Hate to Necro an older thread. But im having the exact same problem as the OP. I have a 1965 F-250 Custom Cab, with a 390v8 in it from a 69.

I re-wired according to the post on page 1


Ok. So now when testing voltage at the battery, I was getting around 13.5. Thinking the problem was fixed I was so happy. Threw the battery on a charger over night, and took the truck to work today. When I took the battery off the charger when leaving for work, the battery was almost full charged and only taking under 1volt of charge. Truck ran great, got it home, threw it on the charger again to see if the battery was draining, and the battery was near dead.

So basically I believe I have the car wired up right now. So im thinking its either the alternator or the voltage regulator correct? Is there a good way to check and narrow it down without just replacing the parts? Im leaning towards the alternator, im not sure how much voltage is suppossed to be coming of the "BAT" terminal on the alt, but it was only around 3.8volts.

Any help would be appreciated.

 

You need to run the engine and check for the 13.5v again. If you have 13.5v or higher, it's charging.

 

I just went out and checked again.

Battery terminals with car off: 13.4v
Battery terminals with car on: jumps between 12.6v and 12.8v
BAT on alternator: 13.4v regardless of off or on.


If that means its charging why did the car battery deplete from near full to near dead while driving for 6 hours? Am i getting drain from somewhere else? I ran all day with no extras (lights, radio, etc).

Am I wrong to be concerned that voltage from the battery drops when the car is turned on?

 

I also just tested the "S" line going into the voltage regulator, it read 13.4volts.

 

You did the right thing and checked the battery with the engine off and the engine running. You are correct, it's not charging.

Since you have voltage on the "s", and probably also on the "a" terminal? of the regulator, you need to see if the alternator is good. I am also going to assume you have battery voltage on the "bat" terminal of the alternator with the engine off, since you ran a new wire.

Take the "f" wire off the alternator and tape it. Then take a scrap piece of wire and hook it to the "f" terminal of the alternator, and lay it over next to the battery. Start the engine, and with your meter on the battery terminals, touch the "f" wire to the battery + terminal. The voltage on the battery should jump way up, and you may even hear the alternator start whining. If it does this, the alternator is good, and I would change out the regulator(before you do that make sure the regulator is grounded well under one of the mounting bolts).

 

I was getting voltage on the A, but now im only getting around .003volts. So im going to replace that wire (was the one and only wire I re-used the original).

Ill check the alternator and the ground of the VR when I get home.


BTW: THANK YOU! so much for being patient and helpful.

 

Franklin2,

When you say to make sure the VR is grounded well under one of the mounting bolts, are you saying to run a ground wire from the GND terminal on the Alt. to one of the VR mounting bolts? I am showing 13.4-13.6 on my Alt. / batt. while running, but my gauge is only reading around 9-10v. My instrument cluster has a light and a gauge. This is beginning to look like an epidemic....

1988 F-250 351W, swapped in 1976 300I6

 

The voltage regulator case needs a good ground. Originally they usually got this from where it's mounted to metal, or sometimes the factory added a terminal under one of the mounting bolts and it was hooked to a ground wire.

You can ground it however is convenient, I personally would not run a ground wire across to the alternator, there is already too many wires in the way going back and forth across to the engine. Depending on where it was mounted, I would run a ground wire directly to the negative terminal of the battery, or somewhere else where a good ground could be found.

 

Franklin2,

You're a gentleman and a scholar, hooked up a ground wire from the battery to one of the VR mounting bolts and it started reading correctly. You saved my truck from being destroyed by the wrath of my ball pein hammer...