Rich’s Repair Ramblings #6: Electrical 102, The Test Light

Last time, we covered battery basics. Picking up from there, we will presume that your battery is charged, and the terminal connections are clean and tight. Santa has granted your biggest wish, a 12V test light! (If you got coal, you can pick one up for $15 or less.) Now you ask, how does it work, and how can you use it to diagnose an electrical problem on your old sled? Test lights are simple and effective, but first we need to understand electrical flow.

For an electrical device to work, electrical current must flow from the battery positive terminal, to the device or “load” (light bulb, dash gauge, motor, whatever), and back to the battery negative cable. The “hot” side of this path is from the positive terminal to the device; the “cold” side is from the device back to the negative terminal. Each device has a dedicated wire on the hot side; but the metal chassis and/or body of the car is used to conduct current (“juice”) on the cold side back to the battery. This is why there isn’t a separate wire for each and every electrical device running back to the negative battery terminal, which would double the size of the wiring harness. You only need to ground the load on the cold side to complete the circuit.

A switch in the circuit allows an intentional interruption, so that the device can be turned on or off. Any unintentional interruption in this flow from positive back to negative, such as a shortcut [“short circuit”] or a break in the path [“open”], will prevent the device from operating. Most circuits include fuses; the fuse acts as a fail-safe in case of a short, so that the fuse “blows” before the device can be harmed.

A test light lets you check for current at any point along the hot side of this path. It’s a go/no-go check: if the test light illuminates, you have current; if it does not, you don’t. For much electrical fault-tracing on our old cars, this is all you need. The test light has a sharp pointed probe on one end; a light bulb inside its clear case; and an alligator clip on a wire at the other end. With the clip attached to any ground point, the test light bulb will illuminate if the probe touches any positive or “hot” 12V source. Let’s see what the test light can do. (The following applies to 12V negative-ground systems only.)

Start at the battery to become familiar with the test light’s operation: attach the clip to the negative battery post; then touch the pointy end to the battery positive post; the test light should illuminate. If it does not: are you sure the battery is charged? Are the clip and the probe actually touching the posts? Are you sure the test light works? Try a different battery if necessary.

Once that test is done, move the alligator clip to a ground point other than the battery negative post. You may ask “how do I know what is ground?” This is a valid question, and it can be a matter of trial and error. In theory, any unpainted metal surface on the engine, body, or chassis should be ground. Try the engine block, an unpainted fender washer, or a bolt along the firewall. In each case, after attaching the clip, touch the probe to battery positive. If it lights up, you have found a good ground. Avoid anything that might be insulated: paint, rubber, and plastic will not conduct electricity well enough for our purposes. So avoid hose clamps, plastic shields, and any painted surface. (Guys with Corvettes and Avantis play by a different set of rules with their fiberglass bodies).

Moving away from the battery, let’s say that a device on your car doesn’t work, and you want to check the fuse. A test light allows you to check the fuse without removing it. This also serves as a preliminary check of the circuit entering and leaving the fusebox. NOTE: you need to know if the ignition key must be “on” for the circuit to be live. I confess that I’ve tested circuits which I thought were dead only to realize that the ignition was off and needed to be on!

With the test light’s clip attached to a good known ground (re-check at the battery positive if you’ve moved the clip), touch the probe to either end of the fuse. (In the photo with the modern blade-type fuse, there are exposed metal points in the top which allow this.) The test light should light at both sides. If it lights on one side and not the other, there is a good chance that the fuse is bad. Try a new fuse. If the test light doesn’t light on either side, it is more likely that there’s an open circuit in the wiring to the fuse. Remove the fuse and touch the probe to the fuse box terminals one at a time. Power at one terminal means that you’re getting juice to that terminal. Lack of power at both terminals means that there’s a break in the circuit between battery positive and the fuse box.

You may need your vehicle’s wiring diagram for the next step. Find a wire which feeds the circuit you’re testing. With the pointy end of the probe, pierce the insulation until the tip is touching copper. BE CAREFUL! That tip is very sharp, and I’ve stabbed myself more than once doing this. For practice, try a working circuit so you get a feel for how far to insert the sharp probe. If the test light lights, you have juice in the wire. If it doesn’t, you’re starting to narrow down the problem.

All photographs copyright © 2023 Richard A. Reina. Photos may not be copied or reproduced without express written permission.