How to check for short circuits in a car

Written by cassandra tribe
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Trying to check for short circuits in a car can be a mechanic's nightmare. A short circuit is usually not something you can easily find. It may not be caused by a bare or exposed wire touching metal. It can also be caused by something as subtle as the insulation on a wire hardening with age and becoming incapable of protecting the wire, allowing a short circuit to form when that wire passes over metal. Follow these steps to check for a short circuit in a car without losing your mind.

Skill level:
Moderately Challenging

Things you need

  • Multimeter
  • 2 alligator clips
  • #12 wire
  • Electrical pliers
  • Razor blade
  • Electrical marking labels (if desired)

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  1. 1

    Set up your multimeter. Read the manufacturer's instructions to connect the probes to the meters. Follow those instructions to also turn it on and set your meter to ohms.

  2. 2

    Cut a piece of #12 electrical wire about 20 feet long. Strip an inch of insulation off both ends. Attach one alligator clip to each end.

  3. 3

    Select an accessory to begin with and remove the power feed wire from it. For example, pull the wire off your right rear turn signal (i.e., a brown wire). Clip one end of the wire you made in Step 2 to the end of that wire. Attach the other end to either one of the multimeter probes.

  4. 4

    Check for short circuits. Moving down the wire, pierce the insulation of the wire with the multimeter probe that is not attached to the alligator clip. Do this every 10 inches. If you have difficulty piercing the insulation with the probe, use a razor blade to scrape off enough insulation to reveal the wire and then touch the probe to the wire. If the numbers on your ohm display go up, then you have no short circuit between the beginning point (where the alligator clip on the wire is) and the probe. Keep moving down the wire (away from the clipped end) and testing the wire every 10 inches. If you pierce the wire and your ohm display does not change, there is a short circuit in that section.

  5. 5

    Locate the short circuit. Examine the area that just tested as having a short circuit (the area between the most recent test point and the last). Look for obvious things like bare wires, burnt insulation or broken areas of the wire--remove and replace the wire to correct it. If you see nothing, assume the insulation on the wire has failed and remove and replace the entire section.

Tips and warnings

  • As you identify each wire, put an electrical marking label on the wire. Electrical marking labels are sets of small labels with the same number or letter printed on them. If you were marking the wire for your left turn signal light, you would place a label (marked "1") at the beginning of the wire and then label the wire with "1" everywhere the wire joins other wires or passes through the firewall. This makes later identification of wires easier.
  • Only handle the probes of a multimeter by the plastic or rubber grips. If you hold them by the metal of the probes and touch it to a live wire, you will receive an electrical shock.

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