AUTOMOTIVE ELECTRICAL CIRCUITS AND WIRING

The starter ground circuit test checks the circuit between the starting motor and the
negative terminal of the battery.
Using a voltmeter, connect the leads to the negative terminal of the battery and to the
end frame of the starting motor. Crank the engine and note the voltmeter reading. If it
is higher than 0.5 volts, check the voltage drop across the negative battery cable. The
engine may not be properly grounded. Clean, tighten, or replace the battery cable if
needed. A battery cable problem can produce symptoms similar to a dead battery, bad
solenoid, or weak starting motor. If the cables do NOT allow enough current to flow,
the starter will turn slowly or not at all.
IGNITION CIRCUIT
Learning Objective: Identify ignition-circuit components, their functions, and
maintenance procedures.
The ignition circuit supplies high voltage surges (some as high as 50,000 volts in
electronic ignition circuits) to the spark plugs in the engine cylinders. These surges
produce electric sparks across the spark plug gaps. The heat from the spark ignites the
compressed air-fuel mixture in the combustion chambers. When the engine is idling,
the spark appears at the spark plug gap just as the piston nears top dead center (TDC)
on the compression stroke. When the engine is operating at higher speeds, the spark is
advanced. It is moved ahead and occurs earlier in the compression stroke. This design
gives the compressed mixture more time to bum and deliver its energy to the pistons.
The functions of an ignition circuit are as follows:
Provide a method of turning the ignition circuit ON and OFF.
Be capable of operating on various supply voltages (battery or alternator voltage).
Produce a high voltage arc at the spark plug electrodes to start combustion.
Distribute high voltage pulses to each spark plug in the correct sequence.
Time the spark so that it occurs as the piston nears TDC on the compression stroke.
Vary spark timing with engine speed, load, and other conditions.
PRIMARY AND SECONDARY CIRCUITS
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