Here is the basic, simplified 70 / later VR and isolated field alternator diagram
Here is basically what you should have
From the ignition switch, top of diagram, power comes down to ballast, junctions and branches off and feeds battery power to one field terminal (matters not which) and to the "I" (IGN) terminal of the VR
The VR MUST be grounded.
The green "F" wire comes out of the VR and goes to the remaining alternator field. This wire controls the 'amount of ground' so to speak on the field.
So ignition (switched 12V) feeds the field, and the green F wire controls the ground side.
There are several possibilities:
The VR may have lost ground
The VR connector may be internally broken or corroded.
The brushes / field may not be continuous
The alternator may have a different problem.
The battery charging wire from the output of the alternator to the battery may be loose / intermittent/ broken. If you have factory wiring, this is suspect
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To get started. Remove the green field wire from the alternator field Disconnect the VR plug. If colors are wrong, then pick one, turn the key to run, and see if the remaining field wire still hooked up has 12V. If not, pull that wire off and put the other back on, and recheck
At this point you should have the key in "run," you should have one field wire dangling, and you should have 12V at the one left connected.
Now take a clip lead and connect to the bare field you just unhooked. Touch it to ground, if possible, in subdued light. You should see a small spark. Hook it up
RE measure power at the opposite field wire. You should still have 12V.
Start the engine, slowly bring up RPM and see if it charges. If it does, great
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Now, re--connect the other field wire, so both are "normal" at the alternator. Move to the VR connector. With the key in "run" both terminals at the VR connector should have 12V. "Make" a jumper (nails, small screws) that will contact the VR plug terminals, and jumper them together.
Go back to the alternator. Disconnect the field wire which you had hooked up in the first test. If colors are correct, this is blue Ground that alternator terminal.
Now check for power, key on, at the green wire field terminal. You should again have 12V. Repeat the charge test. If it charges, these two tests show the field circuit wiring is OK
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If in the first test, you got no output, this might be
A bad alternator
Or a broken output charging wire
Unhook both field wires. Take one clip lead and ground either field. Take a second clip lead and run over to the "big stud" on the starter relay. Repeat the engine run test
No charge?
With the engine running at a fast idle, check battery voltage. Below 13.XX v means no charge. If it's low, probably 12. something, move your meter over to the alternator stud. If it is about the same as battery, then you have a bad alternator
If this stud voltage is very high, varies with RPM, might be VERY high IE over 17V you have a broken output wire problem
Post back here. I may be gone a day or two. Got some things came up here
