Tech Tip: Testing Voltage Drops Effectively

Voltage drops are one of the least obvious problems to “look” for with the eye, but one of the easiest to troubleshoot when you have a Digital Multimeter (DMM). It’s one of the electrical problems that can develop over time, even if no other changes have been done to the installation.

Conditions of humidity and temperature can create corrosion on electrical contact surfaces. In addition, insufficient wire gauge, poor electrical grounding surfaces and just plain loose connections are other contributors to voltage drops. Excessive voltage drops also generate excess heat. Of course, the manufacturing process of today’s unibody vehicles certainly has not helped voltage drop problems where grounding to the vehicle’s sheetmetal is concerned.

Causes and Suggestions
Voltage drops are especially troublesome when you look at the circuit as a series path. Each component or element of the path can be responsible for a drop. When you think of the series path, each point-to-point connection is a series path of current flow. The positive battery post through the battery terminal itself is a series current path. Then from the battery terminal to the cable. Then from the cable to the first connection point, which is probably a fuse, circuit breaker followed by a switch. Then so on, until you complete the whole path to the component (amplifier, electric motor, light bulb, etc.) and back through the chassis ground to the negative post of the battery.

Voltage drops can be tricky to measure because normal DC voltage measurements may not show a severe voltage loss until the circuit is loaded. Whenever you make voltage drop measurements and you don’t want to waste time, remember that the measurement will be most accurate when the circuit is loaded to reproduce the same normal conditions under which it normally operates OR the conditions that bring on the problems you have identified.

For example, if you are testing voltage drops in an audio system, play some loud music or run some sine wave tests (being careful not to burn out speakers) to load the audio system enough to show voltage drops at various points. The more severe the voltage drop, the less loading it takes to show. Repeated loading usually confirms your suspicions if there is, in fact, a measurable voltage drop between the two measured points.

Measurement Procedure
When you’re using a DMM to search for voltage drops, you will set it to the DC volts mode and place the probes between the two points you want to measure. You can measure two points close together like switch contacts or far apart like each end of a long wire run. The accepted rule of thumb is that you do not want any more that 0.5 volt drop in the complete circuit, so a low measurement in several places could easily add up to 0.5 volt or more. Ideally the measurement will be 0.01 volts or less at each point. That means there’s virtually no loss whatsoever – at least not one you are going to notice.

Keep in mind that all of these measurement points should be measured while the circuit is active. That means the lights are ON if you are testing a voltage drop in a lighting circuit or the audio system is working hard if you are testing the power and ground wiring of the audio amplifier.

When you make loaded measurement on a high-current device like an audio amplifier, using a test tone signal provides a more consistent load on the amplifier than if you are playing music, again being careful not to damage speakers. You can always disconnect the speakers temporarily if you are worried about testing that way because the amplifier will still have a measurable change once the volume is increased. Keeping all the factors the same though, that will provide the most accurate measured results and point you toward a problem faster.

What you can determine with using a test on an amplifier is whether the wire used was too small, if the fuse (or fuse holder) is limiting current flow causing the voltage drop, or if the ground connection between the amplifier and the chassis of the vehicle is inadequate.

Some technicians like to check point-to-point connections using the continuity setting on the DMM, but that’s intended for testing connections in circuits where there is no power applied. When you need to test a circuit in real time, the voltage method used in these activities (when you are looking for a voltage drop) is the most accurate way to measure.

In the voltage drop test image pictured, the process of testing an amplifier for voltage drops is illustrated, including the parts of the chassis. All of these points are important test points. One of the most important upgrades that gets ignored is the last link in the circuit – the chassis to negative battery post connection. Even if you leave the original wire and simply add a second wire equal to the size of the amplifier wiring, you will have completed the circuit path without compromising any other current flow for the original equipment in the vehicle that uses that factory-supplied negative battery connection wire.

Practice this technique, and you will find that troubleshooting voltage drops is an easy, methodical process that will save you hours of wasted time in the install bay “chasing gremlins.” It’s yet another way to use a DMM in the install bay. Oh, and you can’t really do that with a test light!

Technical data and illustration provided by Ramsey Consulting Group, Inc. www.ramseyconsultinggroup.com.