I have a one wire alternator on 350 Chevy V8 in a 57 Belair. After charging the battery and running the car for about an hour the battery will run down. Changed the alternator but no difference.
Any clue on what is wrong?
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The one tool you will need to purchase / borrow is a Volt Ohm Meter (sometimes called a Multimeter). Then you need to use it to read voltage AT THE BATTERY, before the car is started and after the car is started. My guess is you won't see any voltage difference. Before the car is started, I would guess you'd see voltages like 12.6v or so. After the car is started you SHOULD see 13.7v or so.
If you see no difference in voltage between car off and car running, that means the alternator isn't working to put out a charge (or its working great, but you've got a short in wiring external to the alternator somewhere.) Don't forget that with a one wire alternator, the ground circuit is made through the alternator's mounting bosses to the engine block. Do you have any pretty components in that chain that have powder coated paint? Ouch. That won't work.
When you installed the engine where did you place the engine block ground strap? Usually its a large braided and flattened copper wire, attached to the engine block (on an unpainted boss) and to the frame of the vehicle somewhere. If its bolted to the frame of the car, there should also be another ground strap that goes from the frame to the body. That's pretty important, and its easily overlooked. That provides the electrical current a return path from the alternator to the battery. Check that ground strap carefully. Is it secure at both ends?
Another item that may come into play is the voltage regulator. Newer cars have an internal regulator inside the alternator. The regulator controls how much voltage the alternator puts out. If you are putting out 18 volts DC (Direct Current) that's too much. If you are putting out 11 volts, that's not enough. Must cars today put out 13.5 volts to 14.5 volts. The fact that you have a one wire alternator indicates you've got an internal regulator. You're not routing the current to an older external regulator are you? You don't need to do that. You can safely route the single output wire from the alternator directly to the battery positive terminal.
One other thought. Do you know the history of ALL the pulleys on this vehicle? Its possible that somehow pulleys got changed around, and your pulley drive ratio is no longer appropriate to put out enough electrical voltage. (I'm guessing you would be aware of other drive related issues, like slipping fan belts.. If that were the case you'd hear the belts making a terrible screech noise.) Can you measure the RPM of the alternator pulley? Tools for this include a variable strobe light setup or an optical tachometer or a temporary hall effect sensor pickup.