Fluke Multimeter FAQ: Used 79 and 23 Series II, Pen Testers, and ATS Checks

I've been an industrial maintenance tech for 12 years. I don't hold a master electrician's license, so I won't pretend this page replaces a qualified person's lockout/tagout procedure. What I can give you is what I've learned by troubleshooting controls and equipment with a Fluke multimeter, including the mistakes that made me more careful.

The questions below are the ones I actually put on a shop FAQ after a few conversations about old meters, electrical pen testers, automatic transfer switches, and a diesel-engine search that confused one of my guys.

Are the old Fluke 79 and Fluke 23 Series II multimeters still worth using?

Yes, if they're in safe condition and you're honest about their limits. I bought a used Fluke 79 in 2019 and carried it for years. On a 120 V lighting circuit it read 120.6 V; our calibrated bench meter read 120.5 V. That's a one-time comparison, not a calibration certificate, but it told me the meter wasn't junk.

I also keep a Fluke 23 Series II multimeter in my travel bag. It's not loaded with features, but it's straightforward and simple enough to hand to someone who only needs a quick voltage or continuity reading. The problem with older meters is that people assume they're indestructible. They aren't. Condition matters more than the badge.

Which is better: a Fluke 79 or a Fluke 23 Series II multimeter?

I won't give a spec-by-spec comparison because the reality depends on the exact version and year. In practical use, the Fluke 79 is more of a general-purpose troubleshooting meter. It has more functions on the dial, which is why it became a favorite for plant work. The Fluke 23 Series II is simpler. It does the everyday basics well and leaves less room for a newer tech to fumble with the wrong mode.

If I could only keep one for industrial electrical work, I'd keep the 79, provided it's in good shape. But if the job is a handful of basic checks and I want minimal setup, the 23 Series II is often the one I grab.

What should I check before buying a used Fluke 79 multimeter?

Start with the body. Look for cracks, burn marks, or a missing battery door. If the CAT rating label is worn off, I skip the meter. That rating tells you the maximum transient voltage the meter is designed for, and if you can't read it, you're guessing.

Open the battery compartment. Corrosion is a deal breaker because it can cause intermittent readings. Check that the fuses are present and are the correct type for the AC/DC current inputs; a missing or wrong fuse can make a meter appear healthy on voltage and still be unsafe. Then do a simple lead test: set it to continuity, touch the probes together, and listen for the beep. Bent or loose input jacks can change a reading, so wiggle the probes slightly while you watch the display.

The same checklist applies when someone offers a Fluke 23 Series II multimeter.

What is an electrical pen tester?

An electrical pen tester is a non-contact voltage detector. If the tip is near an energized wire, it lights up, beeps, or both. It's a convenient screening tool for a quick 'is this likely to be live' check.

The mistake I see is treating it as a measuring instrument. A pen tester senses electric field, not actual circuit voltage. That means it can beep on ghost voltage—induced voltage on a wire that is disconnected but sitting next to an energized wire. And depending on cable shielding and tip position, it can stay quiet when a real voltage is present. I never use it to make the final safety call.

For that, I use a proper multimeter set to volts, verify the meter on a known live source, test the target, then verify again. That's more steps than waving a pen, but I've seen the shortcut cause a long afternoon and worse. Put it this way: the pen is fine for the first pass, not the final answer.

What is an automatic transfer switch?

An automatic transfer switch, usually called an ATS, selects between two power sources and feeds one set of loads. In a typical building, it watches the utility source. If utility voltage drops below a set point, the ATS signals the backup generator to start. Once the generator is ready, it transfers the load to the generator. When utility power returns and stays stable, it moves the load back.

The ATS is not the generator and not a breaker. It's the switching and control logic that decides which source feeds the load.

This belongs in a multimeter FAQ because ATS troubleshooting is mostly contact and voltage checking. I look for control power at the ATS controller, verify that the source sensing inputs are reading correctly, and check whether the main and generator contactors are closing in the right order. When I work on the load side, I also verify that no backfeed voltage is present before opening anything. That's a multimeter job, not a pen tester job.

Why is a DD15 fuel filter housing diagram part of electrical troubleshooting?

It looks off-topic until you're standing in front of a diesel engine with a wiring problem. The DD15 fuel filter housing on certain applications includes electrical components: a fuel heater and a water-in-fuel sensor. A diagram becomes valuable because the harness connector has multiple pins and not all of them do the same thing.

I'm not a Detroit Diesel specialist, so I won't quote a pinout from memory. What I can tell you from a multimeter user's perspective is this: get the diagram before you probe. The fuel heater should show a low resistance, and the water-in-fuel sensor should change state as its float moves, but you need the OEM values to know what normal means for your exact setup.

Opening a service manual to the DD15 fuel filter housing diagram is not a sign that you're in over your head. It's a sign that you're using the meter efficiently. I've watched a technician guess at terminals and replace a good sensor because he measured the wrong pin. The diagram would have prevented that.