Transportation: Automotive, Diesel & Small Engines Resources

To diagnose and repair automotive electrical problems, technicians must be able to test the systems for malfunctions.

Now that we have an understanding of volts, ohms, and amps, how do we test for those attributes during diagnosis?

Generally, the majority of our electrical testing will be done with a digital multimeter (DMM), sometimes also called a digital volt-ohm-meter (DVOM). A DMM is a versatile tool used for measuring voltage, current, resistance, capacitance, frequency, pulse width, and other aspects of electricity.

DVOMs come in many different sizes and capabilities; however, for our purposes we will use the following example which has features and operational characteristics that are typical of today’s meters. Students should familiarize themselves with both the representative meter on the next page, and the one they will actually be using for circuit testing.

We recommend getting your personal DMM or one from the shop to compare it to the simulated meter as you study this section. You should quickly discover that the operation of the meters are very similar regardless of the DMM manufacturer.

A large variety of DMMs are available, but most of them include as least eight common features as shown on the representative example below.

As with most things, meters come with different levels of features, quality, and prices. A technician considering the purchase of a digital meter should keep three things in mind:

• What category meter do I need? This is a major safety issue. The meter should be a category 3 or 4 for today’s vehicles.
• Does it have all the functions I need?
• What is its input impedance?

Impedance is the measure of a meter's input resistance. A higher input impedance will prevent the current flow in the circuit from changing when connecting the meter to a live circuit during testing, and affecting the circuit operation.

Some lower quality meters have a tendency to 'load' the circuit being tested and thus affect not only the operation of the devices itself, but also cause incorrect readings to be displayed. To prevent this 'loading' problem from occurring, always look for a meter with a high input impedance, preferable in the range of 10,000,000 ohms (10MΩ) or higher.

Input impedance becomes even more critical when measuring electronic components, as some low impedance meters can actually cause damage to those circuits by lowering the overall resistance and raising the current flow to a dangerous level during testing.

The saying, a chain is only as strong as its weakest link is true when discussing meter leads. Putting category II leads on a category IV meter creates a category II meter. The meter is only as good as the leads you are using with it.

Before using the meter, inspect the leads for damage. Look closely at any nicks or cuts on the leads. Good quality leads are double insulated. This means there is a layer of white insulation under the red or black outer plastic insulation. If this white is visible, the leads are not safe to use and should be replaced before making any measurements. In fact, any lead with a nick or cut in the outer insulation that is large enough to feel with your fingernail should be replaced instead of used.