I'm running a dual field alternator on a 69 super bee with the original wiring harness. I think you're supposed to ground the second field terminal. Where is the best place to run the ground to?

Shortest route is to the Alternator itself. Make a jumper from either 1/4" spade terminal to the Ground spot.

What Pynzo said, ground it with a ring terminal under one of the (3) mini case bolts & a short pigtail & 1/4" female terminal. OR remove the insulating washers on the brush holder etc so the brush is now grounded to the case nub which'll give you a neater appearance. Dont forget that the brush is grounded if you ever go back to an isolated field circuit!

Guys . . .remember that the field is setting up a magnetic field that the rotor (connected to the shaft which is turned by the belt) moves around. It needs a ground connection and positive connection. The voltage regulator senses the output voltage and varies the amount of voltage and current going to the field.

The stator is the stationary coil of wire around the outside of the rotor. As the rotor turns, the magnetic field induces current in the stator wires - although it is producing alternating current (AC, which goes positive and then negative, just like in the house), in what is called 3 phase (house is single phase).

Since the current is alternating between negative and positive, you have to use diodes to rectify the current into DC (direct current) that is used in the car. With 3 phases, you need 3 positive and 3 negative diodes. When you combine the output of each 3 diode sets, you then have 12volt DC current output from the alternator. This is a more efficient producer of electrical energy than a generator - i.e. it puts out current at low rpm (like at idle), that is why it was a standard years ago.

So with the single wire field alternator, one of the field connections was directly connected to ground and the other to positive from the voltage regulator.

For the dual wire field alternator, one wire (positive) is connected to the ignition circuit, and then the other wire is connected to the voltage regulator - which then varies the amount of current via resistance to ground.

You can test your alternator yourself (briefly), by putting battery positive to the field wire from the alternator (usually a blue with white tracer) and then ground the other field terminal. You should see FULL output and charging on your ammeter (if you still have one) . . . . if this is the case, then you have a bad regulator. If not, then get another alternator. Needless to say, this is with the engine running.

I have also heard of taking a screwdriver (metal one) and holding it to the back of the alternator, where the rear bearing is. If it is held in place, then the alternator is working, as a magnetic field is present from the rotor.

So the back to the question at hand - why would reversing the field wires on the test stand make a difference ??? You are basically causing the rotation of the magnetic field to be set up 180 degrees out - doesn't matter much as you are rectifying the AC signal back to DC through the diodes, however, you are getting one set of diodes working before the others. But it really doesn't matter, so long as you have output from the alternator

Hope that makes it all clear as mud . . .

Cheers

Mark


I found this post from 2003. I hope this helps. I'm not sure how to acknowledge the original poster, but I think Mark does a good job explaining how the system works...

Interesting. Then the original poster has to determine if his car has wiring for a dual field alternator or a one wire. If it is a one wire, then a jumper is necessary to ground one of the fields. If it is a two wire,(one green wire and one blue wire with white tracer), then both wires get hooked up to the field terminals and no grounding is necessary....cool.
Thanks

My alternator wiring is for one field terminal as original and the wiring harness is new. I'm using a later 78 amp, dual field alternator. I now know that the second field terminal has to be grounded, somewhere. Now, to follow up, is there any voltage to ground from that second field connection? I'm getting some, so I hope so.

you have the early box reg & only a green wire from reg to alt correct? plug in the green wire female terminal to one of the alt field terminals & ground the other male field terminal & you're set. Amperage comes to the alt internal field via the green wire/enters either field terminal that you are plugged into & come out at the other field terminal that you are to be grounding. Yes the terminal will have some voltage & the reg determines how much or how little depending on the voltage at the blue "sense" wire at the reg on the other side of the "box".

Thank you for your information. I have it hooked up exactly as you described. Thanks again.