
► The original contact breaker ignition system was replaced with an electronic ignition system of the
distributor type.
► Modern systems are direct ignition systems (or coil-on-plug), which have a dedicated coil for each
cylinder.
► The amount of available voltage should always be higher than the amount of required voltage.
► As ignition components age and become worn, required voltage increases and available voltage
decreases.
► Spark timing is affected by any of these factors: air–fuel ratio, detected knock, engine speed, engine
load, engine temperature, and throttle position.
► Spark timing must be correct in order to give the air–fuel mixture enough time to burn.
► Modern vehicles have electronically programmed spark timing via the PCM; older vehicles mechanically
advance or retard the spark timing.
► Common points on an ignition switch are the lock and off functions and the accessory, on/run, and
start/crank positions.
► Automatic transmission vehicles may employ a transmission shift interlock device to ensure that the gear
selector is in park before key removal can occur.
► Standard ignition coils contain primary and secondary windings around a rod-shaped laminated iron
core.
► The main difference among types of ignition systems is the method used to control the primary circuit to
produce the secondary spark.
► In the primary circuit, a magnetic field develops that is interrupted by the ignition triggering device, thus
collapsing the field and returning stored energy to ignition coil terminals.
► Because the secondary winding has approximately 100 times as many turns as the primary winding, it
can produce 100 times greater voltage.
► Basic contact breaker point ignition systems are comprised of the battery, ignition switch, ignition coil,
contact breaker points, capacitor, distributor, and appropriate voltage-connecting wires and leads.
► The purpose of the distributor is to transfer the spark to the spark plugs with the correct sequence and
timing.
► The distributor cap provides rotor and spark plug lead connection.
► The distributor controls ignition timing in relation to speed via a centrifugal advance mechanism and
ignition timing in relation to vehicle load via a vacuum advance mechanism.
► Vehicles regulate voltage to the ignition system via a ballast resistor, which is inserted in the primary
circuit and lowers the voltage.
► Spark plugs all have at least one, and up to four, side electrodes, as well as an internal resistor to
suppress voltage spikes and prevent radio frequency interference.
► Spark plugs are identified by thread size or diameter, reach or length of thread, and heat range or
operating temperature.
► Spark plug components include the metal case, insulator, terminal, side electrode, and center electrode.
► To work effectively, spark plugs must have proper reach (distance from sealing area to end of spark plug
threads) and proper heat range (operating temperature, generally 746°F to 1,460°F [400°C to 800°C]).
► High-tension leads carry a high-voltage spark from the ignition coil to the distributor cap and on to the
spark plugs.
► Engine timing can refer to valve timing or ignition timing.
► How long the contacts are closed and current is flowing in a contact breaker system is determined by the
dwell angle.
► Electronic ignition systems electronically trigger the primary circuit (rather than using a contact breaker).
► The triggering device of an electronic ignition system can be a magnetic pulse generator (pickup coil) or
a Hall-effect switch.
► Ignition modules process information from various sensors and then interrupt the signal to the primary
winding of the ignition coil.
► Induction-type systems use a magnetic pulse generator with a stator and a reluctor attached to the
distributor body and shaft, respectively.
► Induction-type systems produce an alternating current voltage.
► The internal module control circuit receives the trigger signal.
► The reluctor teeth spin past the stator, turning on and off the primary winding and causing the secondary
windings to generate a high-voltage spark at the end of each cylinder’s compression stroke.
► Hall-effect systems use a potential difference, or voltage, which is used as a switch device.
► In a Hall-effect system, the magnetic field is alternately blocked and exposed by an interrupter ring.
► Some ignition systems use a phototransistor to receive light from an LED and transform it into a voltage
output signal.
► In distributorless ignition systems, the distributor is eliminated so crankshaft and camshaft position
sensors help determine when to send a signal to the coil’s primary windings by monitoring which
cylinder is approaching its power stroke.
► In a six-cylinder, waste spark engine, each set of two cylinders is paired with an ignition coil. The
cylinders alternate between an event cylinder and a waste cylinder.
► The PCM controls spark timing for distributorless ignition systems via information from the following
sensors: mass airflow sensor, manifold absolute pressure sensor, throttle position sensor, cam and crank
angle sensors, engine speed sensor, knock sensor and coolant temperature sensor.
► Special tools and equipment for ignition system diagnostics include the spark plug socket, spark plug
gapping tool, high-tension wire puller, spark tester, test light, a digital multimeter, scan tool and an
oscilloscope.
► Oscilloscopes test the condition of the secondary circuit and can show if voltage to any of the cylinders
is too high, too low, or nonexistent.
► The first step in diagnosing ignition system issues is to conduct a visual inspection to look for loose
connections, broken or corroded terminals, or any components that need replacing.
► Inspect the primary and secondary circuit wiring for cuts, abrasions, and signs of arcing.
► Test the ignition coil for primary and secondary winding resistance.
► If the vehicle will not start, performing a spark test is a good step.
► Test to ensure that the spark plug wire, distributor cap and rotor, crankshaft and camshaft position
sensors, and ignition modules are in proper working order.
► Always follow manufacturer-recommended procedure for diagnosing each ignition system component.
► Spark plugs and spark plug wires will wear over time and require replacement; check the manufacturer’s
recommendations for replacement schedules.
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