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Alternator or batteries??

 
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Old 03-17-2019, 11:44 AM
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Alternator or batteries??

ok so I hopped in my SD to go to Lowe’s this morning it started fine but turned a little slow. I drove to Lowe’s and went in and got what I needed and came back out, no start. It turned really slow and would not start, I have a small capacitor so I connected it to the cranking battery and it fired right up. Now going back to the house I noticed that the battery gauge was steadily dropping the longer the truck ran and when I did get home it was dead to the point that it wouldn’t even turn over. My question is, would this point to a bad alternator or bad batteries? The batteries are around a year old and I have no idea about the age of the alternator? Thanks for any help...
 
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Old 03-17-2019, 12:03 PM
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If the battery voltage is decreasing while the engine is running, generally that means the alternator isn’t charging it.
 
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Old 03-17-2019, 01:40 PM
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Originally Posted by seville009 View Post
If the battery voltage is decreasing while the engine is running, generally that means the alternator isn’t charging it.
i replaced the alternator and charged the batts everything seems to be ok now. The alternator is putting out 14.6 volts and the batts are maintaining while the truck is running on 1100 rpm fast idle with the A/C, headlights and sound system all on.
 
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Old 03-17-2019, 03:42 PM
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Sounds like you fixed it.
 
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Old 03-17-2019, 04:52 PM
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did you check running voltage at batteries before changing anything?
 
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Old 03-17-2019, 06:16 PM
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Originally Posted by kbeefy View Post
did you check running voltage at batteries before changing anything?
i had the parts place check the alternator on their bench. It wasn’t putting anything at all out. But to answer your question, no I didn’t check anything before the swap. Both batts were down below 10 volts and had to be charged for a couple ours before they had enough juice to start the engine. I then checked the alt output and it was good. The batts are both on chargers now to get them all the way up.
 
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Old 03-17-2019, 10:05 PM
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Load test those batteries before using them, so you don’t get stranded. Alternators are known for taking them out. If your batteries are going bad, they will load down your new alternator.
 

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