If you own a classic car and plan to join the “fix it yourself” club, at some point you’re going to have to track down wiring problems. And to do that, one of the things you’ll need to know how to do is to test for continuity in a circuit or wire.
Today’s small digital multimeters make continuity testing a simple task. My multitester happens to be the uber-common GE Digital Multimeter GE2524, which I probably picked up at Wal-Mart or Home Depot at some point. That bright yellow thing has come in handy more times than I can count.
Here’s how I think of it: Continuity means you have an unbroken electrical connection between two points. For example, a three-foot strand of 12-gauge wire has continuity if electricity can go from one end of it to the other. That same wire, cut in half, would not have continuity, assuming you’re still trying to get electrical power to the same end, three feet away.
And obviously, a four-foot roll of rubber tubing does NOT have continuity, because rubber doesn’t conduct electricity.
Continuity Testing: An Example
Suppose you have a wiring harness which contains three separate wires — red, green, and blue — and which runs for, say, twenty feet. Consider a harness like this:
The harness is wrapped inside plastic sheathing, so visually inspecting for wire breaks and such would be a pain — particularly so if it’s already installed in your car. But if you have a multimeter and can test for continuity, then it’s a snap to determine whether each wire is “closed,” or continuous, and whether any of the wires are broken inside the harness.
Obviously, when you touch one end of the harness’s red wire to the multimeter’s red probe, and the other end of the red wire to the multimeter’s black probe, you want to see something on the screen to indicate continuity. If you don’t, then somewhere in that harness is a break in the red wire.
Further, suppose you’re wondering whether one of your wires is coming into contact with another wire inside the harness. Maybe you suspect that the green and blue wires have chafed somehow, and are contacting each other inside the harness somewhere. By touching one multimeter probe to an end of the green wire, and the other multimeter probe to an end of the blue wire, you can see whether the circuit is continuous — which, of course, would validate your suspicions. Naughty, naughty wires.
Testing for Continuity: Multimeter Settings
Here’s a shot of the settings I need to select on my GE multimeter in order to test for continuity:
Note that the dial is turned to the figure which looks kind of like an arrow (→) overlapping a plus sign (+). The red probe wire is plugged into the “VΩmA” port, and the black probe wire connects to the COM port.
Continuity: What I’m Looking For
Note the screen readout on the multimeter pic below: The screen reads simply “1”, which shows on the very left side of the screen. This “1” tells me that there is no electrical connection between the two probes — the circuit is “open.” They’re not touching each other, and they’re not connected by a completed wire or circuit which conducts electrical power from one end to the other.
Now, if I were to touch the two probes together, this would complete the circuit in the simplest way possible, thus making it “continuous” or “closed.” The same thing occurs if I touch the probes to either end of a good, non-broken wire.
How does the multitester do this? Well, when testing, the multimeter sends out small-voltage signals through one probe, which, if the circuit is continuous, it then picks up in the other probe.
In cases where “continuity” exists, then, I would expect to see something else — well, in the case of the GE multitester, pretty much anything else — in the multimeter screen. For example:
In the above image, the two probes are touching, which completes the circuit. Thus, we have continuity, and the multitester displays something other than “1” on its screen.
On the GE tester, when continuity exists, the screen shows a wide range of numbers on its right-hand side. What these numbers mean, I have no idea. All I really need to know is that there’s continuity in the circuit because something other than a left-side “1” appears on the tester screen!