1984 GMC Sierra 6.2 Needs Alternator - But Which One?

I've been putting up with my '84 Sierra's lackluster alternator performance for a long time now. Unless driven very regularly, it would have dead batteries. Now it doesn't even really charge them when driven or run...the
output starts out strong after starting (14.5 volts) but soon drops to 12.8 with next to no load (everything that can be turned on is turned off).
The battery connections look good at both ends, and neither battery is very old. The drive belt is tight, so I think the alternator is sick and dying. It is original to the truck as far as I know. I've been looking for a new one, and the consensus at various auto parts stores is that the alternator is rated for about ~75 amps maximum output on these trucks. This can't leave much left over for charging both batteries when you factor in headlights, radio, blower fan and all that stuff being turned on. It really seems like such a "small" alternator would lead a very hard life.
However, the clerk at Advance Auto Parts showed a number of beefier choices, going all the way up to a 140 amp rated unit. I'd tend to go for the larger alternator...the idea being that a larger unit would be under less stress and have more ability to take on heavy loads when they appear and while still charging the batteries.
What I'd like to know is if I'm on the right track here--if I should spend the extra money for the large alternator or choose something smaller (there are a few choices between the stock unit and the higher rated ones)? Also, would the larger unit be compatible with my truck (belt and mounting wise)?
Thanks in advance for any info, thoughts, whatever.
William
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On Jan 23, 8:53�pm, "William R. Walsh"

========== ==========
Willy............. you looked so lonely sit'n on the board with NO ONE reply'n..........so.........
:)
anywhooooooo..........
Sounds like the thing does ok as long as it's being driven. (?) Sounds like it's causing the problem after sitting for a day or so. (?)
Sounds........like you should check for an amperage draw BEFORE you go buying any new altenator dude.
The output should be around 14.2 or a little better on initial startup, with the voltage dropping back to 12.8 or so shortly after if you don't have any, many, acessories running. That's normal op. I know you well enough to realize you prob know that......
We've had a couple trucks outta the shop FLEET recently doing the EXACT same thing....amperage draw on various circuits has been the culprit so far.
Anywhooo..........i'd ck that first.
Could be the Alt is working. Could be the electrolyte has accumulated on the plates. Could be a current draw when sitting.
jest some guess's..........
anywhooooooo........... Who'd you piss off...so no one wanted to talk to you??
:)
~:~ MarshMonster ~takes a toke......pours one fer WillyWalsh the WashRack man~ ~:~
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Marsh Monster wrote:

Something looked familiar abotu this post. Then I looked at the poster . . .
Nuf said.
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Marsh Monster wrote:

From anyone else I would guess a typo. From you, showing off your ignorance.
SULFATE from electrolyte is accumulating on the plates.
http://www.powerstream.com/1922/battery_1922_WITTE/batteryfiles/chapter10.htm

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============== ==============
yeah....yeah.........yeah...........
like yer the FIRST one to EVER point that out to me.
:)
~:~ marsh ~takes a toke.....exhales in Dobony's direction.........who said 2nd hand smoke was bad for you~ ~:~
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Marsh Monster wrote:

So you admit you are too stubborn to learn.

Typical ignorant bully.
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In article < snipped-for-privacy@h11g2000prf.googlegroups.co m>,

This is backwards... Charging voltage will be a bit low after initial start up as the alternator works to replenish the battery after a large amperage draw from the starter, the voltage will slowly ramp up as less current is used to recharge the battery. Delco alternators -do- have temperature compensating circuitry, charging voltage will tend to be higher in cold weather conditions and lower in hot weather conditions.
Naturally, any accessory load from the truck will raise amperage and lower voltage.
The voltage shouldn't go under 12.8 when the alternator is being loaded at its maximum output rating.
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aarcuda69062 wrote:

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You need to go out tomorrow and connect the carbon pile across the battery, amp meter to the alternator output and a volt meter to the battery. Take an open circuit voltage reading from the battery. Start the engine and maintain 2000 RPM, note volt meter reading. While maintaining 2000 RPM engine speed adjust the load across the battery with the carbon pile until the amp meter reads near maximum alternator output rating, does the volt meter read higher or lower than the no load voltage? If you adjust the carbon pile for less amperage output from the alternator, does the voltage reading go up or down?
I suspect what you and Marsh are seeing is the effect of tapering off the engine speed from a cold start.
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On Fri, 25 Jan 2008 10:10:33 -0600, aarcuda69062

