When it comes to performing electrical tests on commercial vehicle systems, Purkeys Fleet Electric recommends the following testing guidelines to help ensure accurate readings and a quick diagnosis.
- Avoid using test lights – Do not use 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 are important for electrical diagnostics of any late model 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 the 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 the DVOM or clip-on ammeter annually. Batteries in a low state of charge cause incorrect readings, and leaking batteries can damage the equipment.
Date the new battery with a permanent marker so you know how old it is the next time you check.