74 dart sport won't charge

So, what is now left?

What you have proved so far is that the alternator is capable of charging

It is getting 12V field voltage

And the output wire to the battery is probably OK

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What to do next?

Very first thing I'd try is dismount the VR, clean (scrape) around the mounting holes and remount using star lock washers. The VR MUST be grounded. MUST

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Next, look over the VR rubber connector. Are the terminals corroded? "Work" the connector on/ off the VR several times, feeling for "tightness" and "working" this helps scrub the terminals clean.

Run it and try again, see if it works

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If it does not, time for more (simple) tests. Please follow exactly. You are trying to determine if the field wiring is OK

Remove the VR connector. With the key in "run" check voltage from the VR harness connector terminals to ground. BOTH should show "same as battery."

Now "rig" a way to jumper across the two terminals in that harness connector. Machine screws are one way.

With the terminals jumpered, this time unhook the BLUE wire at the alternator field, and connect that alternator field terminal to ground.

Again, run the engine, looking for charging.

If it does charge, replace the VR

If it does not, you have a field wiring problem. Post back