When it comes to performing electrical tests on heavy-duty, commercial trucking systems, following the below testing guidelines will help ensure you get accurate readings and a quick diagnosis:

Avoid Using Test Lights
Avoid the use of test lights for performing electrical tests, unless instructed by the vehicle service manual. Most diagnostics procedures require voltage, amperage, frequency, duty cycle or other values to determine the condition of a circuit or component. Test lights cannot provide these values, and in some cases test lights can damage high impedance circuits and components.

Test lights, however, still have value for checking some circuits, such as incandescent light circuits. A high quality DVOM (digital volt ohmmeter), clip-on ammeter, and background knowledge of how to use these meters is important for electrical diagnostics of any late model heavy-duty truck or trailer.

Use a DVOM
A DVOM with an illuminate screen and a mechanism for mounting the meter, such a magnetic strap, can be very useful. This will make reading the meter possible in poorly lit areas, or when it is impossible to hold the meter for viewing.

Test the Meter’s Internal Fuses
Prior to using the in-line ammeter function of the DVOM, first test the internal fuses of the meter to make sure they are not burned open. Testing fuses will prevent incorrect readings. Most meter manufacturers have a process in the owner’s manual on how to quickly test the fuses.

Zero Out Your Ammeter
A clip-on (inductive type ammeter) must be “zeroed” out prior to installing it on the circuit to be measured. Failure to wipe the meter or “zeroing” after it’s installed on the circuit will cause incorrect readings.

Close the Clamp
Be sure the clamp of the clip-on ammeter is completely closed when in use. If the clamp is open, even a slight amount, the ammeter will read lower than the true value.

Replace Batteries Annually
Replace the internal battery in your DVOM or clip-on ammeter annually. Batteries in a low state of charge cause incorrect readings, and leaking batteries can damage your expensive equipment.  Date the new battery with a permanent marker so you know how old it is the next time you check.

Do you want to learn more about electrical system testing? Are you having troubles with faulty testing equipment? Do you have questions about DVOM or clip-on ammeters? We welcome your comments and questions below.

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