Electrical testing can require all five senses. The taste of a still healthy nine-volt battery. The smell of the Christmas tree while swapping endless bulbs to find a string that works. The feeling of pulling a failed spark plug wire to find a cylinder misfire. The sound of an arcing wrench across a battery terminal. And lastly but most importantly, what you can, or can’t see. The results of electricity are easy to find but it is otherwise invisible.
In order to see what’s going on, we need the proper testing tools. Test lights, multimeters, oscilloscopes, jumpers, noid lights, inductive, non-intrusive, the list could go on. No tool is obsolete when it comes to getting a quick and accurate test. The most essential of the arsenal is the digital multimeter (DMM). Electrical testing, in any capacity, can be daunting because it is unseen, hypothetical, and knows how to bend the rule book better than we realize. A quality digital multimeter is the diagnosticians best friend. And just like a best friend, the DMM never lies and tells us the truth even if we don’t want to hear it. However, just like a best friend, it’s only as good as our understanding of what the friendship is based on. Voltage, Resistance, and Amperage are all relational and each serves a unique purpose. A DMM has the ability to quickly and accurately measure voltage, resistance, and amperage. These measurements are displayed in numbers and/or expressed through a line graph vertically and over time horizontally. The line graph, or graphing multimeter, is more useful when monitoring a circuit that has a variable value.
A quick check of available voltage at an automotive battery would show you around 12.6v. No need for a graph to show a static number. However, if checking to see how much voltage signal is being returned from a throttle position sensor (TPS), a graph would be quite helpful. This voltage signal would likely travel upward vertically from 0vdc to 5vdc as the pedal is depressed to the floor and back downward as the pedal is released. This graph should look like a smooth sloped mountain. Up one side and down the other. If you held the pedal down longer, the mountain would get wider and wider and only as tall as the maximum voltage of the signal. Essentially, this full throttle signal would flatten out and be more of a plateau. Hence the phrase, “plateaued” or reached a level of no more growth or change. If there was an issue with the TPS, there would be a jagged area of that smooth accent and again on the descent. That jagged area couldn’t be seen with a numerical DMM as the numbers would be climbing in increments that wouldn’t show the deviation from the norm. The voltage expressed as a line would show the deviation and alert the technician to an issue.
When testing the resistance of a component, the parameters and testing are similar as with voltage. If checking a coolant temperature sensor (CTS) for resistance, a quick numerical/static check can suffice but monitoring the sensor for dropout with a graphing multimeter can help speed the diagnostic process. Additionally, checking the resistance of a particular wire in a harness can be faster when monitoring the results with a graphing multimeter. If a wire’s conductivity is in question, graphing the resistance can allow the technician to quickly identify a failed area during a ‘wiggle’ test.
Another frequent test that the DMM is called to perform requires some heavy lifting. Checking amperage requires the DMM to wired in series, or in line, with the circuit to be tested. Most DMMs can handle up to around ten amps. This restriction is due the internal circuitry and its ability to carry the load and keep its cool. If a vehicle has a battery that fails to start the vehicle after sitting overnight, there is likely an excessive parasitic draw. Some causes could be a dome lamp left on by a passenger, a greedy phone charger, or perhaps a module that is staying awake for some strange reason. Testing for parasitic draw requires a DMM to be wired in series with the battery. This forces all outgoing electrical power to be measured by the DMM as it passes through. If the results are displayed on a graphing multimeter, the technician can watch for draw as well as spikes or drops that maybe keeping things awake. Once the vehicle has settled, the technician can determine based on the vehicle, if there is a draw or not. If there is an excessive parasitic draw, the next steps require meticulous inspection and searching by process of elimination for the components that are causing the draw. Once the excessive draw has been repaired, the DMM can be referenced to know the job was done right.
Electrical diagnosis has taken many shapes and forms through the years but is here to stay and getting more complex by the year. If an engineer can dream it up, the technician must be able to understand it, dissect it, and repair it. Nobody said it will be easy but with the right tools, training, and practice, being competent and proficient in the ways of the DMM will guarantee a long and prosperous career.
- Ben Edwards