You need to know how to read and trace electrical and vacuum diagrams and relate them to the car in order to troubleshoot the car's systems. There is more to troubleshooting, however, than being able to understand the system that has failed.
The complexity of a modern car's systems demands that you approach any problem in a logical, organized manner. To help you develop this good habit, here is a 10-step checklist for troubleshooting a tune-up-related problem.
1. Ask the owner or driver
If possible, you should talk to the person who was driving the car when the problem first occurred. Questions to ask include:
- Does the problem occur only under certain conditions, such as a cold start, hot idle, or hard acceleration?
- Were there any noises, smells, or other unusual signs?
- Has this problem occurred before? If so, what repairs were made?
2. Know the car
Review the appropriate manufacturer's shop manual to get a general idea of what symptoms could be involved. For example, does the car have an air injection system, a spark timing control system, solid-state ignition, or any other emission control devices? This is good time to use the electrical and vacuum diagrams to help you.
3. Operate the car
Now that you know what the general problem area is and what systems might be involved, you can begin to examine the car. You should make sure that the customer or service writer described the situation accurately and completely. For example, if the work order says "Rough idle," you should run the engine at idle and check for other symptoms. Is the exhaust black? Does the car overheat at idle? Does the problem go away when the car is warm, or when it is accelerated from idle? These related symptoms can give you clues to the unknown problem.
4. List the possible causes
In your head or on a piece of paper, make notes on the possible causes. This should include any ideas you gained from steps 1 through 3. By looking at the electrical and vacuum diagrams, you may see other possible causes. For example, if the car has a spark timing control system, figure 1-12, what does the system include? It may control vacuum flow with an electrical solenoid that is in turn controlled by a coolant-operated switch. A problem in the cooling system could affect the switch, or an electrical problem could affect the flow of vacuum. A list of possible causes will help you to organize your thoughts when you begin to actually test the system.
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Figure 1-12 This simple spark timing control system relies on the electrical system,
the cooling system, and the vacuum system.
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5. Isolate the problem area
If your list of possible causes includes several items, you must narrow the list. In step 4, our sample list included the cooling system, the electrical system, and the vacuum system. To narrow this list, you must make some simple tests and observations. Then you can eliminate the systems that are working properly. If the car has not been overheating or running too cold, then the cooling system is probably not at fault. If there is vacuum at the carburetor port and the hoses are in good condition, then the vacuum system is probably all right. This leaves the electrical system as the problem area.
6. Know the problem area
Now that you know what area is to be tested, you can study that area in detail. If it is a vacuum system, the vacuum diagram will show you:
- The source of vacuum
- Where specific hoses should be connected
- What switches and solenoids control vacuum flow
- What devices are affected by the presence or absence of vacuum
- Where the circuit receives battery voltage
- What switches control current flow
- What devices use current flow to do a job
- Where the circuit is gounded
7. Test Systematically
Once you understand the system's pattern, you can start at one end of the system and test small areas one by one. In our example of an electrical problem in the spark timing control system, figure 1-12, you could start at the electrical circuit's ground connection. This is the coolant-operated switch. If there is no voltage present at the switch, figure 1-13, step 1, then the problem lies between the switch and the battery. You would then move to the next unit nearer the battery, which is the solenoid, step 2. If there is voltage present at the solenoid, then the problem lies between the solenoid and the switch, step 3. By testing the system in orderly steps, you can find the problem without wasting time.
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| Figure 1-13 By testing in logical steps, you can isolate the area of the problem |
8. Verify your findings
Once you find the trouble area, be sure of the failure. In our example, figure 1-13, you might decide that the problem is in the wire between the solenoid and the switch. Before you replace the wire, check that the electrical connections at the solenoid are clean and tight. Checking and tightening a loose connection takes much less time than replacing a wire that was never at fault.
9. Repair
You should begin repairs only after you have narrowed the problem down to a specific point. Much of the manufacturer's shop manual covers repair procedures for specific parts.
10. Test your repair
Many times this will be the last step of your troubleshooting procedure. If you drive the car and the problem symptoms are gone, then you have succeeded. Sometimes, a system will have more than one problem in it. For example, our vacuum solenoid could be bad as well as having loose connections. If tightening the connections does not solve the problem, you must return to step 7 and test the rest of the problem area between the solenoid and the switch. Testing your repair work can avoid added frustration for you and the customer.
