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How To Test Electrical Circuits

Vehicle electrical systems can be frustrating when they don’t function as expected. To diagnose electrical problems, it helps to know how basic systems actually work. It’s an old joke that electrical systems work by running smoke through the wires, because when the smoke comes out, the system quits working.

Electricity works through the invisible flow of electrons. All matter is composed of tiny particles called atoms. An atom has a nucleus, which has a positive charge, surrounded by electrons, which have a negative charge. Electrons jumping — or flowing — from one atom to another is the basis for electricity. Simply stated, electricity is the flow of electrons in a circuit.

Copper is one of the most common materials used to conduct electricity. What makes copper a good conductor is that its outer shell has only one electron. A lone electron is easily motivated to jump from atom to atom along the conductor — the wire. (See the copper atom diagram.)

There are many ways to encourage electrons to move. Simply waving a magnet over a wire will cause them to start, providing there is a complete circuit. In fact, magnetism is the most common way to generate electricity in a vehicle. That is how the vehicle’s alternator works. Another way vehicles generate electricity is by chemical reaction, which is how the battery works.

With that basic understanding of what electricity is, we need to introduce three characters into the picture: Mr. Volt, Mr. Amp and Mr. Resistance. Mr. Volt provides the force that pushes the electrons through the wire. In a battery, a 12-volt force pushes harder on electrons than a six-volt force.

Mr. Amp measures of the flow of electrons. This measurement is what determines how bright the lights are or how powerful a starter is when cranking the engine.

Mr. Resistance, like the name implies, slows down the flow of electrons, reducing the number of amps flowing in a conductor. A light bulb is a good example of resistance. The restriction caused by the filament causes the electrons to bump into each other causing friction, heat and light. Resistance is measured in ohms.

The relationship between these three — volts, amps and ohms — is always predictable. Doubling voltage without increasing resistance doubles the amperage. If the resistance doubles but the voltage stays constant, current flow is reduced by half.


Now that you can visualize what is happening in a wire, how do you test a system? There are three useful meters for doing that: a voltmeter, an ohmmeter and an ammeter. Fortunately, they are often packed into one instrument called a multimeter. In recent years, multimeters have become very affordable.

The voltmeter can be used in a couple of different ways. First, it can perform an open circuit voltage test. This is useful when looking for a break in a wire. This test, however, will not indicate how much current will flow when the break is found and repaired.

Second, it can test voltage drop. That will show how much voltage has been lost as the current flows through a circuit. A voltage drop test is ideal for checking starting or charging circuits to find dirty and corroded connections. High voltage drops indicate high resistance in the wire. The more resistance in a circuit, the less current flow there will be. (See the voltmeter photos.)

The ohmmeter function is less useful when checking large circuits. An ohmmeter uses a small battery inside the meter to send some current through the circuit being tested. It then gives a resistance reading of how much current made it through the circuit. Unfortunately, most circuits used on farm equipment are much too large for the ohmmeter to give an effective reading. However, you can use an ohmmeter for testing fuses, checking the continuity of block heaters or checking the operation of a fuel-level sending unit. The ohmmeter reads in ohms. The lower the number of ohms, the more current flow there will be in a circuit.

The ammeter function is used when checking current flow to a load or from the alternator. Most multimeters have ammeter functions, but they are limited to a maximum flow of 10 amps. This is enough to check the flow to a couple of lights or to the clutch on your AC compressor, but it is not enough to check the alternator output. To check starter draw or alternator output you will need an inductive pickup/amp clamp. Unfortunately, these tools can be quite expensive.

Specific instructions on testing starting and charging systems will be discussed in the next instalment.

Dietrich Schellenberg, Marty Zuzens, Peter Lung and Dennis White are instructors at Assiniboine Community College’s School of Trades and Technology in Brandon, Man. If you’re interested in more information about the training programs offered there, check out

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