Ask the Mechanic

Steve Thompson, Contributor

READER: I bought a digital multimeter to check for a bad wire, and I know that an ohmmeter is used for checking resistance in a wire. After setting the meter to ohms, how do I use it? It seems today that there are a lot of hidden problems inside the wires of my equipment, and an ohmmeter can help locate these problems. I have problems all the time finding bad wires on my equipment. I've always wondered how the ohmmeter knows if the wire is bad. Can you explain how it works?

STEVE: The ohmmeter uses a battery in the meter to cram electrons through the wire when you place the leads on each end of the wire, and it also powers the display light. Never use an ohmmeter on any wire that has voltage. It's better to disconnect the wire at both ends when checking for wire resistance. Place the meter leads on each end of the wire. If electrons flow freely through the wire, the meter will register no resistance (000), meaning the wire has no resistance and is a good wire. If there is a break in the wire, the meter will show on the display window as it does with the meter leads held apart, usually 1 or OL (out of limits), according to your meter. It is cheaper than using an X-ray machine.

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-- Email Steve Thompson care of dan.miller@dtn.com, and be sure to include your contact information and phone number.

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