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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. :confuzeld
 

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Whistle Pigs Are Cool
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Maybe the later models go off of the crank sensor.:confuzeld
 

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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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Discussion Starter #6
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!:thumb:
 

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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.:HiHi::HiHi:
 
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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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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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Discussion Starter #11 (Edited)
IIRC Racer55 posted it a few times that the pulleys were different up to a certain point.
I did a search using different keywords Diaric, none of his posts were close enough to the top that I saw, and assuming that the FAQ sticky's would be correct.:confuzeld I just cant help but think of all who have seen that, members or non- members that may be running the wrong size pulley on their later model truck. Just glad its fixed now, seeing as we send all newbies to those FAQ's stickys at the top, and they see them as the 6.5 bible (so to speak).
 
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Another thing to keep in mind is that the smaller pulley may not allow the belt to get the proper amount of tension from the tensioner.
 

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THe 96+ belt routing and tensioner is a flawed design from the start. Don't know what they were thinking. Most of the time the tensioner tends to flip over center and over tension the belt. I tried different length belts and had no success. A few washers under the AC compressor did the trick. Its held fine for like 2 years now. Before with a new belt it would take a few days to over center itself.

The -95 design is not a lot better but does not seem to be as bad. Getting the bigger alternator pulley on that did seem to help get the tensioner into what I assume is the normal range on its marks.
 

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IIRC Racer55 posted it a few times that the pulleys were different up to a certain point.
I did a search using different keywords Diaric, none of his posts were close enough to the top that I saw, and assuming that the FAQ sticky's would be correct.:confuzeld I just cant help but think of all who have seen that, members or non- members that may be running the wrong size pulley on their later model truck. Just glad its fixed now, seeing as we send all newbies to those FAQ's stickys at the top, and they see them as the 6.5 bible (so to speak).
First of all, good catch Mike. :bow:

Did some digging for Racer55's posts. Here is what I found:

http://www.dieselplace.com/forum/5877034-post6.html

The research I found says that in 96 the alt pulley was standardized across the board?
So it's pre 96 that has the size difference to contend with.
Does it really seem that fast?
Does a scan tool agree with the tach?

There is an issue where if the wrong pulley is installed on the alternator the tach will read higher than actual.

The diesel pulley is bigger than the gasser pulley that is normally on replacement alternators.

This is supposed to be correct:
241250 6-Groove Pulley for Delco 10Si, 12Si, 15Si, 17Si, CS130, CS144 Series Alternators
And one more post on the subject:

The OD of the pulley for the outside lip/flange is 2 and 5/8 inches. The top of the grooves the belt acually rides on is 62mm or 2 and 7/16 inches. This is for the 92/93 mechanical engines, the newer engines changed thier belt drive systems a couple of times and maybe different than this. I'm certain of these figures since my alt. is still original with 190,000 miles (knock on wood). The typical small block chevy alt is 2 and 7/16 inches for the outside lip/flange OD. and is 2 and 1/4 inches (57mm) to the top of the grooves the belt rides on

Assuming a correct engine speed of 2200rpm the SB chevy alt. pulley would show 2400rpm and at correct engine speed of 3600rpm and the SB chevy alt pulley would show 3925 rpm.
What Racer55 said happened in 1996 and what Glagulator posted earlier in this thread about his 1996 & 1999 implies that 1996-2000 used the smaller pulley. The suspicion is the larger pulley is for the 1992-1995 trucks with the specs grabowski posted earlier in this thread.

So......anyone friendly with a GM dealer parts guy that can get part numbers and pulley dimension specs for model years 1992-2000? Then we will have the complete picture.

Absent having facts on GM's alternator pulley specs, the best way to confirm correct pulley size is to cross check Tach RPM with a scan tool.
 

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I'll confirm that the '95 is still a 2-5/8" pulley. I have the original alternator on my truck and my RPMs match what I calculated with my tire size.
 
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