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What you need to know about alternators

Paul Abelson
Technical Editor

Today's modern alternator operates on some fairly ancient principles. One ofthe basics taught in high school science classes is that when a coil of wire moves through a magnetic field, electric current flows through the wire. The alternator is a device that converts mechanical energy from a running engine into electrical energy by driving coils of wire past an array of magnets to generate electrical energy. That, in turn, is used to light lamps, run fans, power instruments and radios and most importantly, to maintain the state of charge of the batteries.

Batteries provide the electrical energy to start the engine, and at low rpm, they supplement the alternator's ability to supply electrical power. With the engine off, batteries supply all the power to operate clocks, satellite communication devices, and even the tiny heaters in computer controls, needed because computer chips cannot operate properly in cold weather. Except for these situations, all the truck's electrical operating needs should be met by the alternator, with enough left over to recharge the batteries, replacing the energy used in starting the engine, or, when the engine was not running.

Not too many years ago, generators and alternators needed to be rebuilt at intervals from around 90,000 miles to as "high" as 150,000 miles (if you were lucky). One reason many drivers idled their engines on a year-round basis was that they thought they were saving their electrical systems from the high demands of starting. To some extent, they were right. Modern alternators can be expected to last 400,000 miles or more without any problems, provided the electrical system is properly spec'd and well maintained. Let's start with maintenance, since it's something we can all work on, starting now.


Drive belts, whether the old multiple V-belts or the new single serpentine style, must be in proper tension. Too tight, and undue stress is placed on shafts and bearings. Too loose, and slippage occurs, providing insufficient drive forces and glazing the contact surfaces of the belts, further compounding the slippage.

When you come across drive belts with glazed sides, resist the temptation to spray-on a belt dressing compound. These chemical treatments work for a limited time, but soon the belts will have to be replaced. If you must, use belt dressing to get you to the truckstop or dealership of your choice, but no farther.

Frayed belt edges are a sign of pulley misalignment. This, too, stresses alternator bearings and shafts, and causes belts to break. When they do, it's always at the most inopportune time, of course. Pulley misalignment can be caused by a variety of causes, from a bent alternator bracket (easy to replace), to worn engine bearings (far more difficult), or by the improper selection of replacement parts. It's a good idea to visually check belt and pulley alignment, using a steel straight edge, after every alternator, fan drive or air conditioning repair, and periodically throughout the year.

Loads and overloads

Excessive electrical loads are often placed on alternators by high resistance in a truck's wiring system. If you're adding lights, stereo systems, refrigerator, microwave, or any new electrical accessories, be sure that wiring is adequate to carry the additional current without overheating. It can be a fire hazard, and it can place heavy loads on the alternator. High resistance also results when corrosion builds up on terminals or within wiring. Probe type current meters should only be used at terminals or connections, never by poking into wires. Even though the holes they leave are the size of pin-points, moisture will penetrate the insulation and create corrosion in the wiring. All corrosion increases electrical resistance, and that places excessive demands on the entire electrical system, alternator included.

If you encounter corrosion
here's what to do

1..  If in a wire, replace the entire length of wire.  If that is impractical, splice in a replacement section using heat-sealing crimp connectors or covering the splice at each end using heat-shrink tubing.

2.  If at a terminal, a connection or a battery post, clean the ocrrosion thoroughly using a wire brush and a paste made of baking soda and water.   Thoroughly rinse and dry the problem area, then use a dielectric compound like TruckLite's NYK (it comes in a handy tube you can keep in your tool kit) to coat the parts. 


Proper specification of components is also critical for long alternator life. That doesn't always mean using the highest amp-rating available. Pulley ratios can have more of an effect than total output. For example, one manufacturer's 100 amp alternator produces more current (up to about 2,200 alternator rpm) than the same company's 145 amp model. Another company rates its alternators at 3,000 rpm, but up to 2,000 rpm, its 100 amp model out performs its 115 amp. With taller gearing in today's trucks, and operations in some areas requiring only 1,200 engine rpm at cruise, the old standard of 2-1/2 to 1 or 3 to 1 pulley ratios (alternator to engine speed) may not be sufficient to get the alternator to produce to its rated capacity.

This becomes important - no, make that critical - in winter when discharged batteries take far longer to return to a 90 percent state of charge. For example, if low cruise speed and improper pulley selection don't allow the alternator to match the high current demand from heaters, defrosters and other high current accessories, the demand will be made up by the batteries.

If the batteries run down to a 75 percent state-of-charge, it takes less than an hour to bring them back up at 80 degrees F, but almost three hours to do the same at 32 degrees F. When the temperature drops to zero degrees, it takes more than eight hours. From a 50 percent state of charge (entirely possible when running through heavy snow) those times increase to 4.7 hours at 32 degrees and 18 hours at 0 degrees. The effects compound as the temperature drops below zero. That's why you never want to operate so that your battery is supplementing your alternator, for any extended time.

When spec'ing an alternator for your truck, ask to see the manufacturer's spec sheet on each model you're considering. It will have a graph of output in amps vs. alternator shaft rpm. Determine your electrical load, adding at least one amp for every extra clearance or marker light.

To determine your truck's electrical demands, you can refer to SAE publication J-1343 "Information Relating to Duty Cycles and Average Power Requirements of Truck and Bus Engine Accessories," and TMC Recommended Practice RP140 "Understanding Key-off Parasitic Loads." Add a safety margin of 20 to 30 percent for any hidden corrosion that may build up. Check the graph to determine what rpm the alternator should run to produce enough current for your needs. By dividing alternator rpm by engine rpm, you calculate the pulley ratio needed to drive your alternator.

With today's improved electrical systems, especially the much improved alternators, the secret to exceptional alternator life is a combination of good maintenance and thorough planning. But isn't that true of all truck systems? LL