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Arlene Spadaro
Arlene Spadaro, Electrical Engineer - Automotive
Category: Car
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Ford: one wire..there is the small screw on the alternator that I

Customer Question

How do you wire a ford motor 351w alternator when there is only one wire attached and the others are cut? The only wire attached is the large battery one. I need to know what to do with the white wire on the plugin thats been cut and there is the small screw on the alternator that I believe says field that needs a wire.
Submitted: 12 years ago.
Category: Car
Expert:  Arlene Spadaro replied 12 years ago.

My answers are general in nature and not for your specific unit.

Alternators transfer mechanical energy to electrical using variable reluctance.

You need to supply a small field current to set up a magnetic field for
the unit to work. The field is regulated to supply the proper
output current to the battery.

What this all means is that the large wire you have attached is to go
to the battery and the field wire will go to an external
regulator. That regulator must also be powered from the battery
and so it will have a ground connection( may be through the frame of
the regulator), a battery connection , and a field connection to the

THe regulators job is to watch the battery voltage so that when the
battery voltage drops , it will supply a field voltage or current to
the alternator. The alternator then takes the field current and
amplifies it and supplies a much larger current through the large wire
to the battery. When the battery voltage is too high , the
regulator will turn off the field and in turn , the alternator will
stop supplying the battery current through the large wire.

Now , for your unit. Attach the alternator to the motor and start
it up. Without field attached, there should be no current to the
battery. You test this by directly measuring the large wire
current of indirectly by looking at the battery voltage. Apply
the power to the field by either grounding the alternator field OR by
attaching it to 12 volys from the battery. One of these
connections will provide full alternator current and you will see the
battery voltage go high and hear the alternator load down and
whine. Don't hold the field on too long. Now attach the
field to the field connection of the regulator and power the
regulator. You should now see the battery voltage slowly run up
to 14.0 to 14.5 volts and no higher when it is all working
properly. Turning lights and fans on in the car will cause the
voltage to drop some but it should not go below 13.8 volts.

Other connections from the alternator plug are used to supply the alternator light on the dash board.