Battery Drain Question

[YourBuddy'sTruck]

Something was draining my battery down overnight.
With a little research here, I found how to test what it was at the fuse block.
One by one, nothing was pulling any current.
So then I tested Voltage OFF, Idle, and at 800RPM.
My battery is fine and the Alternator is charging while running.
So I then performed the test with the meter between the Negative Post and the Cable and it was showing a full 12V draw.
Next - Start unplugging things till you find it, right?
First thing I see in front of me is the pigtail on top of the Alternator and think that can't be it, but do it anyway to rule it out.
Low and behold, the draw went away!
So I left it unplugged overnight and sure enough the battery was not drained this morning.
This 3-Wire Alternator is Internally Regulated.
Is this the Voltage Regulator that is bad?
Can it be replaced or should I get a whole new Alternator.

It's a 350 with HEI and old Voltage Regulator Bypassed.
No other electrical or ignition modifications on this vehicle.
Thanks for you thoughts before I reach for a wrench.

[chevyguyase]

The volt reg. is internal. the problem is probably a diode trio and or the diode bridge (rectifier) . That style alternator is easily disassembled and repaired. I'm sure there are videos on YouTube. I retired from auto repair in 2008 and the parts were cheap and readily available. I have a bin full of Regulators, rectifiers, diode trios and brushes. I think most people these days just get a replacement.

[chevyguyase]

I did a quick check, the parts are still available and reasonable. Here is a YouTube video that looked good. https://youtu.be/SyAFRyaU7dI

[Dead Parrot]

2nd the thought about bad diode(s). Do homework before buying an alt repair kit. Some will turn your alt into a 1 wire but fail to mention that is what they are doing. 1 wire is ok but the voltage sense wire adjusts the output to compensate for any voltage drop in the main charge wire.

If you opt for a new alt, consider going to a 12SI as it has better cooling and is a drop in replacement for the 10SI.

[YourBuddy'sTruck]

Thank you, Fellas.
I'll pull it and see what I come up with locally for parts to rebuild before going for a replacement.
And I'm definitely not going to have a one-wire alternator when I have a gauge I like to watch.
Cheers!

[chevyguyase]

I agree, I have a '54 with a sensing wire and a '66 with a 1 wire. I like the sensing wire system better, both work great. My amp gauges won't handle the output of my alternators, I use volt meters on both of them. I agree with dead parrot about the kits, I would take it apart and test the components and replace them individually. Some of the parts exceed replacement cost.

[chevyguyase]

I watched the video that I recommended and he did not show how to test the diodes. Here is a video that shows testing the components. https://youtu.be/hYfam5f57Ho