How's the DMM Work?
Shown in the picture is our Fluke Model 26III. It is a good quality, basic meter that is ideal for use in a vehicle. It features easy to use controls, long leads, and has a ruggedized case to protect it from being damaged if dropped. It also has a large 4-digit display that is easy to read. This particular DMM will measure voltage, resistance, current, and frequency.
Voltage 
Voltage is simply defined as electromotive force or a potential difference measured in volts. A DMM set to read voltage is simply telling you the difference in potential (volts) between two points (or wires). In a vehicle, we would typically be measuring the potential difference between positive and negative, which is, generally speaking, approximately 12.6-14.4 volts DC (Direct Current). AC (Alternating Current) voltage is also present in your vehicle on some of the wiring having mainly to do with the computer module and computer controlled accessories. AC is beyond the scope of this primer and will not be discussed.
To measure DC voltage in your vehicle, first make sure the probes are inserted in the proper jacks on the meter. Usually, the red probe is inserted into the jack
labeled "+" and the black probe is inserted into the jack
labeled "COM" or "-". Set the DMMs controls to DC volts, place the black probe to chassis ground, and use the red probe to check for voltage.
Current
Current is simply defined as a flow of electric charge measured in amps. A DMM set to read current is simply telling you how many amps a particular accessory needs from the supply (battery) to operate. DMMs will have a maximum current rating. This denotes the maximum
amount of current that the DMM will safely measure. Our particular meter will read a maximum of 10 amps. This means that before you measure current draw in your vehicle, you need to have a good idea as to how much current you expect to read. Trying to read too much current with your meter can damage it.
To measure DC current in your vehicle, first make sure the probes are inserted in the proper jacks on the meter. Usually, the red probe is inserted into the jack
labeled "A" or "mA" and the black probe is inserted into the jack
labeled "COM" or "-". Set the DMMs controls to DC amps. Be sure to choose the proper range if your meter does not have the auto-range feature. To measure current, you must have your meter in between the accessories source and load. This usually means cutting the "+" wire going to the accessory, stripping back a small portion of coating from both of the wire ends, and placing a probe on each wire.
Resistance 
Resistance is simply defined as an opposition to current that results in a loss of energy as heat or another form of energy. Resistance is measured in Ohms. A DMM set to read resistance is simply telling you how much opposition to current flow that a particular accessory has. Before reading resistance, check the wiring in question to make sure that no voltage is present. Reading resistance on an electrically "hot" wire could result in damage to your meter.
To measure DC resistance in your vehicle, first make sure the probes are inserted in the proper jacks on the meter. Usually, the red probe is inserted into the jack
labeled "ohms" and the black probe is inserted into the jack labeled "COM" or "-". Set the DMMs controls to ohms. Place one probe on each side of the accessory. Polarity (red or black) is not important.
Frequency
Frequency is simply defined as the number of repetitions per unit time of a complete waveform. Frequency is measured in Hertz (Hz).
To measure frequency in your vehicle, first make sure the probes are inserted in the proper jacks on the meter. Usually, the red probe is inserted into the jack
labeled "+" and the black probe is inserted into the jack
labeled "COM" or "-". Set the DMMs controls to Hz. Place the red probe on the positive "+" and the black probe on the negative "-" terminals in question.
Range
Most DMMs will have a selectable range of values that can be read. Setting the DMM to the proper range will ensure the accuracy of your measurements. You should choose a range that is nearest to the value you expect, but not less. For instance, if you intend to read DC voltage that you expect to be near 12 volts, you would pick the range that is capable of reading two digits to the left of the decimal point. This range would be capable of reading up to 99.99 volts on the meter. Our particular DMM has an auto-range feature which means that, when activated, the meter itself will pick the proper range for you.
Making the DMM Do Something Useful
The DMM can be used for a multitude of tasks, a couple of which will be covered here. As stated earlier, the DMM will make almost any accessory installation (or troubleshooting task) quicker and easier.
Checking Your Amplifier (and amplifier wiring) for Excessive Voltage Drop
As enthusiasts are searching for more dBs from their audio system, the audio amplifiers are getting larger. Larger amplifiers can require massive amounts of current to produce more dBs. Set your DMM to DC voltage for a reading in the 12 volt range. To check to see if your amp is getting enough voltage, crank your vehicle, turn off all accessories, and check voltage at the battery between the positive terminal and the negative terminal (ex. 14.41 DC volts). Next, with the engine still running and all accessories off, take a reading at the amps input terminals. The voltage should be nearly the same as at the battery (ex. 14.41 DC volts). Now, if you have a test tone CD pop it in your player and crank it up. Playing a test tone will give you a more accurate representation, but a bass heavy music CD will work good enough to give you an idea. Recheck voltage at both locations, battery and amp (ex. 14.24 and 14.24 DC volts), and compare the readings to the ones before.
Consider our example readings:
Amp Off
At the Battery - 14.41 VDC
Amp Cranked
At the Battery - 14.24 VDC
Amp Off
At the Amplifier - 14.41 VDC
Amp Cranked
At the Amplifier - 14.22 VDC
Our readings look good. There is a voltage difference between the readings taken with the amp off and amp on, but they are negligible. These readings denote the voltage drop of your amp on your vehicles charging system. A large difference in voltage in this case could mean that your charging system or battery is not able to keep up with your amp.
Amp Cranked
At the Battery - 14.24 VDC
At the Amplifier - 14.22 VDC
We'll now look at the readings in another way; If you did have a large voltage difference between these two readings, generally a lot lower voltage at the amp than at the battery, you would have found a voltage drop on the wiring. In this case, you would probably need to increase the size of the wiring to your amp.
Checking a Subwoofers Voice Coil DC Resistance
This is a simple check to aid you in finding a voice coil problem with a subwoofer. We're using a SVC (single voice coil) 4 ohm sub for this example. We also have the DC resistance reading from the manufacturers spec-sheet as a reference. To perform this check, the subwoofer must be removed from its enclosure, placed cone up on a flat, hard, and level surface, and be away from wind and vibration. Set up your DMM to read DC resistance. Attach the DMMs leads to the speaker terminals and leave them still as you watch the DMM display. The display will eventually settle, as long as the sub is not disturbed. Our example reading is 3.67 ohms which is in line with the reading from the subs manufacturer. If the reading you obtain is 0 or near 0, then the voice coil is most likely shorted (bad). If the reading is high (infinity, shown on the DMM display as OL), then the voice coil is most likely open (also bad). If the reading is near (within a few tenths) the manufacturers specs, then the voice coil is most likely OK.
Conclusion
Hopefully this primer has helped you to gain some insight into how to use a DMM and how it can help you with your vehicle accessory installation/troubleshooting. What we've covered here is very basic, but should give you a good beginning on the uses of the DMM.
Disclaimer: This primer was written for informational purposes only. While we believe the information here to be accurate, we cannot accept any responsibility for damage caused to your vehicle or accessories. Any use of this information is at your own risk.
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