Ignition and Electrical Systems

The electrical system is where Auto and Shop meets Electronics Information. The components are different but the ideas are the same ones, and the diagnostic questions here are the best in the subtest.

Battery and charging

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A vehicle battery is nominally 12 volts and supplies direct current.

The starter draws an enormous current - hundreds of amperes - for a few seconds, which is why its cables are so thick and why a weak battery cranks slowly. Thick cable means low resistance, which is the conductors lesson applied.

The alternator produces alternating current and rectifies it to DC internally, using diodes. That is the rectifier from the Electronics lesson doing a real job, and it is why the component is called an alternator rather than a generator.

The diagnostic pair

This is the best question in the topic, and it is pure reasoning.

A dead battery with a working alternator: the vehicle will not start on its own, but once jump-started it keeps running, because the alternator supplies everything.

A failed alternator with a good battery: the vehicle starts normally and then stops after a short time, once the battery has been drained by the ignition and accessories.

So: dies while driving points at the alternator; will not crank points at the battery.

A warning light for the charging system is telling you the alternator is not keeping up, not that the battery is flat.

The ignition system

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The job is to produce a high-voltage spark at the right cylinder at the right instant.

The coil is a transformer. It steps voltage up by the turns ratio, exactly as the Electronics lesson describes - a small primary winding and a very large secondary. A spark needs tens of thousands of volts to jump the plug gap, and 12 volts will not do it.

A spark plug fires across a gap between a center electrode and a ground electrode. The gap size matters: too wide and the spark may not jump, too narrow and it is weak.

Modern engines have no distributor. A coil per cylinder, or per pair, is fired directly by the control module, which is more precise and has nothing to wear out.

Timing

The spark fires just before the piston reaches the top of the compression stroke, so that the burn is developing as the piston starts down and pushes hardest at the right moment.

Firing too early or too late both cost power, and firing much too early causes knocking - the mixture igniting ahead of the flame front.

Timing is advanced as engine speed rises, because the burn takes a roughly fixed time while the piston is moving faster.

Sensors and the control module

Modern engines are controlled by a computer reading sensors and deciding fuel and spark.

The oxygen sensor closes the loop on the mixture. It measures what came out and the module adjusts what goes in, which is feedback - the same idea as the amplifier lesson, used to hold something steady rather than to amplify.

The check engine light reports a stored fault code, which is read with a scanner. It means the module has detected something outside its expected range, which is not necessarily an emergency but is not nothing either.

Know it by sight

What you can skip

Across the 59 questions on this topic in our bank:

• Breaker-point ignition. Points, condensers and dwell never appear. Know the distributor as the older system and electronic ignition as the modern one.
• Timing in degrees never appears as a number.
• Hybrid and electric vehicle systems and on-board diagnostic codes never appear.

Where people lose points

Saying the battery powers the vehicle while it runs. The alternator does.

Saying the alternator starts the engine. The starter does.

Thinking the coil generates electricity. It transforms voltage.

Saying a dead battery stops a running engine. A failed alternator does.

Thinking the spark fires at the very top of the stroke. It fires just before.

Confusing an alternator with a generator. An alternator makes AC and rectifies it.

Work one in under a minute

A vehicle starts normally, runs for about fifteen minutes, then loses power and stops. It will not restart. What is the most likely fault?

The alternator.

It started normally, so the battery and starter were fine.

It ran and then died, which is what happens when nothing is recharging the battery: the ignition and accessories drain it until there is not enough to keep the engine firing.

It will not restart because the battery is now flat.

Every part of the description follows from one failure, which is what makes this the cleanest diagnostic shape in the subtest.

Where this leads

The coil is a transformer and the alternator contains a rectifier, so this lesson is Electronics Information applied - and the oxygen sensor's feedback loop is the link to emissions.