Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw
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Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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02.10.2022 Views

The Ignition: Primary and Secondary Circuits and Components 229Figure 11-1. The ignition coil produces high-voltage current in the secondary winding when current is cut off inthe primary winding.cuit, the magnetic flux lines cross over into thesecondary windings of the coil and induce aneven greater voltage charge. The amount of voltagein the primary circuit builds up to 240 volts.The secondary windings meanwhile are building200 times the primary voltage, or 48,000 volts.This is enough voltage to ignite the air-fuel mixtureunder most operating conditions.This collapsing magnetic field can be observedon an oscilloscope as a primary waveform patternand is often referred to as the firing line. The voltagefluctuations vary by manufacturer, but mostignition systems range in amplitude between 250to 400 volts. The secondary windings within theignition coil are much thinner and have between100 to 200 times the amount of windings. Thevoltage induced by the collapsing magnetic fieldis in the kilovolt (1000 X) range.This kind of ignition system, based on theinduction of a high voltage in a coil, is calledan inductive-discharge ignition system. Aninductive-discharge system, using a battery asthe source of low-voltage current, has been thestandard automotive ignition system for abouteighty years.BASIC CIRCUITSAND CURRENTThe ignition system consists of two interconnectedcircuits:• The low-voltage primary circuit• The high-voltage secondary circuitWhen the ignition switch is turned on, batterycurrent travels:• Through the ignition switch or the primaryresistor if present• To and through the coil primary winding• Through a switching device• To ground at the negative and the groundedterminal of the batteryLow-voltage current in the coil primary windingcreates a magnetic field. When the switching deviceinterrupts this current:• A high-voltage surge is induced in the coilsecondary winding.• Secondary current is routed from the coil tothe distributor.

230 Chapter Eleven• Secondary current passes through the distributorcap, rotor, across the rotor air gap,and through another ignition cable to thespark plug.• Secondary current creates an arc across thespark plug gap, as it travels to ground.PRIMARY CIRCUITCOMPONENTSThe primary circuit contains the:• Battery• Ignition switch• Pickup coil in the distributor• Coil primary winding• Ignition control moduleBatteryThe battery supplies low-voltage current to the ignitionprimary circuit. Battery current is availableto the system when the ignition switch is in theSTART or the RUN position.Ignition SwitchThe ignition switch controls low-voltage currentthrough the primary circuit. The switch allowscurrent to the coil when it is in the START or theRUN position. Other switch positions route currentto accessory circuits and lock the steeringwheel in position.Manufacturers use differing ignition switchcircuitry. The differences lie in how battery currentis routed to the switch. Regardless of variations,full system voltage is always present at theswitch because it is connected directly to thebattery.General Motors automobiles draw ignition currentfrom a terminal on the starter motor solenoid.Ford Motor Company systems draw ignition currentfrom a terminal on the starter relay, Figure 11-2. In Chrysler Corporation vehicles, ignitioncurrent comes through a wiring splice installedbetween the battery and the alternator, Figure 11-3. Imported and domestic automobiles may usedifferent connections. Ignition current is drawnthrough a wiring splice between the battery andthe starter relay to simplify the circuitry on somesystems, Figure 11-4.SWITCHING ANDTRIGGERINGAll ignition systems regardless of design use aswitching device usually called an ignition controlmodule (ICM) or igniter that contains a powerFigure 11-2.Ford products draw ignition current from a terminal on the starter relay. (Courtesy of Ford Motor Company)

230 Chapter Eleven

• Secondary current passes through the distributor

cap, rotor, across the rotor air gap,

and through another ignition cable to the

spark plug.

• Secondary current creates an arc across the

spark plug gap, as it travels to ground.

PRIMARY CIRCUIT

COMPONENTS

The primary circuit contains the:

• Battery

• Ignition switch

• Pickup coil in the distributor

• Coil primary winding

• Ignition control module

Battery

The battery supplies low-voltage current to the ignition

primary circuit. Battery current is available

to the system when the ignition switch is in the

START or the RUN position.

Ignition Switch

The ignition switch controls low-voltage current

through the primary circuit. The switch allows

current to the coil when it is in the START or the

RUN position. Other switch positions route current

to accessory circuits and lock the steering

wheel in position.

Manufacturers use differing ignition switch

circuitry. The differences lie in how battery current

is routed to the switch. Regardless of variations,

full system voltage is always present at the

switch because it is connected directly to the

battery.

General Motors automobiles draw ignition current

from a terminal on the starter motor solenoid.

Ford Motor Company systems draw ignition current

from a terminal on the starter relay, Figure 11-

2. In Chrysler Corporation vehicles, ignition

current comes through a wiring splice installed

between the battery and the alternator, Figure 11-

3. Imported and domestic automobiles may use

different connections. Ignition current is drawn

through a wiring splice between the battery and

the starter relay to simplify the circuitry on some

systems, Figure 11-4.

SWITCHING AND

TRIGGERING

All ignition systems regardless of design use a

switching device usually called an ignition control

module (ICM) or igniter that contains a power

Figure 11-2.

Ford products draw ignition current from a terminal on the starter relay. (Courtesy of Ford Motor Company)

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