Testing Wires

by: Andrew Krause

The most important thing in automotive electronics for an installer to know is what wire does what. From having to hardwire a radio into what's left of a factory electrical system, to installing an alarm or even troubleshooting problems, it is important for someone to be able to tell what a wire is and what it is for.

First,you'll need the proper tools. The proper tools are:

A Digital Multi Meter is quite a handy tool, can be purchased at any hardware store, Radio Shack, Wal-Mart, or automotive store. With a basic DMM, you can measure AC and DC voltage, AC and DC current (up to a certain number of amps), and Resistance. Really good multimeters will let you measure other shit, but if you're having to read this, than you don't need to worry about that crap until much later. The 1.5 volt cell is mostly for speakers. You should not use this until you have positively verified that you most likely have a speaker using your DMM. The cell can help you determine the polarity of the speaker, and which speaker in your car you're onto. Finally, I cannot stress the importance of a good pair of strippers. Especially blonde ones.....no wait....wire strippers. GBH, Klein, or Blue Point make very good ones, which are small, and have a spring to open them up. They should have a cutting edge, small gripping flat, and a series of graduated holes to strip insulation off.

Now, before continuing, I'd like to cover test lights. A test light is basically a metallic probe with a light in it, and a clamp on the other side which you attach to the vehicle ground. When you find a wire carrying sufficient voltage and current, the light will glow. DO NOT EVER USE A TEST LIGHT! THIS IS AN IDIOTS TOOL!!! These lights are only good for detecting current. Also, they draw current, and they can easily draw enough current to fry the sensitive electronics on today's modern cars. A DMM will draw almost no current whatsoever (such a small amount, it can hardly be noticed at all), and is safe to use on modern electronics. Also, do not use logic probes. They work in much the same way, for the most part, but even though they are digital, they often draw enough current to damage sensitive electronics.

Now, we are going to look at the basics. First off, we are going to assume your DMM is an "autoranging" model. If not, you will have to select the range in which measurements are taken. This is mostly older DMM's, although a few very high end meters still need to be told this. If you have a meter which is not autoranging, set the range to something within what you are testing for. For instance, if your meter is set up in 10 unit separations, and we are going to test for voltage, then you would set your meter to 20. Also, when we take resistance measurements, if your meter is not autoranging, start on the lowest setting. Even DMM use a small amount of output voltage to check the resistance of something. Starting at the smallest setting ensures the least amount of voltage. Now, for testing!



Testing for voltage is easy. Set your DMM to Volts DC, if you are testing for DC (like, from a battery, or at your fuse box, or at the back of the radio), and Volts AC for AC (like tachometer wires, audio outputs, or your wall socket). When testing voltage, you always place your leads in parallel with the circuit you are testing. For instance, if you want to check the voltage of a battery, place the positive lead to the positive terminal on the battery, and the negative lead to the negative terminal on the battery. Even if the battery is hooked up to something else, you'll still read the same voltage as what every other device is getting. Voltage stays the same in parallel.

Now, in a car, most often you are checking to see if wires have voltage. Most cars out there have a negative ground chassis, which means all metal on the body of the car is connected to the negative terminal of the battery. So what you can do, is just touch your negative lead from your DMM to any metal on the body to get a negative connection. Then, you can probe your wire to test for voltage. Usually, people strip the insulation back, test the wire, and if it's the wrong wire, they tape the wire back up. However, one thing I do is to sharpen my positive DMM lead to a needle point. This way, I can pierce the insulation to read the wire, then remove the point and allow the insulation to seal back up. If you are testing a wire that runs into a harness, you can also probe where the wire goes into the harness. Don't force the lead, however, as you may damage the harness.

Once you have probed the wire, the display on your meter will change. Often, the meter may take a half second to read the proper voltage, because the electronics are testing it. However, soon a reading will come up. For instance, when you test your battery, you may see something like "12.50 V".

Often, when you are testing for voltage, even if you aren't touching a wire, you may get small readings on the order of millivolts or microvolts. This are called ghost readings. For the most part, they are small voltages that occur for a number of reasons. One reason you can quickly have fun with is the voltage of air. Connect the negative lead of your meter to something like a vehicle chassis, or even your plumbing. Then, touch your finger to the other lead, and stick it up in the air. Depending on how dry the air is, and where you are, you can read several millivolts of electrical potential in the air.


Current is measured differently from voltage. For current, you have to switch the positive lead to the special plug for current. This measurement makes the meter a short circuit. Often, you are limited to about 10 amps or so. For instance, if you suspect that your amps are killing your battery, you would use your meter to test for current draw. Disconnect your positive terminal. Set your meter to DC Amps. Put the negative lead on the positive terminal of the battery, and the positive lead to the battery terminal. Make sure the terminal is not connected to the battery in any way other than through the meter. Observe the reading. Just so ya know, you will always have some current draw, but with your door closed, and everything off, it should never be more than .25 amps.


To measure resistance, you simply place your leads in parallel, like you're testing for voltage. However, be aware that if you are testing in a circuit, you are not only measuring that component, but everything connected to it. If, for instance, you suspect a speaker is blow, disconnect it from your amp. Otherwise, you will be testing the combined resistance of the amps output transistors and the voice coil of the subwoofer. Resistance is measured in ohms. If you don't get a real number (may meters will say O.L), you either have an open circuit (one where there is no connection between the leads through the item your testing), or the resistance is so very high, that it exceeds the meters ability or the range on which you are set. Most DMM's go up to mega ohms. If you read just 0, or something close, you have very little resistance.