I see this guy is up to old tricks and passing bad info. Battery charge does not determine voltage of charge, only amperage rate. Regulator will hold voltage constant within limits of output abilty of alt. If voltage climbs when charged it is a sign of a weak alternator or bad regulator because voltage should be contastant or tend to decreases as underhood temps rise, not increase. SnoMan
Let the Trolls desend as their egos and insecurities need to be feed....
----------------- TheSnoMan.com
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snipped-for-privacy@snoman.com says...

[snip]
Yes you are. Lets remind everyone why YOU left for a few months. ___________________________________________________________ SBJ: 4T65E http://tinyurl.com/2lcjkv (Confuses a 4T65e with a 4L65e and won't admit it.) ___________________________________________________________ SBJ: Dumb brake question http://tinyurl.com/2ya3wo (Discribes the wrong brakes and won't admit it.) ___________________________________________________________ SBJ: Front wheel bearings-2000 Blazer?? http://tinyurl.com/2j44zv (Claims torque specs are wrong when they are not.) ___________________________________________________________ SBJ: Snoball Defense System v1.01 http://tinyurl.com/2okyfx (Snoball breaks these out when he knows he's wrong and doesn't want to admit it.) ___________________________________________________________ Snoman these are things YOU wrote. Don't like what YOU wrote? Then YOU should stop posting.
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snipped-for-privacy@snoman.com says...

it's back.......
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snipped-for-privacy@nohow.com says...

Should we point out to it that its signature field is grammatically flawed or should we keep laughing at it? I mean it kinda highlights the incompetents of the troll nicely. Then again if we do point it out, it will never be corrected... http://tinyurl.com/2okyfx
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Good thing you were able to come by 4 weeks later and straighten that out... ...the OP might have froze to death otherwise.

"need to be feed?"
What 2 year old wrote that for you?
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William R. Walsh wrote:

Take the batteries in and have them checked first. I suspect that one of the batteries is not in the best condition. When you disconnect the batteries check the resistance between the leads. There should be very high resistance. If the needle moves very much then you have an short somewhere. Disconnect a fuse at a time and find out which one is shorted out. Then find the direct culprit.
Mike D.
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Hi!

I don't know. I did find out recently that if I bring them up with an external battery charger, they will stay up.
I'll do the resistance check you suggest. I'm also going to pull the alternator and have it tested to see what happens. It seems weird that it drops down to 12.8 volts with nothing happening and even less when loaded. Revving the engine up doesn't help, and I don't really know exactly where 2,000 RPM is. I'm also not sure how you'd hook a tachometer up to a Diesel engine. The ones I have are electronic and actually hook up to an ignition coil on a gasoline engine.
The alternator's behavior strikes me as weird. I have a number of other GM vehicles and not a one of them ever drops much below 13.5 volts output, even when fully loaded.
I don't have a carbon pile, so I can't make any of the tests you suggested.
William
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"William R. Walsh"

The alternator should be able to do that also.

Good idea.

Nothing weird about it, that's what dead alternators do.

2000 RPM can be guesstimated.

There are a number of ways to measure RPM, a photo tachometer, thru the alternator or by connecting a magnetic tach pick up in the holder above the harmonic balancer.

Nothing weird about it, 12.8 volts and dropping is a dead alternator. And when I say dead alternator, that doesn't automatically mean the component itself is bad, you still need to verify the associated circuits to it.

Have you tried full fielding it? Have you checked for battery voltage at the output stud on the back of the alternator and at the red wire in the plastic 2 wire connector? Is their voltage at the brown wire with the key on?
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========= ========= On Jan 25, 10:52�pm, "William R. Walsh" . It seems weird that it

======== ========
WILLY !!!!!!!!!!!!
..........you omitted that from your original post!
Yank the Altenator and carry it in to have it checked.
Carry yer credit card with you. Like Cuda said....symptoms of a bad altenator.
~:~ marsh ~takes a toke.......goes back up to exhale in Dobony's face~ ~:~
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Marsh Monster wrote:

And still refuses to learn.

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