Lesson 8: The Electrical System

This lesson is about the electrical system, or more specifically the engine electrical system. The main parts of the electrical system are the battery, alternator, and ignition system. The battery is the power source of the whole vehicle. The alternator is a re-charging device which is used to maintain the charge of the battery. The ignition system is used to ignite the air-fuel mixture at the correct time during the Otto cycle of events (review Lesson 5 for the Otto cycle of events).

The “ignition” in this lesson is not the same as the key ignition; it’s about the ignition system that is used in the engine to ignite the air-fuel mixture in the combustion chamber. Prior to 1975, the ignition system consisted of the ignition coil, capacitor, contact breaker, distributor and spark plugs. The ignition coil converts low voltage electricity into high voltage. The high voltage electricity is sent to the distributor through the rotor, then it transmits out the distributor cap through plug wires to the spark plugs. The contact breaker or contact points complete and interrupt the circuit to the ignition coil on the low voltage side. The capacitor absorbs a voltage spike when the points open to avoid burning the contacts of the points.

In a modern vehicle the ignition system consists of a crankshaft position sensor, which sends a signal to the PCM (powertrain control module or computer). The PCM interprets the signal and sends a signal to the ignition coils on each cylinder. Then the ignition coils fire through the spark plugs. Some vehicles will use one coil for two cylinders, and some have a separate coil for each cylinder. When one coil is used for two cylinders, the spark plugs for both cylinders fire at the same time, but one of the cylinders will be in the exhaust step of the Otto cycle, so it’s kind of a waste spark (not used to ignite fuel).

The ignition system works in conjunction with fuel injection (Lesson 7) so that the correct Otto Cycle of events occurs in the cylinders. The cylinders of internal combustion engines are numbered. And the firing sequence occurs in a specific order. The firing order can vary by vehicle make and model. The engine size is a factor.

For example, in an 8 cylinder Chevy engine the cylinders might be numbered 1, 3, 5, 7 on the driver’s side (or left side of the engine from the driver’s perspective), and 2, 4, 6, 8 on the passenger side. The firing order for this particular engine would be 1, 8, 4, 3, 6, 5, 7, 2.

Spark Plug Gap:

The purpose of the spark plug gap is to create an electric arc or spark of high heat to ignite the air-fuel mixture in the combustion chamber at the correct time. If the gap is too small then the spark is not hot enough to ignite the fuel. If the gap is too wide, then the voltage is not high enough to create a spark. Either condition can create an ignition misfire, causing poor performance and high emissions.

How to Safely Jump Start a Vehicle with a Failed Battery:

When a battery loses its charge then the vehicle won’t start and it becomes necessary to use another vehicle for a jump start. Since jump starting a failed battery with a charged battery involves creating an electrical circuit, it is important follow the right steps to perform a jump start safely:

• Do your best to line up the vehicles so that the two batteries are close enough to connect jumper cables.
• Turn off the active vehicle
• Verify there is no odor (like sulfur) coming from either battery. If there is a sulfur odor do not attempt to jump start the vehicle. The battery could explode due to the gas that is released (hydrogen gas).
• The next steps involve connecting the cables. When connecting cables to the first vehicle, which is the dead vehicle, make sure the other ends of the cables are not touching each other.
• First connect the red cable to the positive (red) battery terminal on the dead battery or to the jump start locations specified in the owner’s manual.
• Connect the black cable to the negative terminal of the dead battery or to the jump start locations specified in the owner’s manual.
• Connect the positive (red) cable to the positive terminal of the working vehicle (or to the specified jump start location in the owner’s manual of the working vehicle).
• Connect the negative (black) cable to the negative terminal of the working vehicle (or to the specified jump start location in the owner’s manual of the working vehicle) or to anything metal. The metal must be bare or without any paint that would inhibit conduction of electricity. The engine block is sufficient. Yet, the negative terminal is often the most accessible.
  • (More briefly the order is: dead red, dead black, live red, then live black. Red is always positive, black is always negative.)
• Start the working vehicle and allow it to run for approximately 5 minutes.
• Attempt to start the other vehicle. If the vehicle fails to start, there may be additional problems or the battery is no longer rechargeable.
• Take off the cables in reverse order (always remove negative first) and make sure the cables don’t touch each other as you’re removing them
  • Removed the negative cable from the jumped vehicle
  • Remove the positive cable from the jumped vehicle (make sure they don’t touch each other)
  • Remove the negative cable from the previously working vehicle
  • Remove the positive cable from the previously working vehicle.

That may seem like a lot of steps above to memorize, so here are some tricks to remember the process: first connect the Dead Red, Fred. This phrase will help you remember that when connecting the cables you’ll start with the positive cable on the dead battery. The order is dead red, dead black, live red, live black. Then it’s the reverse order to remove the cables after the dead battery is jumped: first remove the Jumped Black, Jack. Remove the jumped black, jumped red, then the previously live black, then the previously live red.

Never attempt to jump start a diesel vehicle. Since a diesel engine battery requires much more power, attempting to jump start it could damage the running vehicle.

Typically you can attempt to jump start a hybrid car, but if the high power battery is dead, the vehicle will not start. A hybrid can be used to jump start a regular gasoline vehicle.

Lesson 8: The Electrical System is one of twelve lessons that are also available as a free ebook download which is complete with illustrations. These lessons are based on the outline for the Boy Scout Automotive Maintenance merit badge and adapted by Crawford’s Auto Repair for a general audience. This article is for informational purposes only and the author does not assume responsibility or liability for any accident that may occur when working on motor vehicles. By reading this page you accept the condition that you are ultimately responsible for your own actions. For additional information on the topics on this page see Electrical System Services, Alternator Repair, Engine Tune Up, and Battery Services.

Copyright © 2014, Jeff Crawford. Permission is granted to republish this article for personal or commercial use as long as the content, citation, and copyright notice all remain intact and unchanged. There must be an active “follow” link to CrawfordsAutoService.com.

Lesson 8: The Electrical System is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.