Despite its longstanding and near universal use in motor vehicles, DC electricity is a maintenance issue with an image problem — especially among managers of school bus fleets. According to industry reports, electrical glitches account for a substantial amount of bus shop work, yet most mechanics say they feel unconfident when troubleshooting such problems. This is a predicament often filled with frustration and added expense, the result of improper diagnosis and unnecessarily replaced parts. Short of a glitzy public relations campaign, the best remedy for the situation is good, basic training.
Start with the basics
“Electricity is one of those areas where you really need to understand the fundamentals,” says Carl Malecha, a bus mechanic and technical school instructor in Faribault, Minn. “People can always do the easy jobs — swap parts, for example — but when that doesn’t help they have to start diagnosing the trouble. They need to read and understand wiring diagrams, and know how current flows. These things can be confusing.” Malecha says a wiring diagram, or schematic, should be the starting point for every electrical task beyond a bulb change. He compares its importance to that of a roadmap on a cross-country trip. All modern buses come with diagrams, usually mounted inside or near the fuse panels, but older models occasionally lack them. Charles Bishop, an engineering manager for Thomas Built Buses, says it is possible to retrieve some documentation from factory records if the units aren’t too old. He says his company’s customer support department regularly handles those kinds of requests. Diagrams are nothing more than fancy line drawings of electrical systems, showing sources of power, ground locations and shared circuits. Although essential for locating trouble, they can also be a little overwhelming to unskilled technicians. “They’re full of information,” Malecha says, “and they’re coded with symbols. Experience is a big factor in successfully reading them.” A legend is equally important. In theory, DC electricity is a reasonably simple concept. The conventional explanation is that electrons move from the battery’s positive post through the system and back to the negative post, energizing every activated, grounded device along the way. This means there can be only three causes of any electrical failure: lack of power (current), lack of ground or defective devices. Determining which of these conditions exists isn’t too difficult, but pinpointing the precise trouble spots can be a challenge.
Grounds for confusion
Insufficient grounds are probably the hardest things to find, says Larry Kramer, maintenance director at Rochester School Bus in Rochester, Minn. “They’ll get lights doing all sorts of goofy stuff,” he says. “Current can ‘back feed’ to different circuits, making clearance lights blink in time with the signals or eight-way systems.” Kramer says the worst cases can take days to diagnose and fix, but most are finished in less than an hour. “The first thing we do is check a bus’ service records to see what’s been done to it lately,” he says. “A lot of times there will be a direct link between past repairs and a pres-ent problem: Maybe a screw got too close to a wire or a connection wasn’t solidly crimped.” The most common failures Kramer sees are bulbs and switches. “We also get into a lot of relays,” he says. There are 15 to 20 on a typical bus. “They’re either good or bad. We’ll check how much power is going in and how much is coming through. Sometimes it’s easier to just plug in a different one and see if it works.” Carl Malecha agrees with Kramer’s approach. “Always try the simplest things first,” he says, “and you don’t even need a new part.” Light bulbs, for instance, can be borrowed from another lamp, relays from another circuit. Just make sure the pieces used for testing are similar to the inoperative ones. If the problem persists, you’ll have to dig into the system — unless, of course, you spot loose connections or massive corrosion on the mounting fixture or nearby wiring. Malecha suggests starting any in-depth investigation “at a point where you’re sure to have power.” This normally means the fuse panel. First, make sure there’s adequate current (voltage) going to the fuse controlling the circuit being tested. Next, check the fuse itself. Then isolate the circuit and follow it toward the failed device(s), testing for current at each connecting point along the way. You’ll probably need to unplug a few wires, so a diagram and numbered or color-coded wiring harness are essential. Electrical failures often provide helpful clues to guide troubleshooting efforts. Some of the more common problems and their hallmarks include:
SHORT CIRCUIT: Current from two (or more) hot wires coming together. Devices cease operating. Sparks, tripped circuit breakers, burned fuses and, sometimes, melted wiring result.
HOT TO HOT: Current from two or more hot wires coming together. Devices cease operating.
SHORT CIRCUIT: Current going to ground before reaching a device (or “load”). Devices cease operating. Sparks, tripped circuit breakers, burned fuses and, sometimes, melted wiring result.
