Want to quickly see if you’re getting power to your starter solenoid? A test light will show you. (Image/OnAllCylinders)

Whether it’s for a holiday or birthday, this Tools Under 30 Dollars Gift Guide Series shows you important, yet somewhat uncommon, tools that any gearhead would want.

Today, let’s talk about automotive electrical test light circuit checkers.


We’ve covered multimeters earlier in this gift guide series, and for a darn good reason: multimeters are invaluable electrical diagnostic tools, and handy for a wide range of home and auto electrical troubleshooting.

But on some electrical jobs, you don’t need a tool that fancy.

In fact, sometimes a simple test light circuit checker is all you need to track down and defeat electrical gremlins.

There’s a small light bulb in the wand and an alligator clip coming out of the other side. Super simple. (Image/OnAllCylinders)

Think of a test light (sometimes called a test probe) as a gussied-up light bulb. The bulb is housed in a wand with a metallic probe, and there’s a wire coming out of the other end with an alligator clip attached. You simply attach the alligator clip to one side of the circuit (-), commonly chassis ground on a vehicle, and use the metallic probe to connect to the (+) side.

With the circuit you’re checking energized, the bulb in the wand will light up when you place the metallic probe on the (+) side to complete the circuit (like a light bulb socket or fuse clip).

When it lights up, you know you’re getting power and there’s continuity in the circuit—in other words, you don’t have any broken wires or bad connections up or downstream of your test location.

We’re troubleshooting a vehicle’s parking light here, with the probe’s alligator clip (circled in green) connected to the metal body/frame for the (-) side of the circuit, and the probe lead placed in the bulb socket (circled in red). Since the probe lights up, it tells us the problem is likely a bad bulb. (Image/OnAllCylinders)

While this tool won’t tell you much in the way of precise electrical voltage or current, it’ll quickly tell you if the circuit itself is intact.

And you can infer some things from the intensity of the probe’s light too.

For instance, if it’s unusually dim, then you may have a break, corrosion, or poor connections somewhere in the circuit. Or, if you notice the light flicker as you move the probe or surrounding wires, it can indicate a local break in the wire or dirty contacts on a connector.

Here, we’re using a test light to trace a circuit in an old electromechanical pinball machine. But WE’RE EXTREMELY CAREFUL to only work around the low voltage (6 volt) areas of the game. Other parts of this machine feature live 110 volt circuits. (Image/OnAllCylinders)

And a test light has tons of other uses outside the garage too. BUT BE CAREFUL! A circuit checker like this should only be used on low-voltage circuits (like, 12 volts or less). This is critically important, because you’re working with live voltage here.

Up until now, we’ve been talking about direct current (DC) applications, but many test lights will work with alternating current (AC) as well, as evidenced by this track test of a model train layout that’s running around 12 volts AC. Note the test light’s alligator clip connected to the other side of the track to complete the circuit. (Image/OnAllCylinders)


But the best part of a test light is that the basic ones are pretty darn cheap, so you’ll have no problems finding a quality circuit checker for under 30 bucks. And considering how easy they are to use, and how handy it is to have a spare one in a vehicle’s toolbox, automotive circuit checkers are great gifts to give and get.

Author: Paul Sakalas

Paul is the editor of OnAllCylinders. When he's not writing, you'll probably find him fixing oil leaks in a Jeep CJ-5 or roof leaks in an old Corvette ragtop. Thanks to a penchant for vintage Honda motorcycles, he spends the rest of his time fiddling with carburetors and cleaning chain lube off his left pant leg.