Using a Test Light for Electrical Problems

A test light is a simple device that can tell you if power or ground is present at a particular circuit or fuse. The test light consists of a sharp probe, indicator light bulb and a wire lead with a clip. This tool is easy to use and is reasonably priced on Amazon (about $20.00 US) and pertains to all 12 volt systems. EV's use a 12 volt system to run everything but the main drive motor, and most have a conventional 12 volt battery including Tesla.

test light

Lest Jump In!

Before you begin testing it is good practice to make sure the test light works. This is easily done by connecting the test light lead to the negative side of the battery and then touching the point of the probe to the positive side battery terminal, the test light should light up.
test light testing

A popular use of a test light is to check for power and the condition of the fuses in the fuse panel or power distribution center (PDC), this should be the first check when a particular accessory such as door locks, lighting or power windows are not working. If you need the locations of a fuse or electrical item such as a fuel pump relay you can ask our experts to supply the information (free), or consult an online resource such as AllData DIY (pay). Once the test light is properly grounded (negative side of the battery or any solid metal part), touch each side of the fuse, both sides should light up which is telling you both the fuse is good, and there is power present in the fuse panel. Note: Any solid metal part of the vehicle such as the frame, engine, radiator core support is grounded naturally as part of the electrical systems design.
test light power check

If the fuse is good the next step is to see if a particular accessory is getting power such as a power window, with the test light grounded check for power at the switch, and then the window motor. If there is power at the switch but none at the motor with the switch activated, you know the window switch is bad, if there is power at the motor the motor is bad, assuming the grounds are good, which we will show you in the next steps.
window switch test light

Watch the Video!

Please watch this video of the job being done, then continue down the guide to glean additional helpful information.

Here a mechanic is testing the power feed to the fuel pump from under the vehicle, with the test light grounded the sharp point is pushed through the wire insulation to contact the internal wire. This is done by locating the wiring harness headed to the fuel tank and while using a wiring diagram provided by our expert to confirm the color of the wire, in this case blue, if power is present as in this case the fuel pump motor is bad.
fuel pump power test light

When a bulb is dim such as a headlight and brake light it usually means the power side is okay and there is a bad ground. To confirm this, connect the test light lead to power, which then changes the test to find ground. Here a mechanic is testing the light bulb ground which is bad by showing dimly lit bulbs for both the brake light and test light. Follow the ground wire to the body and repair the connection.
bad ground test light

When power tests okay at the fuse yet power is not present at a particular accessory such as the cooling fan, a test light can trace the problem down through the wiring harness to find a melted or bad connection.
burned wiring

This electrical connector tested good while probing the wires before the connector, it wasn't until the technician removed the connector was the problem discovered.
rusted electrical connector

By probing the wires at a fuel injector you can see if the PCM is producing a signal to open the injector.
probe injector wire


Our certified technicians are ready to answer electrical questions for free. We hope you saved money and learned from this guide. We are creating a full set of car repair guides. Please subscribe to our 2CarPros YouTube channel and check back often for new videos which are uploaded regularly.

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