Finding a bad wire in a car wiring harness often requires testing with a volt ohm meter or a homemade tester. With a little practice the average weekend mechanic can learn to locate trouble in a wire with minimal testing. Knowing where to look and what to look for will be learnt. Smell and feel also play an important part in troubleshooting wiring faults. The average weekend mechanic can learn the skills in about an hour and a half.
Put on the safety glasses. Select the wire to be tested. Pull firmly on each end as if trying to stretch the wire. Trace along the wiring harness feeling for rough spots in its outer tape or wrapping. Check for wires that have been pulled out and spliced with non weatherproof connectors. Smell the rough spot or spots for a burnt smell.
Cut the outer cover on the wiring harness open at the suspected bad spot. Find the wire being tested by its colour. Look for damage to the wire.
Turn on the ohm meter and place the dial on the ohms reading. Clip the two test leads together and record the reading which should be infinite ohms. Disconnect the test leads from each other.
Disconnect any devices from each end of the wire being tested and place them where they are not touching any other wires if no visible trouble was found. Place either test lead from the ohm meter to either end of the wire using its alligator clip. Place the other test lead on the other end of the wire in the same manner.
Place the ohm meter in the ohms position and read the dial. Consider the wire bad if the reading is zero or a low number of ohms. Consider the wire good if the reading is infinite ohms or the same reading as when the test leads were placed together.
Remove one test lead from the end of the wire and connect it to each of the other wires one at a time and read the meter for each one. Consider the wire good if the reading is zero when the test lead is connected to another wire. Consider the wire bad if it is any number of ohms or infinite ohms when the test lead is connected to another wire.