Big Rig Basics

John Baxter | March 01, 2011

Getting Your Power Back

If your engine won’t run and your lights won’t turn on, it may be time to replace your alternator


Replacing an alternator is a simple job, but diagnosing electrical problems is complex. Don’t replace an alternator until you’ve had it tested and know it’s bad.

Most trucks today use a pad-mount alternator, like the one shown, for more rigid mounting. Consult your truck’s technical manual or the supplier of the unit for information on bolt torques and for further information on replacing older-style alternators mounted on adjustable brackets.

What you’ll need

• Basic socket wrench

• Open-end wrench

• 1/2-in. socket drive breaker bar

• Impact wrench

• Torque wrench


1.  With the engine cool, disconnect the battery cable connectors at the batteries.

2. Note the routing of the alternator drive belt around the tensioner, crankshaft pulley and driven accessories. Draw a picture if necessary. Using a breaker bar with a half drive, rotate the belt tensioner away from the belt to release tension. Hold it in position, remove the belt from the alternator pulley and release the tensioner.

3. Note and document the positions of the battery positive connection and smaller electrical connections on the rear of the alternator for proper polarity. Using an open-end wrench, disconnect each connector, including the ground. If the alternator does not have a separate ground cable, consult factory information for the proper procedure.

4. While supporting the alternator, remove the four mounting bolts attaching it to the engine block. Remove the alternator.

5. Support the alternator in a soft-jawed vise. With a screwdriver, hold the pulley stationary via the cooling fan, then use an impact wrench to remove the pulley-attaching nut. Slide the pulley off the armature shaft.

6. Slide the pulley onto the new alternator’s armature shaft. Install the attaching nut and, while holding the pulley stationary, torque the attaching nut to the specification recommended by the manufacturer.

7. Locate the alternator on the block with all four bolt holes lined up with the corresponding holes in the block and support it. Install the top two bolts first, threading them in just far enough to hold the unit. Then install the remaining two bolts. Tighten the bolts alternately in several stages to the recommended torque.

8. Install each electrical connection, including the battery positive connection, onto the rear of the new unit, observing the polarity determined prior to removal. Torque to specification.





9. Inspect the drive belt and replace if necessary. Use the breaker bar to rotate the pulley tensioner to the untensioned position and hold it there while routing the drive belt around all the pulleys. Once the belt is in place, release the tensioner and remove the breaker bar.

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.