Electrical troubleshooting fundamentals key to diagnostics

Unfortunately, some technicians can be as mystified as their customers when strange electrical behavior confronts them in the service bay. Fact is, electricity behaves in very predictable ways.


The world of electricity has a vocabulary all its own.

Voltage: This is electrical pressure, the potential "push" of electrons through a circuit. For example, a voltmeter reading that displays 12 volts actually represents the potential difference in energy between the meter's two connections.

Amperage: This is current flow, which is the flow of electrons through a conductor. Measured in amperes, the flow through any given circuit can be used to check the electrical consumption rate of certain components to see how they stack up against specifications.

Resistance: Simply put, resistance is a circuit's opposition to the flow of current. All conductors resist the flow of current to some degree, some more than others. The unit of measurement for resistance is the ohm, which is the amount of resistance that allows one ampere of current flow when the electrical potential is one volt.

Ohm's Law: A basic rule about the relationship of volts, amps and ohms in electrical circuits. The law applies to all circuits, large or small, simple or complex. When two of the three values are known for a circuit, the third can be determined through some easy math. The conclusion that you arrive at can give answers to seemingly mysterious electrical problems.

Ohm's Law states that voltage (E), is equal to the current (I) times the resistance (R), E = I x R. To put things in more comfortable, everyday terms, we'll use V for voltage, A for amps and O for ohms: V = A x O.

Here's how to put Ohm's Law to work when any one of the three values are missing.
1. Use V = A x O to find the unknown voltage.
2. Use A = V ÷ O to find the unknown amperage.
3. Use O = V ÷ A to find the unknown resistance.

For example, if you want to know what the resistance is of a circuit with a current flow of 3 amps and a push of 12 volts, here's how you would get your answer:

O (?) = V (12) ÷ A (3)

In other words, 12 ÷ 3 = 4 (the resistance). As you can see, Ohm's Law can help provide answers to your questions about a circuit's vital signs. It's a good ally to have in your battle against electrical problems.


Here are the most common types of basic electrical test equipment.

Unpowered test light: This inexpensive tester 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, don't use them because the resulting hole in the insulation can lead to future circuit problems.

Powered test light: Almost the same as the unpowered test light, the 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.

Short finder: This type of tester can be very effective when diagnosing short-to-ground problems, which I'll address a little later. Generally, the tester consists of a low-amperage circuit breaker with leads and alligator clips, and a magnetic field meter. The idea behind the short finder is to connect the circuit breaker to the affected circuit, then trace the problem with the meter. As the circuit breaker opens and closes from excessive current flow, the magnetic field around the good portion of the circuit will increase and decrease. When you get to the location of the short, the meter's needle will stop oscillating.

Multimeter: The multimeter is, by far, your best equipment investment towards electrical troubleshooting. Today's DVOMs (Digital Volt-Ohmmeters) measure AC and DC volts, amps, resistance and much more. Many now have frequency, duty cycle and diode check functions.


Electrical circuits suffer from these common ailments: short-to-ground (before consumer); short-to-ground (after consumer); short-to-voltage; open circuit; and high resistance circuit.

When a short-to-ground occurs before the intended consumer in a circuit, the resistance to current flow is low and current flows excessively until the circuit protection device — be it circuit breaker or fuse — opens the circuit to prevent damage. An example of a short-to-ground before the consumer is a chafed circuit feed wire that touches ground.

Not as common as the other types of shorts we've discussed, the short-to-voltage occurs when a circuit that's normally off receives voltage from another circuit. This sort of condition can occur when wiring insulation inside a harness melts, effectively jumping one circuit over to another.

The open circuit is extremely common. An open circuit occurs when the conductive path between the source and the consumer is disrupted for any reason. Examples of an open circuit include a broken wire on the positive or ground side, a bad switch, or bad connection.

The last type of circuit problem, a high resistance circuit, develops when current flow through the circuit is inhibited. Circuit resistance can result from poor connections, damaged or inadequate cables, faulty switches and solenoids, and the like. High circuit resistance impairs proper current flow and circuit operation. High resistance circuits are best found using voltage drop testing.


1) Confirm the complaint. Never attempt to diagnose a problem without first confirming that the problem actually exists. If you can't duplicate it, you can't fix it.
2) Understand the circuit. A few minutes spent understanding circuit operation can save hours of troubleshooting time. Know what, how and why the circuit operates first.
3) Test the circuit. Use the equipment and procedures recommended by the vehicle manufacturer and never skip steps. Proper troubleshooting takes discipline, but saves time.
4) Make the repair. Once you've concluded what the circuit fault is, fix it the right way. If the repairs require splicing or disassembly of connectors, follow the manufacturer's procedures to achieve a reliable, long-lasting repair.
5) Check your work. After repairs have been completed, make sure you try the circuit in all modes of operation. It's one thing to fix a headlight circuit that doesn't work, but if there are no high-beams, you're not finished.