Test Light
A simple electrical diagnostic tool with a bulb or LED used to check for the presence of voltage in a circuit.
A test light (or circuit tester) is a simple, hand-held diagnostic tool used to quickly check for the presence of electrical voltage or ground in automotive wiring, fuses, switches, and relays. It consists of a sharp metal probe housed in a plastic handle containing a light bulb or LED, connected to a long wire with an alligator clip.
To use a test light, the alligator clip is attached to a known good ground (such as the vehicle frame or battery negative post), and the probe is touched to various points in the circuit. If voltage is present, the bulb inside the handle illuminates. This allows a technician to quickly verify if fuses are blown or if power is reaching a component like a fuel pump or starter solenoid.
While traditional test lights with incandescent bulbs are useful for heavy-duty circuits, they can draw enough current to damage sensitive computer circuits (ECUs) if probed directly. For modern electronic systems, technicians use digital test lights or LED test lights that have very high internal resistance, protecting computer circuits from damage.