Precise Engine Repair Notebook

How to use a 12 volt Test Light

12 volt test light

The simple 12 volt test light is a very effective tool for troubleshooting many basic electrical problems. It is not a voltmeter, so you mainly are checking for voltage present, not the precise amount. For many troubleshooting applications, this is an important first step and the test light can do it quickly. My test light pictured here is one I bought from Cornwell over 40 years ago and it still works fine. I have had to change the bulb in it several times over the years, but that is not a big deal.

The 12 volt test light can be used for many basic checks and I have listed a few below that we will talk about.

  • Check battery for having voltage, the light should light brightly.
  • Quickly check for voltage along any circuit at various points.
  • Check at the starter post to see if it is getting voltage if you have a no crank.
  • A dimly lit light usually means voltage drop in the circuit.
  • Use the test light to check for a draw or drain from the battery.
  • Use the test light to see if a circuit is hot before you start working on it.

A common use of a 12 volt test light is to connect the clip to a good ground and start poking with the pointed end. Start at the battery positive terminal and work up the cable from there. The test light will quickly show when you have reached the end of the line of voltage availability. This might be at a connection point or fuse. If it's at a connection probe until you find the last place that the light lights. The problem is just beyond there. One thing to keep in mind when probing is that if your probe contacts 12 volts and then the tip of the probe touches ground, you can have a bad short very quickly, so be aware of the probe at all times.

You can use the test light on a starting circuit that appears to have no power by starting at the starter solenoid. See if you have power coming from the battery and then if power is going through the solenoid when you are turning the key switch. Don't just assume the solenoid is bad, check the small terminal and see if it receives power when your trying to crank. If not, work back toward the key switch S terminal. You may find some safety switches in the circuit not letting the voltage through. Keep in mind also that some systems use a ground activated solenoid and these will not light the test light when energized, but the test light will be lit when not cranking.

The beauty of testing with a 12 volt test light is the light bulb has a built in resistance of about 100 milliamps or less. This keeps you from jumping something by mistake and letting the smoke out. At 1/10th of an amp, most circuits can stand this kind of a short if you make a mistake. By using the test light to test for voltage on suspect circuits, you can quickly and safely find the problem spot. If your going to test a circuit by jumping it with a test lead, I always used a fused test lead or one with a small circuit breaker in it. That way if you make a mistake, it blows the fuse instead of letting the smoke out of the ciruit, which is not good.

One of the things I like to do with a test light is check for a drain on the system. If you suspect you have a drain on the system, I recommend taking the ground cable loose at the point it attaches to the chassis of the equipment. The reason I suggest the ground cable, is if your wrench touches ground it will not arch. If you take a cable off at the battery and you have a drain, you will have a spark. This is a very dangerous situation and I have seen several batteries blow up. The gas they vent is highly flammable and it's not a pretty sight when one blows up spewing sulphuric acid all over. So be very careful about sparking around the battery.

I got sidetracked, but I felt that it was important to discuss safety around batteries. Now, assuming you have disconnected the ground cable at the chassis, connect the alligator clip to ground and then put the probe in the cable end. If you have a drain, the light will come on. If the light comes on, but is dim, it means your drain is probably under 100 milliamps or whatever the bulb is rated at. If your light does not come on at all, you either have a very small or no drain. Now assuming the light is lit, just unhook or unplug circuits or fuses until the light goes out. At that point you have narrowed your troubled shooting to that circuit and you can look it over until you find the short keeping the light on. Pretty simple huh.

This should give you some pretty good ideas of what you can do with a good 12 volt test light. It is not a replacement for a good accurate multimeter, but in many cases can find a problem quickly. It also is often easier to use for many people than the more complicated multimeter. In closing I want to say that many problems can be easily solved with the test light and others will need to be investigated using a multimeter to get the right answers - Bruce Perrault

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The Precise Engine Repair notebook is maintained by Bruce Perrault