How not to get bitten by electrical diagnostics

At home, in the car we drive, or on the coach we repair and rely on for our daily income, we all experience electricity.

Electricity is a wonderful thing when everything is working correctly, but when things go wrong, it can be frustrating to repair something that you cannot see work. Diagnosing and repairing the system doesn't have to be as seemingly mysterious as electricity itself, as long as we can grasp a few basic principles about how it works.

For a simple understanding of how electricity works, electricity flowing in a circuit is often likened to the flow of water; and even though we cannot normally see electricity flowing, the concepts are still very similar. Water will flow everywhere unless it is contained and directed. Just as we use pipes and hoses for water, we use insulated wires and cables for electricity. The insulation on the conductor keeps the electrical energy from "leaking" out and being unavailable to drive our load device.


All electrical circuits have things in common, whether they are the high current flow of a starter or the mili-volts of an LED light. Every circuit has:

  • A power source — Usually the battery, or alternator if charging, it supplies the electricity to operate the load device.
  • Conductors — Electrically insulated wiring and cables, or the circuit board for electronics. Directs the electricity to the load device.
  • Load — The electrical device being operated, i.e. a light or motor.
  • Switch — For turning the device on or off. It can be manual or automatic, and may be located on either the power (hot) side or ground of the circuit.
  • Ground — The return path for the electrical energy being used in the circuit.



If we want water to flow farther or faster, we have to increase its pressure. In electricity, we increase the voltage, or electrical pressure, to do more work. This electrical pressure is measured with a volt meter.


The volume of water that flows through pipes is closely related to the current found in the electrical circuit. Just as a larger-diameter pipe will carry a higher volume of water, larger cables will carry more current, or amps. This is measured with an amp meter.


With water, if there is a kink in the hose, the water doesn't flow well as it has to pass through the restriction in its path. In an electrical circuit, this resistance is measured in a unit known as Ohms of resistance. If our usable voltage (pressure) is reduced by the resistance, there may not be enough current (flow) to do the job we need. This is measured with an Ohm meter.

The typical modern meter, often called a DVOM (Digital Volt/Ohm Meter) or Multi-Meter, is capable of measuring all of these factors. While an old-fashioned test-light is acceptable for measuring many basic circuits, these modern meters are critical for measuring the very discrete voltage and resistance levels associated with electronic circuits.


Open: an open or break in the circuit path that prevents current flow
An open circuit is just that — an opening in the electrical path. Like trying to cross an open drawbridge, the electricity cannot complete its journey back to the source, and the circuit is effectively dead. Common switches control a circuit by opening (OFF) and closing (ON) the electrical path.

Short: an unintentional electrical connection
Short-circuit problems come in two types, short to POWER and short to GROUND. The type of complaint associated with a short to power varies with where in the circuit the short is located.

If located before the load, it will generally leave us with a circuit that won't shut off, hence dead batteries and related power drains. These shorts are most commonly caused by pinched wires, but an accidental screw through a wire harness can do the same thing.

If the short to a power source is located after the load, we can be left with a path to power on both sides of the load, instead of a ground. This can be tricky if the ground has been lost, since the load requires a ground to operate and can cause a loss of function.

Shorts to ground occur if a power source is shorted to a circuit and the ground is still intact. It will usually blow a fuse or trip the associated circuit breaker immediately. When multiple circuits are attached to a common fuse or breaker, remove them all, and then reconnect one at a time. Once the breaker trips, the specific circuit with the short is located and can be diagnosed individually.

High resistance: a restriction to full current flow resulting in reduced function
If you have ever looked at a flashlight with nearly dead batteries, you can see the effect of reduced current flow. Resistance in a circuit leads to the same results, even if the source voltage or batteries are fully charged and active. Resistance cuts down on the available power to work the load, be it a light or electric motor. It will not be able to function at its intended capacity.

While your electrical load device will have a nominal amount of resistance, this is necessary. You may note that lower-resistance devices like motors and starters require larger cables to carry the current, while high-resistance devices like lights can use smaller-diameter wiring. Any additional resistance beyond what is intended reduces power, and can increase heat in a circuit, resulting in burned contacts and melted connectors.


The sensitivity of modern electronic circuits requires test equipment that is designed for their delicate circuitry. As mentioned before, the DVOM, or multi-meter, is the most common diagnostic tool for modern electrical circuits as well as earlier designs.

But the volt meter is also capable of showing you "cause and effect" on each of the above conditions, if you know where to look. From Figure A we can see that all of our voltage is available to light the bulb, and it is burning bright.

With resistance in the circuit, some of that voltage is lost, or used up, getting through the poor connection. This drop in voltage, known as VOLTAGE DROP, leaves very little energy to burn the bulb. As shown in Figure B, we have lost over half of the available voltage, leaving only 6¼ volts to drive the lamp — not a very bright situation!

While we might first think that measuring the resistance of a connector should be done with an Ohm meter, a volt meter not only lets us see the effect of the loss, but also lets us see it in a live circuit. To use an Ohm meter requires isolating the component being tested from the circuit itself, an often time-consuming process.

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