How to Use a Circuit Tester

How to Use a Circuit Tester

Automotive electrical problems are never fun to diagnose. But getting down to the fundamentals can be a good place to start. Finding out if a circuit is even receiving power can help rule out other problems. A circuit tester is a handy tool that can indicate if a circuit is being powered. Here’s how to use a circuit tester and how to find the right one for your needs.

A circuit tester is nothing more than a portable electrical circuit designed to be easily connected temporarily to a power source and a ground. It helps you see (or hear) the presence of electricity in a wire. There are several kinds of circuit testers for different kinds of diagnostic needs. The most basic and common circuit tester type looks like a screwdriver with a sharp pointed tip. Inside the handle is a light source (bulb or LED). At the end of the handle will be a cord with a clamp of some type to allow for temporary connection to the battery. A simple tool but well worth having when the situation calls.

A simple circuit tester is easy to use. Familiarize yourself with the tool by doing a simple test of the battery. Connect the circuit tester ground clamp to the negative terminal of the vehicle battery. Now touch the probe tip of the circuit tester to the positive battery terminal. The light in the circuit tester handle should illuminate. When electricity is flowing through a circuit that you are testing, this is what you will see. Now that you know what to expect let’s look at a real-world example.

Trailer wiring gets a bad reputation, but making sure you have power at the 4-pin trailer connector is a good starting point for diagnosis.

Start by connecting the circuit tester ground clamp to the vehicle frame or chassis. The battery is grounded to the chassis, so the function is the same.

Turn on the vehicle and switch on the turn signal (left or right) to supply power to the trailer wiring plug. The turn signals on the vehicle itself should be blinking for this test.

Insert the circuit tester probe tip into each of the trailer wiring plug connectors one at a time, watching to see if the circuit tester light illuminates at the same time as the turn signal lights on the vehicle. The circuit tester light should pulse with the vehicle turn signal lights when you find the right trailer wiring plug connector. If the circuit tester does not light up at any point, try changing the ground clamp connection to a different chassis location, or even the exhaust pipe then probe the connections again.

Once you have verified that power is reaching the left or right turn signal connector of the trailer plug, you can move on to other circuits. If you don’t find power, make a note of it and move on to the next step. Turn on the vehicle headlights to test the tail light/side marker circuit. Finally have a friend press the brake pedal to test the stop light circuit.

For a 4-pin trailer connector there should be one ground wire and three power wires. You should find power at all three power wire connectors during testing. If you find power at one wire and not another, then you will need to do a deeper dive into the vehicle’s trailer wiring harness.

There are many types of circuit testers from simple to complex. Here’s a few you might run across in the automotive world.

Standard Circuit Tester – This is about as basic as it gets. A probe, a light, and a cord. Nothing fancy and pretty much unchanged for decades. They don’t cost much and are a no-brainer for adding to your toolbox.

Multi-Function Circuit Tester – This type of circuit tester can test for more than just flowing current. It can indicate whether a connection is hot or if it is grounded. Typically it needs to be plugged into a power source of its own or connected to the battery. It is a step up from the basic circuit tester and actually uses a bit of electronic circuitry to work its magic.

Circuit Tester Pliers – Sometimes it is just difficult to get a circuit tester probe into a tight space. For cramped work spaces you can use circuit tester pliers. These special pliers have a sharp pin in the jaws that can gently pierce the outside insulation of a wire for testing. There is also a normal sharpened probe tip that flips up normal circuit testing use.

Digital Circuit Tester – Now you are getting into the high-tech realm of circuit testers. A digital circuit tester can test components, read out voltage levels, check continuity, check grounds, test relays, and more. Some even step up to complete kits with circuit tracing capability. While not cheap, if you spend any amount of time chasing electrical gremlins a digital circuit tester may be just the ticket.

Trailer Circuit Tester – We’re throwing this one in just so you know about it since we talked about trailer wiring earlier. A trailer wire connector circuit tester simply plugs into the vehicle trailer plug and lights up to show which circuits are working. It is a specialty tool, but it does its one job very well.

Now that you have an idea about what a circuit tester is and the different options available to you, your next electrical repair should be a bit less daunting.