Make electrical system service less challenging

Always buy and use the right tools and equipment


To help pinpoint the location of a short-to-ground, use an unpowered test light in conjunction with the circuit breaker tool. Connect the circuit breaker as usual and probe the various connectors and circuit portions with the test light. If the light flashes on and off at that point, the circuit is OK up to that point. If the light doesn’t flash on and off, methodically probe your way back in the circuit until the light flashes. The short-to-ground problem lies between your present test location and the previous test point.

Q: Sometimes a shop manual specifies a non-powered test light and other times a powered test light. What’s the difference and where does each one apply?

A: An unpowered test light contains a bulb mounted in a clear handle, connected to a probe on one end and to a ground lead on the other. Although there are unpowered test lights available that pierce a wire for testing convenience, you should think twice about poking a hole in the insulation, which can lead to future circuit problems. The unpowered test light is used most often for checking the presence of voltage and ground in electrical circuits, but carmakers may specify this type of tester for checks in electronic systems such as engine controls.

A powered test light is very similar to the unpowered test light, except that a powered test light contains a low-voltage bulb mounted in a clear handle with a battery. The handle also has a probe on one end and a lead and alligator clip on the other.

The powered test light can be used to check specific portions of a circuit that are unpowered, including bench tests of some parts. When a circuit portion or component is continuous and therefore has the ability to conduct current, it has the property known as continuity.

Q: Two technicians in our shop have an ongoing debate about the best way to find high resistance in a circuit. One of the techs claims that using an ohmmeter is the best way to go. The other tech claims a voltage drop test is the best method. Which is right and what’s the best tool to perform that test?

A: Although it’s true that an ohmmeter measures electrical resistance, the ohmmeter is really better for checking the electrical resistance of components than for circuits. This is due to the small amount of current the ohmmeter applies when performing a resistance test in a non-live circuit. It just simply isn’t enough to reveal problems in most circuits. Voltage drop testing, on the other hand, provides a more reliable method of determining circuit resistance. You take voltage drop readings in a live circuit and because of this, this type of test is more likely to show problems in a circuit. Higher voltage readings indicate higher amounts of circuit resistance than lower voltage readings. In either case, an ohmmeter and voltmeter are functions of the same tool: the digital multimeter.

The Society of Automotive Engineers publishes voltage drop specs for numerous electrical parts in its handbook. SAE compiles these specs for an engineering environment and not the repair world, but even here the differences are clear. For example, a battery cable is allowed to have up to 0.2 volt drop with 100 amps of current flowing through it. A 16-gauge wire, on the other hand, with 15 amps of current flowing through it, is only allowed a voltage drop of 8.0 millivolts. Keep this in mind when you check voltage drop on low-current circuits such as those for the onboard computer.

Q: Awhile back, I read something about 42-volt systems as being the “next big thing” in vehicle electrical systems. When are these systems scheduled for introduction and what do I need to service them?

A: For the time being, 42-volt systems are on hold. Initially it appeared as though the automakers were on a crusade to upgrade their electrical systems to this new infrastructure. The downturn in the auto industry sent economic shockwaves throughout the companies, causing them to shelve 42-volt systems for now. You can count on PTEN to bring you any new developments in this area when they happen.

While there’s an almost unlimited array of gadgets for testing electrical circuits, make sure your choice provides the widest range of testing applications. As you consider various tools, carefully research your options so you get the most mileage from your investment. That way, you’ll be able assured of the widest range of coverage while getting the best value from your electrical troubleshooting budget.

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