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.
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.
What happens when a technician goes to check for wiring problems in a trailer lighting system? Typically, he pulls the bulb out and checks for voltage. Often, he gets nearly battery voltage, but when he reinstalls the bulb, it produces a dim light – or none at all. High resistance in the wiring is the cause, Purkey says, and that’s why such tests always must be done under normal load: The tech needs to fire up the bulb and find a way to measure voltage at the socket while normal current is being drawn through.
Purkey recommends referring to Technology and Maintenance Council Recommended Practice 129, which covers electrical testing. The RP shows how to use a carbon pile to put a fixed load on a cable so one can be sure it can handle the load. Train technicians on how to make these tests, get the right test equipment, and establish rigid procedures for doing the tests right, Purkey says: “Don’t let them test any way they want.”
Another mistake fleets make is using dielectric grease improperly to seal electrical connections. The grease offers high resistance, hence the name. “Under the right conditions, these greases work fine,” Purkey says. “But if you use them, make sure to clean the connectors, and make sure the connections themselves are tight. Don’t allow the grease to get in between the parts that actually carry the current.”
Purkey also advises maintenance managers to remember the other side of the circuit. Many times, all the cabling will be good, but the circuit still won’t do the job. There might be good cables, and good connections at the battery and starter solenoid, as well as good connections on the battery ground and frame connection – and still be a problem. Voltage at the battery and starter might be OK, even under load.
Just having a high-resistance connection between an engine ground strap and the frame is enough to cause trouble, Purkey says: Clean and tighten such connections as necessary, and test a questionable ground strap for resistance under load – or just replace it.
Remember when upgrading a major load like the starter, or the alternator, to think about cabling: There is little point in providing more current – or the ability to use more – unless the wiring is upgraded in proportion to the greater flow.
Making the right connection
When replacing batteries, or considering upgrading a set of batteries to increase amp-hour capacity to power the truck during shutdown, remember the difference between series and parallel wiring, says Kalyan Jana, technical product manager at Enersys, maker of Odyssey batteries.
Consider two batteries for purposes of illustration. A series connection would mean taking the positive connection from battery A, and the negative from battery B. There also would be a connection between the negative terminal of A to the positive of B. Such an arrangement will double the voltage and power as compared with a single battery, but provide the same amp-hour rating.
A parallel connection, on the other hand, would mean running a single positive cable to both positive posts, and a single negative cable to both negative posts. This would provide the same voltage, but twice the power and twice the amp hours. Adding one or more batteries in parallel is a great way to increase amp-hour capacity and keep the batteries from running down overnight, Jana says.
A series connection means that the current is forced to go through both batteries, Jana says. “It’s one way to make sure both batteries share the load equally.” However, when using a series connection, it’s necessary to make sure the output of the batteries adds up to the proper voltage, Jana says.
For example, two 12-volt batteries often are wired in series to operate a 24-volt starter. The amp-hour capacity then could be doubled by using a series-parallel connection, where two 12-volt batteries connected in series could be hooked up in parallel with two additional 12-volt batteries connected in series.
This underscores the need, when running the commonly used setup of four batteries wired in parallel, to wire things properly, Jana says. Make sure the batteries are mounted close together so the cable runs are short, and that the cables are more than adequate to handle the load; this will help ensure every battery carries its share of the load.
Jana also warned about the lead connections frequently used on batteries. “Lead is subject to cold creep,” he says. “You need to retighten them on a regular basis. You will lose current-carrying capacity if you don’t tighten them frequently enough.” Odyssey batteries have brass- or tin-plated brass terminals that, once properly torqued, continue to carry current at full capacity, Jana says.
So by purchasing batteries with the right type of connector, fleet managers can save themselves one maintenance headache – and avoid some no-starts, too.