How to: Replace an Alternator

John Baxter | January 01, 2012

When power fades

Signs of impending alternator failure often include a slight reduction in the voltage reading on the dash, or noticeably duller headlights. It’s hard to predict unless you periodically do an output check over a range of rpms. Even then, alternators with brushes can fail suddenly when a brush wears down to nothing.

Replacement should take about an hour. Consult your truck’s technical manual or the alternator supplier for information on bolt torques.


Tools

• Socket wrenches

• Open-end wrenches

• Half-inch socket drive breaker bar

• Impact wrench

• Torque wrench



1.  Shut down the truck and allow engine to cool. Disconnect the battery connectors.


2.  Note the routing of the drive belt around the tensioner, crankshaft pulley and accessories. Using the breaker bar, rotate the belt tensioner away from the belt to release tension. Remove the belt from the alternator pulley.



3.  Note the positions of the battery positive and smaller electrical connections on the back. Disconnect each, including the ground.




4.  Support the alternator while removing the four mounting bolts and remove it.





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


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


7.  Position the alternator on the block with its bolt holes lined up with the holes in the block and support it. Torque the bolts alternately in stages to the recommended torque.


8.  Install each electrical connection, including the battery positive connection, onto the rear of the new unit. Torque to specification.


9.  Inspect the drive belt and replace it if needed. Use the breaker bar to rotate the tensioner to the un-tensioned position and hold it there while routing the drive belt around all the pulleys. Release the tensioner.


10.  Reconnect the battery cables and start the engine. The reading on the dash should be approximately 14.6 volts.