New battery. Alternator checks out at the parts store. Put meter on wire coming out of alternator and it is hot. But when running, it dies if I take the pos cable off of the battery. Somewheres between the alternator and the battery is my guess. Any ideas. Starter is also new.

This is on a 70 V8 3/4.

When you checked the alternator output did you get 12 volts or 14.5. I think your alternator might not be getting field voltage from the regulator so all you would be running on is battery voltage. You can lay a hacksaw blade on the outside of the alternator and if it is magnetic then the alternator is putting out voltage.
Do you still have the external voltage regulator and do you have points ignition?

VettVet is correct about the rotor field being established IF you can get something ferrous to stick to the rear bearing cap. This indicates the "FIELD" circuit is complete and has no problems.

You should also have 12 volts from the battery at the "STATOR" or output terminal of the alt with the key in the "ON " position but not running.

You should get @12.6 volts NOT running and 14.2 volts across the battery terminals when the motor IS running IF the alternator is producing.

If the first 2 checks are bad, work backwards from the alternator to the battery and check each wire, terminal and connection to be sure its all tight.

Go to "madelectrical.com" for an in-depth read on charging circuits.

I was only getting 12.3 from the battery and the alternator so I took the alternator in and had it checked. When I check the output of the alternator on one of the other trucks, it was 14 something. They gave me a new alternator, but it is outputting the same things. So could something be bringing it down? Still dies if I take the pos cable off of the battery.

Which wire from the alternator should I check and where does it go?

It's gotta be the regulator or somewhere in the sensing circuit to the regulator. I's try the regulator first or else convert to an internally regulated alternator. Do you know if you have a external regulator or if it's been converted to internal?

I swapped out the regulator Sunday. At least the black box on the rad support that has a wide connector and a condensor attached. No change and $13 dollars poorer. Couldn't change the condensor since they didn't have one with the correct plug.

Here is a crop of the wires that come off the alternator and the regulator.
The blue and the white wires off the regulator go to the small plug on the back of the alternator and the blue one is the one that charges the field of the alternator.
The larger red wire ties into the junction with the red wire off the regulator and they both connect to the positive battery post.
check for battery voltage on both these red wires. The only other thing I can figure is that the regulator must be grounded really well so the contacts inside can close and send power to the alternator and battery.
If you go to the sticky at the top of the electrical forum and open it and scroll down to the electrical diagrams from farside you will find a larger diagram that I cropped this from. I hope this helps. VV

Rickstrucks, I'm having the EXACT problems your having. It's finally killed my battery. I have a new uncut (used) harness (mine is all cut up) that I'll be installing in my truck, I'll also be swapping to the internal regulated alternator. I've changed, checked, and tried everything I can think of, this is the last thing I know I can do :confused:

I know it is something from the two wire plug that comes from the alternator thru the regulator. I pulled that wire from my 69 and it would die when I pulled the pos batt cable, so know it is in that circuit. I think I wil try taking a regulator and condensor off of another running truck and make sure the new regulator or old condensor aren't bad.

IT WORKS. Was talking to Earl (earl87gta) yesterday and he mention he had a similar problem. He said it was the red wire going to his ignition switch. Well I got under the dash and saw that several wires, going to the ignition switch, has small exposed areas - not sure if any had been touching. But I taped them all up and started the truck. Removed the pos cable and it didn't die. Measure the output of the alternator and it was finally up to where it should be. Thanks Earl.

Oh, forgot to mention about the new regulator. Once it started working I saw some smoke coming from the new regulator, so I shut it off. Thinking that my son may have zapped it when he crossed the battery cables, I purchased another one. It did the same thing, but I waited a little while and it quit. Must have burnt of some stuff underneath - I think the ceramic piece. Works fine now. Hope that is normal.