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.