Electrical circuits suffer from these common ailments: short-to-ground (before consumer); short-to-ground (after consumer); short-to-voltage; open circuit; and high resistance circuit.
When a short-to-ground occurs before the intended consumer in a circuit, the resistance to current flow is low and current flows excessively until the circuit protection device — be it circuit breaker or fuse — opens the circuit to prevent damage. An example of a short-to-ground before the consumer is a chafed circuit feed wire that touches ground.
Not as common as the other types of shorts we've discussed, the short-to-voltage occurs when a circuit that's normally off receives voltage from another circuit. This sort of condition can occur when wiring insulation inside a harness melts, effectively jumping one circuit over to another.
The open circuit is extremely common. An open circuit occurs when the conductive path between the source and the consumer is disrupted for any reason. Examples of an open circuit include a broken wire on the positive or ground side, a bad switch, or bad connection.
The last type of circuit problem, a high resistance circuit, develops when current flow through the circuit is inhibited. Circuit resistance can result from poor connections, damaged or inadequate cables, faulty switches and solenoids, and the like. High circuit resistance impairs proper current flow and circuit operation. High resistance circuits are best found using voltage drop testing.
1) Confirm the complaint. Never attempt to diagnose a problem
without first confirming that the problem actually exists. If you can't duplicate it, you
can't fix it.
2) Understand the circuit. A few minutes spent understanding circuit operation can save hours of troubleshooting time. Know what, how and why the circuit operates first.
3) Test the circuit. Use the equipment and procedures recommended by the vehicle manufacturer and never skip steps. Proper troubleshooting takes discipline, but saves time.
4) Make the repair. Once you've concluded what the circuit fault is, fix it the right way. If the repairs require splicing or disassembly of connectors, follow the manufacturer's procedures to achieve a reliable, long-lasting repair.
5) Check your work. After repairs have been completed, make sure you try the circuit in all modes of operation. It's one thing to fix a headlight circuit that doesn't work, but if there are no high-beams, you're not finished.
Remember, if you’re curious about a given tool or equipment application, please submit your question to PTEN so it can be considered in an upcoming Tool Q&A. This month covers your questions...
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Voltage drop as a theory and voltage drop testing are often ignored because the test and its usefulness are often misunderstood. Let's clear up that misunderstanding.