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Old 05-03-2015, 07:29 PM   #1 (permalink)
dieselmike1
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Do we need to re-visit alternator pulley sizes

I just got to thinking about my alternator pulley size. My old alternator is the same as my new one, they measured 2.36" with the micrometer across the face, according to the FAQ's it should be 2 5/8". I got out my scanner and plugged it in and the tach and the crank sensor signal are within a few rpms of each other. Does anyone know if the later model trucks the alternator pulley size was changed? I also run about the proper rpms for my gearing at 70mph which is about 2500 to 2550 rpms.

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Old 05-03-2015, 07:55 PM   #2 (permalink)
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Nah:
https://www.dieselplace.com/forum/63-...ley-stuff.html

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Old 05-03-2015, 08:09 PM   #3 (permalink)
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Quote:
Originally Posted by grabowski View Post
That doesn't address why my crank sensor is agreeing with the dash tach with the smaller pulley.
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Old 05-03-2015, 08:15 PM   #4 (permalink)
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Maybe the later models go off of the crank sensor.
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Old 05-03-2015, 08:45 PM   #5 (permalink)
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My 99 and 96 both have the 2-3/8" gasser pulley and both show verified correct RPM's. I suspect the larger pulley was last used in 94. My 94 GMC had the larger pulley, I still have the original alternator. I have searched for a definite answer to this and have not found one yet. The gauge cluster changed in 95 so maybe a few owners of 95's might measure theirs and post up the results. All 6.5's get the tach from the alternator except for some military vehicles.
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Old 05-03-2015, 08:49 PM   #6 (permalink)
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Quote:
Originally Posted by Glagulator View Post
My 99 and 96 both have the 2-3/8" gasser pulley and both show verified correct RPM's. I suspect the larger pulley was last used in 94. My 94 GMC had the larger pulley, I still have the original alternator. I have searched for a definite answer to this and have not found one yet. The gauge cluster changed in 95 so maybe a few owners of 95's might measure theirs and post up the results. All 6.5's get the tach from the alternator except for some military vehicles.
Thanks glagulator, yes if we can confirm this, we need to change the FAQ's sticky's as it can be confusing to later model truck owners!
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Old 05-03-2015, 08:51 PM   #7 (permalink)
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Quote:
Originally Posted by dieselmike1 View Post
Thanks glagulator, yes if we can confirm this, we need to change the FAQ's sticky's as it can be confusing to later model truck owners!


Changed grabowski's reference to 1994
Changed quadstar87's reference to 1995
Added Glagulator's post above for 1996 and 1999.
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Old 05-03-2015, 09:04 PM   #8 (permalink)
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Thanks for fixing that Freddy, if I hadn't questioned this, I would have run out and gotten a 2 5/8 pulley to fix something that wasn,t broke and broke something that didn't need fixing.
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Old 05-03-2015, 10:28 PM   #9 (permalink)
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IIRC Racer55 posted it a few times that the pulleys were different up to a certain point.
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Old 05-03-2015, 11:17 PM   #10 (permalink)
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My 96 has the smaller one and agrees with the scan tool. I know its off the alternator because when I first installed it I had not yet figured out that I could move the wires in a replacement gas connector to get the tach to work. Why they changed it? Don't know. Why it does not come off the computer? You would think the OBD2 computer had enough extra power to handle it. It was a new style alternator for 96 but seems like the pulleys shaft was the same so its not like they needed to design an new pulley for the diesels.

I don't have a scan tool for my 95 but when I replaced the alternator the new one had a bigger pulley so I am assuming its the correct one now.

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