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Underdrive Alternator Pulley Installation: 4.0 SOHC

aldive

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In my quest for the utmost mileage, I came across underdrive alternator pulleys for the 4.0 SOHC motor. It didn’t take much pondering before I ordered the underdrive alternator pulley from Underdog Performance ( http://underdog-performance.com/index.html ). They have tested this alternator pulley in conjunction with the ASP UDP/harmonic balancer ( 25% underdrive ) and found the charging to be acceptable in street use.

The pulley cost $34.00 delivered to my door.

An alternator pulley is just the opposite of a crank pulley for underdriving. To underdrive the crank, you use a smaller diameter pulley, whereas to underdrive the alternator, you use a larger diameter pulley.

My 130 amp alternator ( http://www.explorerforum.com/forums/showthread.php?t=139789&highlight=alternator ) is already being underdriven ( http://www.explorerforum.com/forums/showthread.php?t=151951&highlight=underdrive ) via the UDP/harmonic balancer; this pulley will further underdrive it allowing more power to the rear wheels. But will it allow the alternator to make enough power at idle and slow speed?

To facilitate access ( my impact wrench would not fit with alternator on truck ) to the alternator pulley retaining nut, the alternator had to be removed.

The first step in the alternator removal was to disconnect the battery. My Mac Intake ( http://www.explorerforum.com/forums/showthread.php?t=130982&highlight=mac+intake ) was then removed to gain better access. The next step was to remove the serpentine belt by rotating the drive belt tensioner counterclockwise with a 3/8” breaker bar. Now the wire harness to the voltage regulator was unplugged and the B + terminal cover lifted and the nut (10 mm) and wire removed. The 3 bolts ( 13 mm ) holding the alternator in place were removed and then the alternator removed from the truck.

Next, the alternator was placed in my bench vice and the pulley nut (15/16”) removed with my air impact wrench.

The new pulley was installed and the nut tightened with the air impact wrench and the alternator was then reinstalled in the truck by reversing the removal procedure. A new serpentine belt was not required.

The motor was started and observed for any maladies; none were present.

After allowing the motor to reach operating temperature, the voltage at idle was 12.5 volts ( as determined with my Dakota Digital [ http://dakotadigital.com ] voltmeter ). Idle voltage prior to the new pulley install was 13.4 volts. The voltage over 1200 rpm was 13.4 volts; the same as before the installation of this larger pulley. A short road test showed the voltage to be as before (13.4 volts) the mod. This test was in the daytime ( no lights ) with the AC on.

This relatively simple mod took about 30 minutes at a leisurely pace.

There was no seat of the britches sense of more power, nor was it expected; the power gain was minimal, but every little smidgen adds up. Gas mileage, which is my goal, will be evaluated on a long road trip.

The low power at idle is not acceptable to me. I shall do some diagnostics on the alternator tomorrow, maybe even upgrade the wiring ( from the B ) terminal to the battery, from the battery to the frame, and from the frame to the motor ). If I am unable to resolve the issues, I will probably go to overdrive alternator pulley.

The results will be posted later.
 
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aldive

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jtsmith said:
Al, I believe this would work on v-8 as well. correct?
Same alternator, But it might need a longer belt

Yes, the V8 and the 4.0 SOHC use the same alternator and it should indeed work. I doubt that the small difference in pulley size would dictate a different belt; I used the same one.
 
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Afboy143

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So if I am running an audio system, would this mod not be reccomended?
 
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aldive

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Afboy143 said:
So if I am running an audio system, would this mod not be reccomended?

Not necessarily. An overdrive alternator pulley is one option ans another is a higher output alternator.
 
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Green98XLT

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Get rid of the UD alternator pulley, they are nothing but trouble. See my other reply to your re-wiring post.
 
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