Testing Voltage Drops
By Todd Ramsey
Ramsey Consulting Group Inc.

Causes and Suggestions
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.

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. It then goes from the battery terminal to the cable. Then from the cable to the first connection point, which is probably a fuse, then to a circuit breaker followed by a switch. And 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.

Measurement Procedure
When you’re using a DMM to search for voltage drops, you will set it to 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 than a 0.5-volt drop in the complete circuit, or 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 and that means there is 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 your 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.

Using the test tone provides a consistent current draw under which problems like power/ground wire that is too small, fuses or fuse holder resistance problems, or even something as simple as bad ground points can be easily identified. 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 upgrade that is 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.” Testing voltage drops is but one of the many techniques that Advanced level installers review in the MECP Advanced study guide. If you have been away from the program for a while or are just in need of some reference material geared for today’s installation technicians, you can order the MECP Advanced installer study guide by calling (866) 858 1555.