Auto mechanic testing an electrical circuit using a test light

How to Use An Automotive Test Light

An electrical system in the modern-day car has many components and is complex in general, so tracking down issues within it can be a hard task. Still, you can check most components with a simple test light. This is a tool used for performing electrical power and grounding testing before performing automotive repairs. It is easy to use, reasonably priced and available in most parts or hardware stores.

An electrical system in any car consists of similar components. There is a battery that is a source of electricity while the engine is off, and an alternator that does the same with the engine running. It all starts at the battery positive terminal. Electrical power flows to different components through auxiliary circuits, which have fuses as a starting point. They protect the corresponding electrical circuits in case there is a short-circuit. Next, there are switches or control units that govern the electrical flow to the appliance, which can be a light-bulb, electrical motor, ignition coil or something else. In the end, the appliance connects to the battery’s negative terminal, either by a chassis or by a direct line. A bundle of wires, called wiring harness, connects all these components.

Usual causes of electrical systems related issues are either a short or open circuit. A short circuit happens when a wire has rubbed through to ground causing a fuse to blow. On the other hand, if a connection stops working because of a broken wire or a bad conductivity, there will be an open circuit.

What Kind of Test Lights Are There?

Auto mechanic testing an electrical circuit using a light bulb test light

Although available in many shapes and sizes, we can divide all test lights into two main categories.

A simple test light has a light bulb within a casing, with one end connected to a metal probe. The other side connects to a wire that has a clip or a clap on the end. The most common way to use a test light is to connect the clip end to the negative end of the battery and start testing connections with the probing side of the test light. If there is a connection to the positive side of the battery, the electric circuit will be complete and the light bulb will light up. Another way to use a test light is to connect the clip end to the positive end of the battery and use the probe end to search for connections that may be connected to the negative end of the battery. If the spot is grounded, the test light will light up.

Newer test lights use LED instead of a light bulb, and there is a good reason for this. The older test lights that use a lightbulb will draw more current than the LEDs. In modern cars, probing a wire connected to a computer might potentially draw too much current. This can damage the circuits inside the computer. Therefore, there is less chance of damaging sensitive electronics when using the test light with LEDs. Besides, some of them even have two separate LEDs. This is a nice feature, as each of them will illuminate depending on whether you are probing a power or a ground connection.

How Do You Check A Fuse With A Test Light?

Auto mechanic removing a fuse

One of the most common applications for a test light is to check for a blown fuse. This will save both time and effort, as there is no need to pull out the fuse. The procedure is simple – locate the fuse box, connect the clip end to a piece of metal nearby and use the probing slide to see if a fuse is good. An illuminated light shows that the fuse is good, while a blown one will not light up the probe.

How Do You Diagnose Electrical Problems In A Car?

Testing a wiring using a test light

The most important aspect of solving electrical issues is a methodical approach. Before starting any tests, you must check the light itself to make sure it works. The quickest and easiest way to do this is to connect the clip the battery negative terminal, and then gently touch the test light probe to the battery positive terminal.

The first step in the process is to test the fuses, as already described in the previous section. This will not only reveal if the fuse is in good condition, but it will also confirm power at the fuse if an accessory has failed. With a positive fuse test, the checking procedure can go further down the circuit, with switches or control units being the next stop.

While performing these tests, it is useful to have a wiring schematic. Sometimes a test will show the presence of electrical power at the source, such as a relay or fuse. And yet power is not present at a particular accessory such as the cooling fan. In a situation like this, a test light can trace down the problem through a process of elimination. You can hunt down many electrical issues as long as you have a wiring diagram and a test light.

Another use of the test light is to check if the alternator is producing electricity. To charge the battery and provide enough electricity for all appliances, power must be present at the large terminal of the alternator. You can use the test light to check this.

You can use a test light to check if a particular fuel injector works. To do this, first, place the clamp to the negative battery terminal. Then use the probing end to check the trigger side of the injector circuit.

The test light is a useful tool when checking for a ground connection. A bad ground can cause various electrical and running issues, so it is important to track these problems down. Place the clip ends on the positive end of the battery and use a probing end to check for a ground in the suspected area.

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