GROUNDED OUT: Current going to ground after a device but before a switch. Devices operate constantly. Circuit breakers and fuses stay intact.
OPEN CIRCUIT: A break, such as a severed wire, that prevents current flow. Devices cease operating. Circuit breakers and fuses stay intact.
HIGH RESISTANCE: Commonly resulting from corrosion in a wire or connection, unwanted resistance cuts the amount of current flowing. Device performance drops (dim lights, for example).
LOOSE CONNECTION: Another producer of high resistance either inside a butt connector or at a terminal post. Devices operate intermittently (flickering lights, for example). Arcing and sparking might occur. POOR GROUND: A disruption of current flow after a load. Devices operate strangely (signaling clearance lights, for example).
EXCESSIVE AMPERAGE: The result of too many devices on a circuit or a problem within one of the devices causing it to draw extra amps (a binding heater fan motor, for example). Fuses fail repeatedly. Diagnosing any of these ailments can be done with a digital multimeter, a handy tool (priced between $100 and $500) that allows technicians to check volts, ohms or amps. Malecha says he still prefers standard test lights for non-computerized equipment, partly because they’re much cheaper and easier to carry around, and partly because he’s been using them for a long time. “It’s really just a matter of what you’re comfortable with,” he says. Some people still make the mistake of probing insulated wires with test equipment. Malecha says that’s a good way to doom a wire to corrosion and failure. Wires that are accidentally opened up should be quickly sealed with silicone. Newly installed connectors need similar protection from moisture. Heat-shrink tubing and dielectric grease are the preferred products for those jobs. Never rely on tape.
Electrical work isn’t for everybody. It is a challenge requiring more brains than brawn. Technicians must think like detectives, constantly using logic to zero in on defects. It’s a process of elimination: testing or disconnecting devices or circuits until a nagging problem either stops or starts. Even experts get stumped at times. “Certain problems simply can’t be explained,” says Malecha. “You might know roughly what and where the trouble is, but can’t quite get it straightened out. When I reach that point, I’m not concerned about the cause anymore. I just want to finish the job and move on. If that means stringing a new wire to bypass the spot, so be it.” In doing this, though, it is important to leave a short length of the original wire attached to both ends of the new one so other technicians won’t be confused later when the unit’s wiring doesn’t quite match its diagram.
Don’t forget safety
The most important thing to remember with electricity is your own personal safety. Electrons move at 186,000 miles per second, leaving no opportunity to protect your eyes or hands if a metal tool accidentally brushes against an unshielded power source. At the very least, sparks will fly and equipment could get damaged. Although DC current isn’t as dangerous as AC, it is still to be respected. Never wear a ring when working around live current. And always disconnect the battery cables before replacing components. If you’re unsure of any aspect of a job, get help from others more familiar with electrical systems. Your pride might droop a little, but you’ll likely avoid some potentially harmful and costly mistakes.
Coming to terms with the terms
VOLTAGE, also known as electromotive force, is the pressure that pushes electrons, or current, through a circuit. It’s often compared to water pressure in a garden hose.
AMPS (technically amperes) are a measure of current flow or volume. One amp equals 6.28 billion electrons passing a point in a wire in one second.
OHMS are units of resistance in a circuit. Resistance cuts current flow. Electrical devices all have a certain amount of resistance. Unintended resistance results from corrosion, poor connections and inadequate wire size.
WATTS are units of power. Knowing the wattage of a device or circuit is important when choosing the correct fuse. Wattage equals amps times volts (i.e. 5a x 12v = 60w). Or in reverse, amps equal wattage divided by volts (60w / 12v = 5a).
CONTINUITY is essentially a working circuit. In other words, a circuit is said to have continuity if it allows electrons to make a complete round from the battery to load(s) and back to the battery.
LOAD is any device (light, fan motor, solenoid, etc.) that uses electricity to do its job.
Failure analysis in four steps
1. Verify complaints. Confirm all reportedly faulty devices by trying to operate them yourself.
2. Check related systems: clearance/signal/brake lights, for example. This will help quickly narrow your search.
3. Check easy things: bulbs and sockets, for example.
4. Start diagnosing. Grab a test light or multimeter and refer to the wiring diagram.
Paul Hartley is a freelance writer in Northfield, Minn.