Big Rig Basics

John Baxter | March 01, 2011

10. Reconnect the battery cables and start the engine. Observe the voltage reading on the dash. It should be about 14.6 volts.

Truckers News thanks truck alternator manufacturer Prestolite-Leece-Neville for providing photos and information for this article.

Brushless versus brushed alternators

When it’s time to replace your alternator, or spec a new vehicle, you’ll have to choose between a brushless alternator and one with brushes.

Alternators have a spinning shaft with a coil of wire wound around it called a “rotor.” The rotor sits inside the “stator,” which is a large stationary coil of wire where the main output current that runs your truck is created.

A standard alternator uses brushes to carry current from the ignition system to the rotor to energize the alternator. The resulting “field” current creates the necessary moving magnetic field as the engine spins the rotor. The brushes are metal conductors that rub against smooth rings and, though they last a long time, they are subject to mechanical wear.

A brushless alternator is larger and twice as complex, with two sections of the rotor and two sections of the stator. The extra stator section is where the field originates on a brushless alternator. Since it is not moving, the ignition system wiring can feed it directly, without brushes.

Alternators with brushes often work only until the brushes fail due to mechanical wear. The brushes are its most vulnerable part. So, without brushes and the resulting mechanical wear, the brushless alternator can last significantly longer. Also, because the field coil can be much larger, brushless alternators can create a more powerful spinning field and make more power at low rpm. This design is more complex to build and takes more material, so cost is a lot higher.

Brushless alternators are best for use in new trucks, especially where minimal downtime is essential. Alternators with brushes will save on initial cost and are ideal on older trucks which may not have many miles left in them, where a few hours’ downtime may not be critical and where the owner can minimize the cost of repair by doing some of the work himself, possibly even replacing the brushes in his own garage.

Big Rig Basics Tip

“If I think I have a charging problem, I check all the battery cables and the grounds to make sure the connections are tight and not corroded. I use a battery terminal cleaner to clean them, and then reconnect them, tightening them snugly with the right size wrench. I also inspect all the cables for chafing and replace any that show signs of wear to the insulation. I always tie my cables down to make sure they won’t chafe anywhere, too.

I also periodically disconnect and clean all the alternator connections with sandpaper, if necessary using a file or gasket scraper to make sure the block is clean where any grounds connect. And I periodically load-test my batteries to make sure they will accept a charge as they should. One key to long alternator life is good batteries.”

— Gordon Bow, owner-operator, Oakfield, N.Y. strives to maintain an open forum for reader opinions. Click here to read our comment policy.