Aviation From My Rocking Chair: The V.O.M. (Not Vroom, Vroom)

This month let’s talk briefly about a piece of electrical test equipment everyone should have. It can be used at home, on your car, boat, or Aircraft! Let’s talk about V.O.Ms.

Before we start, let me see a show of hands. Who thinks of electricity as some kind of voodoo, and electricians as practitioners of a very dark art? Hmmm, that many eh? O.K. then, how many of you are baffled by hydraulics? I see a lot of hands went down. Actually, the hydraulics to electricity analogy is a good one. A hydraulic pump does not make pressure. It provides flow, just as a battery or generator does in the electrical circuit. Hydraulic pressure increases as flow is restricted.

Electrical pressure (or Voltage) is produced when the electron flow encounters resistance. Electron flow is called Amperage, and electrical Resistance is measured in Ohms. Think of the V.O.M. as an electrical pressure gauge or flow sensor. V.O.M. stands for Volt/Ohm Meter or since it can measure Amps, it is also known as a Multimeter.

There are two types of V.O.Ms- Digital and Analog, each having advantages and disadvantages. The Analog V.O.M is more prone to impact damage due to it’s mechanical meter movement and an exotic switching system needed to drive the meter from Volts to Amps scales. As the internal batteries discharge, frequent compensation must be made for voltage drop on the meter. Digital V.O.Ms are harder to break due to integrated circuits and the lack of mechanical parts. The need to watch numbers instead of needle deflections can be a problem, especially when checking continuity on many components in a short period of time. You must also be very careful when using auto-ranging features on digital meters as it is easy to misinterpret the scales. Which one do I use, personally? The easy answer is both. Most of my working life, Analog meters were the only ones available, and I still prefer them. Because most troubleshooting involves simple voltage or continuity checks, I find it much easier to watch a needle swing than to watch numbers flash on the screen. Be aware, however, an Analog V.O.M applies enough current (from internal batteries) when continuity testing to destroy integrated circuit components.

Now, let’s do a couple simple troubleshooting problems. On preflight, you switch on the landing light and get…nothing. You next make sure power is on and either cycle the circuit breakers or check for a blown fuse. Still no help. Turn off all power, get your tools, and bring along that multimeter. Remove the lamp from it’s holder and select the highest DC voltage scale on the meter. Touch one lamp power wire with the black test lead, and one power wire to the red test lead. There should be no voltage. Swap the test leads just to make sure. Now it is safe to select an Ohms scale. A mid-range is usually O.K. Touch the test leads to both lamp contacts. If the needle moves, the lamp is good. Reinstall the lamp. Next step is gain access to the landing light switch. Connect the red test lead to the positive(+) side of the switch and the black lead to the wire going to the light. Select a Volt scale above battery voltage range. Voltage should read the same with the switch on or off. This tests the integrity of the circuit between the switch and the light. Finally, if you still have a problem, disconnect the battery, set your multimeter to an Ohms scale, attach one test lead to one side of the switch, and the other lead to the opposite side of the switch. Actuate the switch. f there is no needle swing as the switch goes on, the switch is bad. Notice the troubleshooting process follows a logical pattern with the most likely problem first.

Let’s try one more and then I’ll let you go. High resistance connections or grounds can cause really strange problems. I chased a taillight problem on my wife’s VW van for days and finally found a loose ground on the right light assembly. O.K., set the V.O.M to a Voltage scale (DC in this case) and attach the red test lead to the component end of the ground wire, and the black lead to a known, good ground. Turn on the system being tested. Any voltage indication on the meter is proportional to the resistance in the ground wire or it’s attachment. More observed voltage = More resistance in the ground.

After watering your eyes with my ramblings, I’ll let you go. Remember if you see someone wandering in your aviation neighborhood, it’s only me trying to come up with next month’s column!

Rick Vaux TC 4130