How to Diagnose Trouble in a Car Electrical System
The electrical system in your car serves a vital function--it connects the various engine sensors with the electronic control module (ECM, or "brain" of the car) and provides power to operate the lights, horn, instrumentation and wipers, among other accessories. Diagnosing trouble in a car electrical system involves determining the nature and location of the problem. Common problems include short circuits, open circuits and high resistance within a circuit.
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Things You'll Need
- Voltmeter
- Test light
- "T" pins (available where sewing supplies are sold)
- Wiring diagram
Instructions
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Determine the type of trouble that you are diagnosing. A blown fuse, burned fusible link or open circuit breaker points to a short circuit, which is caused by a "hot" (current carrying) wire touching ground (any part of the car's body is a ground). An electrical component that doesn't work is likely caused by an open circuit, which can result from a broken wire, a disconnected electrical connection or a bad switch. High resistance in a circuit will result in symptoms like lights being dimmer than they're supposed to be.
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Isolate the trouble. In the case of a short circuit, connect a test light across the blown fuse or open circuit breaker. Then, while the test light glows, disconnect each component of the circuit and watch for the test light to go out. When it does, you've found the location of the short. For open circuits, use a test light or a voltmeter to check for power along the circuit. If there is power at the fuse, but not at the inoperative component (load), the open is between the fuse and the load. This check can be done without disconnecting any of the electrical connectors by using the "T" pins. Push the pins through the insulation at the connection you are testing and touch the voltmeter probe to the pin. To isolate a high resistance problem, measure the voltage drop across each component of the circuit. A voltage drop higher than 0.5 volts indicates a problem such as a burned out switch.
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Use a wiring diagram when you are diagnosing trouble in a car electrical system. The wiring diagram will tell you the color of the wire in the circuit you are tracing, as well as the location of each component in that circuit. It will also show where wires have been spliced. Wiring diagrams can be found in service manuals. To read a wiring diagram, find the symbols representing the parts you are testing then follow the line on the diagram, which shows where each wire goes.
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Tips & Warnings
After using the "T" pins to probe through the insulation of a wire, you might wish to use a product known as "liquid electrical tape" to seal the hole in the wire.
When using the voltmeter to test a connection that is part of a circuit that includes the ECM, make sure you are not using the ohm meter function, because the ohm meter introduces current to the circuit which will damage the ECM.
