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Years ago I had installed a high capacity alternator on my 1958 Oldsmobile 88. The alternator would charge the battery at idle and low rpms. But at highway speed it would not charge the battery and keep up with the load of headlights at night (I was running an aircraft landing light in one of the high beams). I discovered the fan belt would slip. Replaced with a heavy duty fan belt and all was well.

Bottom line check the serpentine belt and the tensioner.
 

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Discussion Starter · #22 ·
Years ago I had installed a high capacity alternator on my 1958 Oldsmobile 88. The alternator would charge the battery at idle and low rpms. But at highway speed it would not charge the battery and keep up with the load of headlights at night (I was running an aircraft landing light in one of the high beams). I discovered the fan belt would slip. Replaced with a heavy duty fan belt and all was well.

Bottom line check the serpentine belt and the tensioner.
Good advice. In my case, the belt, tensioner, and both pulleys are new within ~10k miles.
 

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2018 GMC Sierra 3500 SRW Z71 LTZ
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When it comes to starters and altenators I always pay the difference and buy new not rebuilt.
 

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Discussion Starter · #24 ·
When it comes to starters and altenators I always pay the difference and buy new not rebuilt.
I would agree if I could buy new OEM but I'm not seeing that anywhere. The only new ones I've seen are Optima Brand and another brand I've never heard of. BestStart maybe? I can't recall. Both are cheaper than a remanufactured AC Delco.
 

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I would agree if I could buy new OEM but I'm not seeing that anywhere. The only new ones I've seen are Optima Brand and another brand I've never heard of. BestStart maybe? I can't recall. Both are cheaper than a remanufactured AC Delco.
Check out rockauto.com. It looks like they have new AC Delco alternators in stock for my 2018 3500HD at $247. Hope this helps.
 

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Get the OEM part number and google that for availability and price. Often, not always, Amazon is about as cheap as you will find.
 
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Discussion Starter · #27 ·
Check out rockauto.com. It looks like they have new AC Delco alternators in stock for my 2018 3500HD at $247. Hope this helps.
Good call! I had looked on there before but completely missed the new AC Delco on there. I only saw the remanufactured one. The new one is less than half the cost of the remanufactured option at O'Reilly's, too. Can anyone confirm if this part number is correct? According to GM VIN Decoder's website I have the 105amp alternator and I believe that to be true from when I bought my replacement belt and associated pulleys and tensioner.

AC Delco Part Number - 3351086
 
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Discussion Starter · #28 ·
Get the OEM part number and google that for availability and price. Often, not always, Amazon is about as cheap as you will find.
From my research, it appears to be the part number I posted in my last reply. I've cross referenced that number across a few different sites and it looks like that's the correct part number. If so, that would be great because it's much cheaper than a remanufactured AC Delco at O'Reilly's.
 

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I'm not sure how your 2005 is equipped but my 2015 alternator has a one-way clutch built into the pulley (like a sprag clutch) designed to help the alternator deal with the speed fluctuation of the crank at idle. Every cylinder firing sends an acceleration through the accessory drive and the alternator (runnning at 3 times the crank speed) doesn't handle it well. I wonder if that clutch is failing on your alternator; not handling the high load well but still working under light load. I had to replace my alternator a couple years ago as it failed electronically but I hung onto it as the clutch seemed OK and I don't know if you can by the clutch by itself. Since the new OEM alternator (I believe sold by Nippondenso) came with the clutch pulley and they don't take cores, I hung onto the old one. I was cautioned by the alternator shop not to buy the aftermarket ones as they get a lot of them back. He said the manufacturer doesn't sell the parts to rebuild these as they're patent protected and some of the aftermarket ones don't come with the clutch device built into the pulley. They're just a machined pulley; hence the lower price.
 

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Discussion Starter · #31 ·
Forgot to update the post. It was definitely the alternator. Swapped in a new AC Delco from RockAuto and all is back to normal. Won't be doing any business with RockAuto again due to them lying to me about my package being handed over to FedEx (my part was supposed to arrive on a Thursday, didn't get it until the following Monday and they tried to blame FedEx when they hadn't even given to package to FedEx yet) but regardless I'm back on the road. I think next time I'll just pay the extra money and get one at the local O'Reilly's. They get parts like that same day or next day.
 
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The clutch is an over-running type: when the engine suddenly slows it allows the alt. to freewheel until engine speed catches back up. The clutch can be purchased separately if need be.
 
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