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I recently replaced the alternator in my Caprice--one day it just stopped charging at all. I bought this new Remy 140-amp model with lifetime warranty from Rock Auto: https://www.rockauto.com/en/moreinfo.php?pk=474185&cc=1037461&jsn=1

I noticed tonight that when the tach went past 4000 RPM, the lights dimmed and the voltmeter dropped from the low 14s down to the upper 12s. As soon as RPM dropped below 4000, the lights went back to normal, as did the voltmeter. (My '94 Caprice has an F-body gauge cluster.)

This alternator has only been on the car for less than a month, and maybe 600 miles. Is it likely that anything else is amiss, or should I just contact RockAuto for a replacement?
 

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Flesh, time to break out the VOM. Start by checking voltage at the alternator and work your way to other points. Battery and such. See if there is a drop anywhere along the line. How are all your grounds? Check your hot connections too. You could also take your alternator to Advance or AZ to see if it's your new alternator. How's your belt tension? Might be slipping at higher RPMs. I'm sure you will get more and maybe better thoughts on this, but right now that is all I can think of.
Good Luck Friend,
Mark: Snowman-33
 

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There could be a loose winding, that disconnects at higher RPM. The alternator runs at about 2 1/2 or more times the engine speed. It could also be the regulator that must switch the voltage as the RPMs increase. The regulator is internal for the alternator. The regulator is included in the warrantee.
 

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Not trying to Jack your thread but thought fred might know.can a alternator drain your battery?i installed a used alternator and now my battery goes dead in a few hrs.
 

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Everytime.less than 12 hrs its so dead not even dome light.i installed old alternator tonight.going to see if it starts in the morning.
 

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Meant 'how often do diodes short out rather than burn out open circuit' is was my question. But you might have gotten the lucky one.

You can see if the batt terminal shows a short to case when you take it out. or any of the other terminals to case.
 

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Discussion Starter #9
I received and installed the replacement alternator today and the problem is solved. Voltage while engine running is a bit higher on the dash gauge than before, and it remains consistent all the way to 6000rpm.
 

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What kind of set up do you have to pull those RPMs? I can say I've never touched those levels.
Without digging up the spec sheets to refresh my memory, it has:
--recently rebuilt bottom end with lighter-than-stock pistons, ARP bolts, etc.
--one of GM's upgraded cams (LT4 hot cam, maybe? I forget)
--ported aluminum heads from a Vette
--1.6:1 self adjusting rockers
--appropriate valve springs for the cam

According to the tach (it has an F-body cluster), it shifts at or just slightly beyond 6000rpm at WOT.
 

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Not trying to Jack your thread but thought fred might know.can a alternator drain your battery?i installed a used alternator and now my battery goes dead in a few hrs.
the diodes in the alternator can short out and pass current flow in both directions called leakage. if a person jumped the battery and put positive on the negative [jumper cables] this will damage the diodes..

never use a rebuilt alternator as the winding's do get damaged ..armature winding's ...
 

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Used alternators are not bad per se. They are supposed to check them before sending them out, and if the windings are damaged, they will not perform properly. You hope they put a new voltage regulator in them as well.


If you put a rebuilt alternator in, and it quickly discharges the battery, I would surmise that the alternator is bad, unless you changed something else in the system. A diode can either burn open, or fuse to effectively a wire. If it is installed backwards, it will not do what it is supposed to do either. No matter what, the system will not work correctly. If the alternator is not charging, the parasitic loads in the car can drain the battery in short order, and running it with no charging will only last a couple of hours during the day.
 
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