Multi-Meter Series: Using the Voltmeter and the Voltage Drop Test

We’ve covered some basics by now such as how a multimeter works, and how to get the most reliable results from a multi-meter. We also looked at the functional differences between Alternating Current (AC) and Direct Current (DC). Please feel free to go back for a little refresher if you feel the need.

It’s probably safe to say that the average person willing to use a multi-meter at all knows how to check for voltage and a ground. An easy example would be a burned-out light that may need replacing. Remove the light from the connector, put your voltmeter to 20V or whichever setting is closest to, but over, 12 volts (since we’re talking about automotive) and put one test lead end to each pin. If you read a number around 12.00V then your power and ground are likely okay. If the multi-meter reads -12.00V or something of the sort, it simply means you have your negative lead on the positive pin and your positive lead on the negative. For our purposes the reading still is 12V and your circuit is likely okay.

It’s my experience that everyone instinctively knows this to be true. One problem, however, can be that a wire is mostly burned or frayed such that while there are 12 volts getting from one end to the other, the wire is damaged such that it can’t carry enough amperage to do the work of lighting bulb. The problem could also be a loose connection somewhere. A loose wire connection and a frayed or otherwise damaged wire has essentially the same effect. Not enough amperage getting through. This is exactly why the voltmeter is so important and the test light so useless. While the test light takes so little amperage to light up, it could be misleading in that you one may believe the whole circuit must be in working order.

Beyond the test light

So, let’s take a look at what the voltmeter can do that the test light cannot. Let’s use the simple example we took earlier: the light that we think is burned out. The rear marker light isn’t working. We’ve removed the light, and checked for a solid ground and a positive 12 volts. With the test leads firmly placed on either pin we read 12.6 volts. IT seems to be a perfect circuit. The light must be defective. We replace the light. It still doesn’t work. How is this possible? We saw 12.6 volts and if we use a test light, it lights up. In my experience, this could be anything from a frayed or burned wire, to a faulty switch or even a melted fuse. You wouldn’t think a fuse could melt, but apparently, they can.

One important thing to note is that you can’t put too much faith in new parts. Many mechanics have made the mistake of going through an entire diagnostic, finding the problem, replacing the suspected part, and then assuming that the entire diagnostic was wrong because the results aren’t what we expect. Despite how good the majority of manufacturers are, no company is perfect and your new part could be defective as well. Be it a light, an air valve, or any other component, when this happens, check the new component with known inputs before dismissing your diagnostic. What I mean by known inputs is replace a light (for example) that is working, for the new one to see if it is the circuit or the new light.

How to test wires making up a circuit

Many people would simply take out their multi-meter and set it to ohms. More generally speaking, the ohms function that beeps when a circuit is complete. This is not the way to test a circuit for the same reason that a test light is no good. It takes next to nothing for the meter to read the circuit to be ‘complete’. This doesn’t mean that enough power can pass through a wire to power a light or electric motor.

Wire or cable testing

Now, to test a cable or wire, place the meter leads. One lead should be at the battery, or as close as possible to it. The other lead should be at the load (light, electrical motor, etc.), or as close as possible to it. The multi-meter should be set to whatever is nearest 12 volts and it should be set to DC. Also, the circuit should be complete. If the light, motor, etc. is removed the circuit cannot be complete and the test will not give accurate results. Once all of this is done, the circuit should then be turned on. The reading should be between .00 volts and 0.30 volts. This is where most people get confused. Common sense, or logic, usually tells us that the circuit should read 12 volts. The reason it doesn’t, or shouldn’t read 12 volts is that we are measuring the load, and so we aren’t measuring voltage compared to the ground. If the meter did, in this example, read 12 volts, it means that the wire is broken or damaged and needs to be fixed or replaced.

As far as voltage readings go, the following is essentially what they represent:

0.00-0.30 volts DC = Wire/cable is in good condition.

0.30-11.00 volts DC = Wire/cable damaged, frayed, burned or loose connections.

11.00-+ volts DC = Wire/cable cut, otherwise disconnected.

Now we’ve seen how to measure a positive wire for any possible problems it may have, it’s worth noting that you can’t forget the ground wire or ground connection. The ground is measured the same way as the positive, just on the other side of the load. On lead going to the other side of the load, the other to the ground terminal on the battery.

More and more components are being controlled by the ground. Even starter motors. Some would argue, and I tend to agree, that the ground should be checked first. One reason why I would disagree is that if you take the example of a trailer lighting system, the one ground serves all six other functions. So, if the brake lights aren’t working but everything else is, then it likely isn’t the main ground.

How to do a voltage drop test on a starter motor

Step one: Secure the vehicle (wheel chocks) and make sure the vehicle is in neutral.

Step two: Put one test lead on the positive post of the starter, and the other test lead on a known ground such as the chassis. This is often easier done with alligator clips.

Step three: Have someone turn the key to the ‘Start’ position, or use a remote starter.

Step four: read the results on your multi-meter.

Results: Again, same as with the wires, the lower the reading the better. Over 1.00 volt could mean a faulty starter motor. Over 2-3.00 volts definitely means a faulty starter motor.

Conclusion

With trucks and trailers getting more and more complex electronics knowing how to test and verify circuits is more important than ever. Also, some manufacturers will do anything they can to not pay a warranty. Approaching them with circuit information like that that you learned here today will help. Also, diagnostic by replacing components is expensive. Add to that the fact that the vast majority of electrical components will not be accepted back from your suppliers and all the more reason to learn how to diagnose more effectively.

I hope this helped and that you look forward to the next article in the series on amps and using ammeters!

Drive safe,

Kevin Gauthier