Auto Service World
Feature   January 1, 2006   by Jim Anderton

Current Affairs

Alternators spin out current into ever-higher electrical loads. Here's how.

One of the best, or worst parts about working with electricity is that it doesn’t know where it is. Electrons will flow just about anywhere, from a wiring harness, to the air, or your body if you’re not careful. And while most of us are trained to think in terms of direct current as a consistent flat voltage value as seen by a multimeter, in most applications, “pure” DC is a rarity. In household and commercial power applications, it’s an alternating current world, namely because the cyclical polarity shift of AC current minimizes power loss in long transmission lines. 60 Hertz AC power is the North American standard, but for industrial users, three overlapping AC since waves are used to create “three-phase” power. Why three phase? Cheaper and more efficient motors, wiring and generation make three phase systems the power of choice for industry. In automotive uses, however, the short current transmission distances and relatively low power requirements make DC perfectly adequate. Adequate, except for low speed operation, like idling, where DC generators produce little current. Dimming lights and periodic battery charging was common before alternators replaced them starting in the early ‘Sixties.

That change was traumatic for the repair aftermarket. Alternators were called “sealed units” and “non-rebuildable”. They were also rumored to last the life of the vehicle, and while neither prediction turned out to be true, they have become a “re-and-re” item as quality ‘reman’ and new units became cost competitive with bench repair. That’s a good thing for the industry, but what’s been lost is a deeper understating of what specific malfunctions can do to other electrical system circuits, long before the battery dies.

The diagram at right shows a typical alternator internal winding arrangements, in a “wye’ configuration. If you’ve seen wiring diagrams for three-phase industrial motors, you may notice that they look similar. The details aren’t important, but the key is that each of the three windings will generate its own AC current, a little like three generators in one package. Why not generate DC directly? The picture showing the rotating elements at left shows one reason why. Note the numerous windings separated by the striped commutator at the top of the starter motor armature. DC generators use an identical technology, with carbon brushes making and breaking contact across the commutator segments. Wear, arcing, carbon tracking or contamination would all affect output of a DC generator. In contrast, the alternator rotor at right uses a simpler six-pole winding set that is connected electrically by slip rings instead of a commutator. Slip rings allow continuous contact, reducing the wear and arcing issues that kill both DC generators and starter motors.

The other advantage is in the output. Automotive electrical systems require clean DC, meaning current flat-lined at a nominal 12-13.8 volts with minimal ripple. Alternators by definition produce alternating waveforms that switch polarity at a frequency determined by the speed of the alternator’s driven pulley, the opposite of what’s needed. Three phases provide part of the answer. By superimposing three sine waves, 120 degrees out of phase with each other, the voltage levels average out to a relatively low ripple current, although it still switches polarity. The on-board rectifier pack takes care of the polarity issue using solid-state diodes in a heat sink assembly, usually with an integrated circuit regulator close by. No problem? For a while, yes. If the regulator fails, or a mechanical issue like a bad bearing or bushing crops up, the diagnosis is easy. A cooked battery points to the alternator, as does dimming lamps or intermittent or sluggish accessory operation, all of which don’t require a scan tool to track down.

Codes are normally definitive, but consider what can happen inside the regulator and rectifier units. On the regulator side, modern thick film integrated circuit technology is durable, but it’s not perfect. Outright failures are self-evident, but “intermittents” can creep in due to overheating and physical damage. Keeping regulators/ rectifiers cool requires lots of airflow, and many alternators use wide open “pineapple” housings that are notorious for their ability to collect dirt and debris. They’re also very fragile. Prying or levering units into place is risky, as is deleting factory air ducting or shrouding. OEM’s literally count pennies when designing modern vehicles, so anything designed to enhance under hood airflow is necessary.

Everyone prefers to work on a clean engine, but consider the power of modern pressure washing equipment if you’re working under hood. Driving water, debris or caustic chemicals into the alternator housing is never life-enhancing. On the other hand, a “leaker” that cakes the alternator with enough dirt to block cooling has to be addressed immediately.

If it’s charging the battery, is it still O.K.? Maybe not. ‘Scoping the output, regardless of trouble codes, can be illustrative. How stable is the output? Garbage or ‘noise’ can suggest a small AC component getting past a blown rectifier, which may have still have enough DC current to keep the battery up, especially if the vehicle is highway driven. Modern regulators usually fail with easy to recognize symptoms, normally low or no output, but in cases where the problems show up after the engine warms up, capturing the dropout with the ‘scope helps, as can cooling the regulator assembly with air or spray electronic coolants where the electronics are bolted to the outside of the housing. How hot is too hot? Alternator operating temperature is rarely discussed, but if you have a non-contact thermometer, maybe for A/C service, why not “shoot” a few good units to get a feel for the housing’s surface temperature? Write it down by model and you might get a sense of how alternator location, under hood temperatures and cooling system performance affect output. Similarly, ‘scoping the output waveforms (choose a time base that lets you see the ripple) of electrically normal vehicles in the shop for other service gives a feel for the “normal” level of current cleanliness. It’s not official, but it only takes a second, and if you’re there with the scope anyway, why not? And modern digital multimeters allow techs to check for voltage drop not just between positive high and ground but along the individual wires themselves. Are you sure that all the output is getting to the battery? And is full battery voltage being sensed at the alternator? There’s much more to discuss about alternator technology and much alternator service will still be about bad grounds, weak batteries and corroded connections, but the combination of OBD, common sense and intelligent intuition can prevent unnecessary alternator service … and protect your reputation.

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