Continuity is another way of measuring resistance, but it very different. Normally, when we test for continuity, we are not so much concerned with the resistance, but the lack of it. Normally, we test continuity for a length of wire, or if a circuit connects to the vehicle ground. Most DMM's will emit an audible beep if a circuit has continuity. For instance, if you believe a fuse is blown, but it looks okay, you would put one lead from your meter on one end, and the other lead at the other. If the fuse has continuity, the meter will beep. If there is a break in the fuse, even a tiny one you can't see, there will be no beep.

Testing for Stuff


When something is shorted, there is a path of near zero resistance from one side to the other. If you are blowing a fuse in a particular circuit, you may have a short to ground. Best way to test is to go to the non battery side of the fuse (this is the side that does not read 12 volts). Set your meter for continuity, and place the negative lead on nice shiny metal (the cigarette lighter of most vehicles go straight to ground). Place the other lead on the fuse box contact for that circuit. If it beeps, than check your wires for damaged insulation.

Amp Power

If your amp doesn't come on, but the fuses look fine, you will want to check it (duh). First, make sure you ground terminal on your amp reads continuity to chassis ground. If you have it, also check the resistance. As little as possible is best. Then, place your ground lead on the negative terminal, and check for voltage at the power terminal and the remote terminal. If you don't have any power on the main wire, check your lead and it's connections. Test the power wire fuse at the battery for continuity, and replace it if it is blown. If you have power at the main feed, but the remote is dead, try turning your radio off, then on, or resetting it. Also, check the wire for continuity with ground. You can also check the wire farther up the run. Most radios will shut off the remote output if the wire grounds out or if too much current is drawn off of it.


If you are putting an alarm in, everything is a trigger. It is important to get the right wire for the alarm to work properly.

There are two basic types of trigger. A positive trigger will read 12 volts when something is happening (for instance a + door trigger will read 12 volts when the door is open, but not when the door is closed). A negative trigger will be just the opposite. It will either read 12 volts until the trigger happens, or will read nothing (null), then continuity to ground when the event happens. For instance, most Japanese cars have - door opening triggers. They will read 12 volts, because the potential from the other side of the dome light is 12 volts. However, when you open the door, and the circuit is complete, the potential at the switch disappears. Likewise, with door locks. If you have negative trigger door locks, the wire will read 12 volts. When you hit the lock or unlock button (depending on what wire you are using), the voltage will drop drastically (but not necessarily all the way). Likewise, a positive trigger system (some Ford and GM cars) will read no or small voltages, until you the lock/unlock button, then you will get 12 volts.


If you are ever testing for an ignition line, here's how it works. The ignition wire must provide power to the engine and electronics when the car is running and during starting. With the key in the off position, you will have no voltage. Turn the car to ACC, and you will still have to voltage. Turn the key to ON or RUN, and you will get full voltage. Crank the starter for a few seconds. The ignition line will still get almost full voltage, although it may drop a few volts. The ignition line will normally be found below the steering column, and will be a large wire, like 12 or 10 gauge. For safety sake, whenever sourcing off this wire, use a fuse as close to the connection as possible.


The starter wire only has voltage going to it when you crank the engine, not while the engine is running, or when the car is switched to OFF, ON, or ACC. It may not be a full 12 volts, because of the massive draw of the starter. It is usually found below the steering column.


This wire always has 12 volts. It comes directly from the battery, via the fuse box. Normally, this is the largest of wires below the steering column. If you don't have a really high current application, you can also find it at the back of the radio (for memory), or the cigarette lighter on some cars. For safety sake, whenever sourcing off this wire, use a fuse as close to the connection as possible.


The tach wire will have both AC and DC current on it when the car is running. Use the AC setting to determine if is a tack wire or not. This wire will read a very low voltage at idle, but will increase dramatically when the engine is revved. Commonly, a tach wire will go from a half a volt up to 3 or 4 volts. More is possible. Normally, you can find a tach wire either at the ECM, PCM, or you can also get them at the Cruise Control Module, or they will be found it a little plug in the coil pack or at the distributor.


This wire gets voltage when the car is in ACC or ON or RUN position. It does not have much voltage, if any, when the car is cranking, and should have almost none at all if the car is off.


This wire is tricky, and has fucked up many an installer and DIY'ers day. The dimmer wire will read as a ground, but a weak one, until you turn the dash lights on, then it'll have no ground at all. It'll have 12 volts. A lot of times, an installer will not properly check this wire, put a radio in, and let it roll. At the very least, you'll blow the dash lighting fuse. However, it is not uncommon to burn up the dimmer switch.


A speaker uses a pair of wires. Using your DMM, you will test for continuity between two wires. When you get it, measure the resistance. Speakers will not always be what they are rated at, so if you're speakers are 4 ohm, you may read as little as 3, or as high as 5. Also, many stock speakers can be as high as 10 ohm. However, as long as it is not very high, or very low, you probably have a speaker. Now, you can take you cell and test it. If you hear the speaker pop, you have it.

You can also use the battery to check for polarity. If you hook the battery up, and the speaker pops forward, you have the correct polarity. The positive terminal of the battery is connected to the positive terminal of the speaker, and vice versa. It is important to observe polarity. If you have more than one speaker, and you hook one speaker out of polarity, or out of "phase" with another, it will actually take away from the overall sound (usually, it kills your bass response).

Finally, the pop of the speaker can be used to tell you where the speaker is you're looking for.

This has covered some of the most basic testing skills. However, the highest of skills come with deductive reasoning. To solve a problem, you must know how something basically works, and to observe it's behavior. The majority of problems arise from damaged equipment, poorly connected equipment, or damaged interconnects. Sometimes, a wire will not test the way it's supposed to. You will have to use your noodle and figure out what a wire is doing, and why it's behaving a certain way.