A digital multimeter (Photo 1) is the perfect tool for testing many components on a guitar or bass. We use them in the shop for testing pickups, output jacks, switches, and batteries, and you can also use them for testing cables and wiring harnesses. Best of all, you don’t need to spend a fortune to get a good multimeter. I still use the one I bought at Radio Shack 23 years ago, and such luthier supply companies as Allparts and Stewart-MacDonald have excellent multimeters designed for working on a guitar, priced from $25 to $35.
Let’s explore five ways to use a multimeter:
• Testing pickups (impedance and functionality).
• Mapping out a switch.
• Testing a guitar cable.
• Identifying the lugs on a TRS output jack.
• Checking battery life.
Testing pickups. On many occasions I’ve used a multimeter before buying a used pickup. I learned the importance of this the hard way when I bought a used pickup at a guitar show, only to discover it didn’t work when I got it home. If I’d brought a multimeter with me, I could have tested the pickup on the spot and saved some money.
It’s a simple process to test a pickup. Set the multimeter to the ohm setting and touch its red test lead to the pickup’s primary lead (hot) and touch the black test lead to the pickup’s ground wire.
If you’re testing a humbucker with four conductors, make sure that the wires are properly attached to each other. For example, before you try to measure the impedance on a Seymour Duncan humbucker (Photo 2), make sure that the red and white leads are connected to each other and isolated from any other connection, and the green and bare leads are also connected to each other. Touch the red test lead from the multimeter to the black wire—the pickup’s primary lead. Touch the multimeter’s black test lead to the green and bare ground wires on the pickup. This should give you an accurate impedance measurement.
To make sense of a particular model pickup’s reading, check with the manufacturer for its exact impedance. If the impedance measurement you take is significantly lower than the manufacturer’s rating, then you know there’s something wrong with the pickup.
A multimeter can tell you a lot about a pickup before you wire it up. For example, humbuckers typically measure at around 8k ohms for a vintage-style pickup to as much as 25k for a high-output pickup. The 16.25k reading we see in Photo 2 indicates this is a hot humbucker. Single-coil pickups can range from 6k (vintage) up to 16k (high-output).
Mapping out a switch. If you don’t have a diagram to follow, wiring a new switch can be frustrating. An easy way to save time when tackling a wiring project is to use your multimeter to map out the switch. Simply set your multimeter to the continuity setting, which is marked with a speaker or soundwave icon. This setting will produce an audible tone to let you know when a circuit is closed.
On a 3-way toggle switch, for example, touch either one of the side lugs with one of the test leads from the multimeter, then touch the middle lug—that’s typically the output—with the other test lead (Photo 3). If you hear a tone, then you know the switch is on in that position. If there’s no tone in that position, the switch is off.