Thread: New alternator and good battery but low voltage

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Hello,This is a stock 1980s 305 SBC engine, suddenly I started having this problem with my battery voltage gauge where it would be at 14 volts when I first start the engine and then it would start to drop to about 12v and below while still idling and with no load. I checked the alternator belt tension, tested the voltage to the battery with the engine off and I was getting about 12+v, then I tested the battery with the engine running and nothing else turned on and the voltage was about 11.5v. The fact that the battery voltage was lower with the engine running led me to believe that the problem was the alternator not charging. So I recharged the battery overnight and bought a brand new 100 amp alternator and now when I started the engine the battery gauge was only showing 11 volts. The engine did cranked right up but it appeared that the alternator was not doing anything and revving the engine made no difference. Battery has new posts and secured connections. Could someone please help? Thanks in advance for your inputs!

Check the wire between the alternator and starter/fuse box for a blown fusible link or fuse.

Also check to see if your internal regulator is getting power. It is a 2 wire plug on the side of the rear case of the alternator. Usually you can see it looking at the engine but sometimes they can be on the side where you can't see them as easy.

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Sounds like the two - prong plug to the alternator is bad to me.

I will check on those wires if it ever stops raining around here and will post an update. Thanks!

Also make sure your ground is nice and tight to bright shinny metal.

Don't check them. They can look perfectly OK and still be bad. Just pick up a new plug and splice it in, preferably soldered and shrink wrapped.

I just fought that exact situation recently.

Do you have an underdrive size pulley to gain 2 or 3 hp? Under drive pulleys and poor grounds are causes of most DIY repaits that came thru our shop--------also cheap overseas reman parts-------
what do you mean about the battery has NEW POSTS?
is your guage a dash gauge or a hand held mechanics tool?

run a jumper to the 2 spade plug on the alt. both need to be hot.
if your getting the alt from orilley or autozone it can be junk .

Run a jumper from the alt. case to battery ground see if that changes the output I have run into this problem a few times where the alt. case does'nt have a good ground

Problem solved. A fusible link had split the connection between the 10g alternator wire and the junction point. Thank you all for your help!