Train technicians on how to conduct proper electrical tests.
Wiring is rarely a dream to decipher, but it’s often more like a nightmare once a truck has run long enough for vibration and road salt to start getting through. But take comfort: There’s an old saying that “You probably learned most of what you need to know in high school,” and that’s true for many of the most common wiring problems.
Of course, it also helps to understand the implications of Ohm’s law, which states that in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them.
Testing, testing
Bruce Purkey, president of Purkey’s Fleet Electric, says fleet managers should remember those rules about current flow, voltage, resistance and power, and keep their technicians aware of what they mean when actually testing a circuit for trouble.
“Most fleet managers do themselves a disservice when they put ‘Inspect battery cables’ on a check sheet,” Purkey says. “You can’t look at a battery cable and tell whether it’s good or not. If it looks good, the technician just checks it off.” Even with insulation that looks perfect, the cable underneath might be broken, he says.
“What if all the strands in the cable but one are broken?” Purkey asks. If a technician puts a voltmeter or continuity checker on the connection at the starter with the ignition switch off, he’ll still get full battery voltage. The resistance won’t show up until the starter starts to draw current.
“The only way to check it is under load,” Purkey says. One then can calculate resistance, or measure voltage drop, to see if they meet specification. Remember, too, that a starter at 0 degrees Fahrenheit might pull 2,000 amps, several times what it’ll draw at 70 degrees; those extra amps will drop the voltage, even if the wiring is good.
