two field alternator connections

[moparjohnny]

my harness has one field wire to connect,i have a electronic voltage reg,point distributor,car cranks, run just fine since i had it with that one field wire connection,anything wrong with that set up? thanks 69 b body 383.

 

[Darthomas]

If only one wire is connected to a two connector alternator, no current will flow in the field and no output will be produced from the alternator.
One connection must go to the regulated voltage from the VR, the other must find ground, and if you have an electronic regulator, I would expect you to have two wire in the harness for field connections.
Even so, one is essentially a ground connection.
I have taken two wire alternators and grounded one brush internally to work with the old mechanical style regulators.

 

[bm02tj]

If you have a replacement electronic reg ground one brush if you have late model reg find the other wire

 

[moparjohnny]

If you have a replacement electronic reg ground one brush if you have late model reg find the other wire

this car cranks up and run just fine no problems with it having one field connected.

 

[funknut]

What alternator do you have? Are you sure it is a dual-field model? Some alternators have other connectors on the back besides BAT/FLD.

 

[moparjohnny]

What alternator do you have? Are you sure it is a dual-field model? Some alternators have other connectors on the back besides BAT/FLD.

squareback model,says fld on two prongs.

 

[Shorthorse]

If I read the wiring diagram correctly, the OEM setup was a single field alternator so your original harness will have a single dark green field wire in the harness. The Alternator you have is a newer dual field model so you are going to have to do some wiring. You didn't mention what voltage regulator you have. It makes a difference. Right now your car is not charging.
Honestly, if you have the original harness AND if you have the original voltage regulator AND especially if you are uncomfortable with wiring, go buy the correct alternator.

 

[Pops1967GTX]

On the newer style dual field alternator ,Grounding either field lead will do...
The green field wire from your V reg. hooked up to one terminal. Ground the other field terminal to the alternator case.

I got this from Nacho-RT74 FBBO...

 

[Shorthorse]

Sorry moparjohnny, I missed the part where you said you have the electronic regulator. You need to add the dark blue tracer. It would be connected as pictured or really any 12v source that is activated when the key is in the on/run position.

 

[Nacho-RT74]

On the newer style dual field alternator ,Grounding either field lead will do...
The green field wire from your V reg. hooked up to one terminal. Ground the other field terminal to the alternator case.

I got this from Nacho-RT74 FBBO...

this pic with legend I made ( posted by Pops1967GTX ) is to make the earliers alt conversion to a laters system

Reason why somebody wants to make the conversion I explained there:
-you can keep your actual earlier alt but upgrade to electronic system ( making a cheaper upgrade )
-you can't find a correct 70/71 roundback alt with dual prong setup from factory and gets easily an earlier roundback, keeping the "correctness" on 70/71 system
-your car is any kind of electronic reg system, for example a squareback, but got broken and have handy and cheap an earlier roundback, without care if is the correct unir for the year or not.

Of course requires also the extra wiring ( blue wire added ) and regulator pigtail job


Now to become a dual prong alt into a single prong alt to make it work on mech regulator system is way easier, being enough ground one of the prongs, ( either one ). You can make it in two ways... Jumper wire between prong ( with a terminal ) and chassis AND my favorite one wich makes a clean assembly without any jumper wire added, just replacing the prong/brush isolation washer with a metallic one... And just for safety, cut the prong, to save from connect there accidentally the regulated positive source coming from regulator ( green wire ). This confusion could make a short

 

[Nacho-RT74]

Lets state how it works every system

Mech reg/single prong alt ( earlier roundbacks )

-Reg gets the reference voltages negative from chassis and positive from ign switch ( blue wire )
-once the voltage required is set ( via mechanical relay system inside regulator ) regulator send regulated POSITIVE source ( more or less according with what regulator senses ) to the single prong alternator through the green wire, which is a brush to feed the rotor. This brush is isolated from alternator chassis being a positive and regulated source.
-The other field, negative, is feeded to the second brush via chassis, since this is not isolated and of course is constant.

Electronic reg/dual prong alt ( 70/71 roundbacks and all squarebacks )

-Like the earliers, gets negative voltage from chassis and positive voltage from ign switch ( blue wire ).
-the difference on this system is the ign system positive source ( blue wire ) is spliced and reachs AT THE SAME TIME one of the alternator prongs, of course isolated from chassis. This time the constant source is positive, not anymore the negative.
-regulator sends to the second prong/brush on alternator a REGULATED NEGATIVE SOURCE through the green wire, so these are the reasons why both brushes are now isolated, one positive constant, the other a regulated negative source ( hence the reason why must be isolated from chassis too, because is not anymore constant )

 

[funknut]

Have you verified that it is actually charging? What voltage do you read across the battery posts when running?

 

[1968RRTEXAS]

I have the same setup, single field voltage regulator with a two prong alternator. I know my charging system isn't working. So all I need to do is make a wire with a female terminal, attach it to the 2nd field connector on the alternator, then ground it to the alternator frame? Correct?

 

[Darthomas]

I have the same setup, single field voltage regulator with a two prong alternator. I know my charging system isn't working. So all I need to do is make a wire with a female terminal, attach it to the 2nd field connector on the alternator, then ground it to the alternator frame? Correct?

Yep!

 

[1968RRTEXAS]

Yep!

I'm freaking stoked. I've been trying to figure this out for months.

 

[funknut]

just replacing the prong/brush isolation washer with a metallic one... And just for safety, cut the prong, to save from connect there accidentally the regulated positive source coming from regulator ( green wire ). This confusion could make a short

This is very clever, I like it!

 

[moparjohnny]

This is very clever, I like it!

the field terminal that is not connected has a larger post,what size connector do i need to fit down over the field connector.

 

[Darthomas]

Never noticed different size lugs, hmmmm.
May be easier to remove insulating washer from one brush, whichever does not match your wiring harness connector.
Electrically it doesn't matter which one gets tied to ground,

 

[1968RRTEXAS]

Never noticed different size lugs, hmmmm.
May be easier to remove insulating washer from one brush, whichever does not match your wiring harness connector.
Electrically it doesn't matter which one gets tied to ground,

How hard or how to remove the insulating washer from one brush. Is there a write up on this? It sounds like that would have a cleaner look.

 

[funknut]

I had an alternator in the garage, so I dug it out and took a few pics. The first is no surprise, it's just the field terminal as installed in the alternator:

Here I've removed the 1/4" screw holding the field terminal assembly to the alternator, removed the terminal assembly from the alternator and reinstalled the washer and screw just so you can get an idea of how things work together when installed in the alternator. The white piece that contains the brush is plastic, so it insulates the terminal from the alternator housing.

Now for the exploded view:

The washer is also an insulator, so between the white plastic piece and the insulating washer, the field terminal cannot ground to the alternator housing. To ground the field terminal, you can simply replace the insulator washer with a metal washer. This allows the field terminal to ground to the alternator housing.

Nacho further suggested to clip off the field terminal so nobody would accidentally try to connect a wire to it. Very good advice!