Finding the cause of an engine problem and then fixing that problem can be done in a hap-hazard way or in a logical, organized manner. The hit-and-miss approach to service never works well. You will waste time and effort without solving the problem. An organized approach to service is often called troubleshooting. It consists of a step-by-step examination of the problem, isolation of the cause, and repair.
This article begins with an introduction to electrical and vacuum diagrams, which are necessary tools for troubleshooting. Then, as we progress further, we will discuss some troubleshooting methods that can guide you through almost any service job.
Electrical Diagrams
Electrical diagrams are used to show you, on paper, what the electrical system of a car contains. These diagrams use color coding and symbols that you must understand.
Color Coding
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Figure 1-1 Circuit diagrams often have the name
of the wire color printed directly on the wire. |
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Figure 1-2 Some diagrams use abbreviations for wire colors
and print a table listing the abbreviations
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Symbols
Figure 1-3 shows part of a car's starting system. The starter motor is represented by an electrical symbol. This is not what the motor actually looks like, but it is what the motor looks like electrically. Figure 1-4 shows some common electrical parts and the symbols that stand for those parts. Some more common electrical symbols are shown in figure 1-5.
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Figure 1-3 This diagram of a
starting system shows you how
the parts look electrically.
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| Figure 1-4 Here are some common automotive diagram symbols and the actual hardware that they represent. |
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Figure 1-5 Here are some more
common electrical diagram symbols.
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Tracing Specific Circits
When faced with the system diagram of an entire automobile, it may be hard at first for you to find and follow an individual circuit. However, once you get to know how circuit diagrams are designed, this will be easier. The first step in finding a particular circuit is to check the index to the diagram, if there is one.
Many diagrams are indexed by grids, figure 1-6. The diagram is marked into equal sections like a street map. Each section is lettered along the top of the diagram and numbered along one side. The diagram's index will list a letter and number for each major part and many connection points, such as the fuse panel and the trunk connector. Figure 1-6 shows how to find a part when the letter and number of its grid are known.
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| Figure 1-6 The simple system diagram has been divided into grids so that an index can be used. To find the dimmer switch look in grid E8. However, if the diagram does not have the grid marks, you must search the area of the diagram that represents the area of the car where the part you are looking for is installed. |
If the diagram is not indexed, you must find a major part by its location in the car. Most system diagrams are drawn so that the headlamps are on one side of the diagram and the tail lamps are on the opposite side. The front of the car is usually on the left of the diagram, figure 1-6.
Once you have found a major part of the circuit you are looking for, the rest of the circuit can be traced. Sometimes, it is helpful to lay a blank sheet of paper on top of the system diagram and actually draw the individual circuit. Remember, a complete circuit must include:
- The battery as a power source
- All switches controlling current flow
- The devices within the circuit that do a job
- A ground connection for all lines of current flow
Vacuum Diagrams
Vacuum-controlled systems are used to regulate an engine's operation for better emission control. Vacuum systems are also used to control air conditioners, headlamp doors, power brakes, and other devices. Several of these systems are often used on the same engine. They have become quite complex, and are often connected to each other to change their own operation, as well as engine operation.
The engine compartment of many late-model cars is crisscrossed with vacuum lines and hoses, each of which does a specific job. When you disconnect these lines to test or service engine parts, you must be sure to re-install them correctly. To help you to properly route and connect vacuum lines, manufacturers provide vacuum diagrams for each different engine and vehicle combination.
Vacuum hoses, like electrical wiring, can be color coded for easier identification. The vacuum diagram can be printed in color or the color name can be printed near the line in the drawing.
A vacuum diagram is especially needed with late-model engines. The routing and connection pattern of vacuum lines can vary a great deal on a given engine during a single model year. One diagram may apply to an engine sold in California, figure 1-7. A different diagram is used when the same engine is sold in a high-altitude area, figure 1-8. A third diagram may be used for engines operated in the rest of the country, figure 1-9.
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| Figure 1-7 Vacuum system diagram 1977 Chevrolet 305-cid V-8, California. |
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| Figure 1-8 Vacuum system diagram 1977 Chevrolet 305-cid V-8, high altitude |
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| Figure 1-9 Vacuum system diagram 1977 Chevrolet 305-cid V-8, low altitude, 49 states. |
The problem is even tougher when car makers make running changes in emission systems during a model year. This can result in two identical engines in the same geographical region having slightly different devices and systems. In this case, trying to route and connect vacuum lines properly without the help of a factory vacuum diagram can be next to impossible, even for an experienced mechanic.
Vacuum diagrams have different forms, according to the manufacturer or service manual used. Some offer vacuum diagrams only for individual emission control systems, figure 1-10. Others attach a complete emission control vacuum diagram inside the engine compartment, figure 1-11.
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Figure 1-10 Vaccum diagram for 1976 AMC V-8 EGR system only.
Figure 1-11 Chrysler vacuum system diagram for 1976 360-cid V-8.
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