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Alternator Not Charging At Speed

6K views 28 replies 11 participants last post by  sambor 
#1 ·
This is a weird one for me.

A few days ago I hopped on the interstate around dusk and I immediately noticed my headlights were a bit dim and seemed to flicker occasionally. Glanced down at my dash gauges and the battery voltage gauge was hovering at 12. Not charging it would seem. I flicked through the CTS3 and it confirmed the same.

Not wanting to pull it over and shut it down and risk an expensive tow I just rode it out to see if it would come back up. I tried running with all unnecessary electronics turned off and no change. So then I turned on everything I could to try and see if a big load would get the alternator charging again. No change.

I get off the interstate at my exit and as soon as I start slowing down my voltage starts to rise. Sitting at a dead stop I'm charging at 14.0 volts. Not what it normally charges at (14.3 to 14.7) but better. It's been brutally cold here the past few days so the truck has sat but next week is warming up so I'll be back to work and I'll need my truck for that.

Anyone experienced this before? I've only ever had one alternator go bad and that was on a gas truck and it just plum quit one day.

Any suggestions on diagnosing it to ensure it is the alternator at fault? I'm about 99% certain it is but I hate throwing parts at problems and it's always good to learn new diagnostic techniques. Especially electrical ones because those are not my forte.
 
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#2 ·
Similar, but pretty much the opposite happened to me a few years ago. Road trip to the other side of Illinois from home to pick up some parts, going down the interstate the battery light popped up and went off before I could see what it actually was. Did it a couple times before I figured out it was the battery light. Thought I'd hop off the interstate to stop at a parts house and get it checked- kicked the cruise off and it dropped to battery voltage only. Stopped at the parts store, ripped off the alternator and took it in to test- sure enough wasn't charging high enough to pass. Installed a new one and rolled on.
I would suspect alternator for sure.
 
#5 ·
What happened to yours is what I would expect a failing alternator to do. Charge when under a decent load and not charge when under little or no load. That's what has thrown me for a loop on this one. Seems completely counterintuitive.
 
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#4 ·
Yes, that's probably my next step. The batteries are relatively new and the "Battery Not Charging" message never came up on the dash. The truck started up like normal the next morning after sitting 12+ hours overnight so I don't think I depleted the batteries but I intend to have them tested and put them on the trickle charger before putting the truck back on the road.
 
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#8 ·
Okay, it's definitely the alternator. I've read a bit on here and was just wondering what y'all recommend for a replacement? I've seen some upgrade to the XDP but I'm not a huge fan of buying parts that can't be obtained locally in a pinch. I will buy parts like that if it's a serious upgrade but otherwise I just run down to the local parts house and get the best I can afford. O'Reilly's carries "new" Optimas as well as remanufactured AC Delco units. What do y'all think is best bang for the buck?
 
#9 ·
I grew up with my dad insisting on rebuilt starters and alternators. I cannot really say we had much trouble with them but I have always bought new replacement parts for my vehicles. So, in a way I guess I am saying I don't think you can make a bad choice here unless you go with some off brand, etc. I would stay away from anything Optima.
 
#10 ·
Where and how you get a replacement warranty part if need is always high on my list of what to replace something with.
 
#11 ·
I've never liked AutoZone for electrical stuff, but that was what I had in my situation needing an alternator that time so I used theirs. A few months later that replacement failed, but they gave me another one. Still running that one today
I usually use Napa or Oreilly personally
 
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#12 ·
I can’t remember the brand name but I had problems with the cheap Advance Auto reman alternators. I went through 3 of them in a 2-3 year period on a previous vehicle (95 gas Suburban). Yes they were always under warranty and fairly easy to swap out… once in the Advance Auto parking lot… but it was a pain to deal with and I almost ended up broken down & stranded 2 of those times. And I think the failing charge ruined my battery 1 of the times too.

If reman AC Delco is an option, that sounds like a pretty good blend of value + better reliability. Depending on the price difference between reman & new.
 
#13 ·
Around here we have a shop that actually re spins alternators. They do upgrades as well when they crack it open. Not sure if thats an option by you but that way they can up your amperage a bit as well.
 
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#16 ·
I've tried looking around my area for a rebuilder but haven't found anything yet. I'm in SW Missouri so if anyone knows of a good rebuilder I don't mind making a drive for a quality rebuild job.
 
#14 ·
#15 ·
Those look very nice but at twice the price I don't think that's necessary for my truck. My truck is a daily tow rig and the only electrical add-ons I have is a single Kennedy lift pump. Very cool looking units though.
 
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#17 ·
Seems like it’s the time for alternators to fail. I just replaced mine. Advanced auto.
 

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#18 ·
It's weird how these things tend to go in groups around the same time. A friend of mine with a similar truck called me last night and mentioned his alternator was acting up, too.
 
#21 ·
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.
 
#22 ·
Good advice. In my case, the belt, tensioner, and both pulleys are new within ~10k miles.
 
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#23 ·
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.
 
#26 ·
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.
 
#27 ·
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.
 
#28 ·
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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#29 ·
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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