A broken wire can cause an intermittent failure in your car, appliance or piece of equipment. Especially in a moving vehicle, a broken wire can stop electrical current every time it disconnects from the other piece of wire it is supposed to be attached to, potentially causing a permanent failure. Still, there are certain strategies you can follow to locate an opening in a wire, even if the broken piece is hidden inside the insulation.
Disconnect the power source to the vehicle or piece of equipment that contains the wire you need to troubleshoot. Use a wrench if you need to disconnect a battery cable.
Inspect closely the connections at both ends of the wire for damage, if they are readily accessible. Slightly pull at the wire ends, where they hook to the connectors, to make sure that they are firmly attached.
Trace the length of the wire with your index and thumb, paying special attention to any imperfections around the wire insulation. If necessary, use a small mirror and a flashlight to reach around spots where you have minimum access. You can suspect any problems if the insulation shows signs of damage, including darkened spots that may indicate overheating, which might have caused the wire inside the insulation to break.
Unplug the connector to which the wire is attached, and check for damage.
Get your digital multimeter and set it for continuity, or the lowest range on the Ohms scale.
Turn on the multimeter and touch one of the probes to the metal terminal that holds the wire to the connector, and the other probe to the exposed part of the wire, where it enters the connector. Wiggle the wire to check for a false connection as you probe the terminal. Your multimeter readout should show zero resistance. If the display shows infinite resistance, the wire is not properly connected to the terminal. Test the terminal at the other end of the wire as well, if equipped with a connector.
Connect one of the meter probes to one end of the wire and the other probe to the other end of the wire. Use alligator clips on the probes to keep them hooked to the wire ends. Wiggle or carefully bend the wire at various points. If there is a break at any point along the wire, your readout will display infinite resistance.
Insert a pin about two or three inches away of the point on the wire where you suspect there is a break. Insert another pin on the other side of the wire, where you suspect a break.
Connect the multimeter probes to the pins and wiggle the wire at the break point. If the multimeter display reads infinite resistance as you wiggle the wire, you have found the break in the wire.
Sometimes, especially if you are working on an automotive electrical circuit, you will not be able to reach each end of a long wire with the multimeter probes at the same time. In this case, you can use a jumper wire between one end of the wire and one of the probes. Another alternative for use in a vehicle is to connect the jumper wire to chassis ground and then connect one of the multimeter probes to the chassis ground and the other probe to the other end of the wire to check for continuity.
If you use pins to probe through the wire insulation, make sure to seal the punctured spot(s) with electrical tape to keep moisture out of the wires. This could create electrical circuit problems later on, if not fixed.