Round back alternator

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1965barr

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Hi, I have a one wire field alt. Question is the one field across from the battery post is it supposed to be ground too? The one at the side is grounded and when checked with ohm meter the other one is too to the case . It was pulling 15 volt and the volt gauage was bouncing 13-15 V. For fun tried a square back one 90 amps volts was steady 14 when just fired.

46AD1524-FC34-484D-A8ED-8CB6F3F3912F.jpeg
 
They have different regulators, a square back reg wont work on a round back alt.
 
1965barr said:
The one at the side is grounded and when checked with ohm meter the other one is too to the case
Click to expand...
iirc the field is a coil with the other end to ground, it is what generates a magnetic field for the alternator. So an ohm meter would read close to shorted between field and the case.

How did you wire the square back?
 
I grounded one of fields to the case. I have a 65 Barracuda vr electronic upgrade , done the bypass volt , cracked back’s headlight upgrade with bigger wire from alt.,electronic dist. Mopar , had another round back 60 amp in there first trying to figure out why it was doing this checked everything and regrounded everything was new except dist. Finally took the first alt. To get checked while being checked it was in full field then nothing, got this one same thing I might be the lucky one finding bad refurbished alt. The square back is original 76 year that was upgraded to 90 amp 25 years ago I did. System is still one wire

C9270CE6-155F-4AA3-937C-925B6C55A7C8.jpeg


1B1C4FA6-749D-491E-8D5A-7A6647C5EB68.jpeg
 
Is the round back "new"? It could have a bad diode.
 
The single wire alt regulator varies the voltage on the one brush and the other brush is grounded to case. The 2 wire alt has battery voltage to one brush and the regulator varies the ground on the other brush.
 
You have other issues. Send the round back one to me it’s looks nice!
 
1965barr said:
Hi, I have a one wire field alt. Question is the one field across from the battery post is it supposed to be ground too? The one at the side is grounded and when checked with ohm meter the other one is too to the case . It was pulling 15 volt and the volt gauage was bouncing 13-15 V. For fun tried a square back one 90 amps volts was steady 14 when just fired.
Click to expand...
That particular alternator case was a pre-70.
The ground brush went here.
upload_2019-4-30_10-45-44.png

Regulation worked like this.
upload_2019-4-13_15-33-27-png.png

Yours has been modified for use with both brushes insulated from ground.
1970-72 Roundbacks and all squarebacks had insulated brushes because the regulator controls power for the field by connecting and disconnecting the ground.
See more alternator guts here: Alternator repair, a little show and tell.

The later revised squarebacks don't mount quite as neatly as the first gen squarebacks. Regular ones fit pretty well.
My experience, and Chrysler's own practice, is to take some extra pre-cautions when using the "60 amp" and higher alternators. This is true whichever way you choose regulate.
Be careful not to allow too much high rate charging (you can monitor this with your ammeter).
More about that:
Charging Battery with Alternator - Warning
 
Last edited:
HERE IS the story. Mopar alternaters WERE ALL "round back" up through maybe 72ish or so.

Round backs up through 69 had one insulated brush and one grounded brush

Round backs starting in 70 had TWO insulated/ isolated brushes and used the newer "flat" electronic regulator.

All square backs have two insulated/ isolated brushes

YOU CAN USE ANY isolated brush alternator with a 69/ earlier regulator simply be grounding either brush and hook the other to the regulator.

Some round backs as pictured had provisions for BOTH a grounded brush and two isolated brushes. THIS CAN BE BAD as I've seen "rebuilt" ones come through with ALL THREE BRUSHES installed. This can short the blue field power and burn up a harness

Worse, "rebuilders" used to DRILL THE CASE of older grounded brush roundbacks and install an aftermarket isolated brush so they can sell them as 70/ later. THESE ALSO can come with all three brushes installed
 
Manufactured roundback which can accept either a grounded brush or isolated brush

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Ugly old "rebuilt" roundback which has been MODIFIED for an isolated brush. NOT factory and NOT manufactured

128306-500-0.jpg
 
1965barr said:
Hi, I have a one wire field alt. Question is the one field across from the battery post is it supposed to be ground too? The one at the side is grounded and when checked with ohm meter the other one is too to the case . It was pulling 15 volt and the volt gauage was bouncing 13-15 V. For fun tried a square back one 90 amps volts was steady 14 when just fired.

View attachment 1715326503
Click to expand...

Simple answer is pick one brush and ground it, hook the rest up as per original
 
Thanks for your answers and reading , I did take this alt. Back to get checked and it was bad a diode , so they were kind to me and pick out a new one a square back 55 amps , tried it and it works just like my old original square back ,steady and only pulling 14.3 , yes I grounded one bush, if I were to turn this into a two wire alt which it is ,which bush is negative and positive? And would this be a good plan as I do have the parts just have to wire in. sorry 1969383S
 
1965barr said:
Thanks for your answers and reading , I did take this alt. Back to get checked and it was bad a diode , so they were kind to me and pick out a new one a square back 55 amps , tried it and it works just like my old original square back ,steady and only pulling 14.3 , yes I grounded one brush, if I were to turn this into a two wire alt which it is ,which brush is negative and positive? And would this be a good plan as I do have the parts just have to wire in.
Click to expand...

Brushes do not matter, either one. If you have the newer flat 70/later regulator they are more available, and original ones are better.
 
Tried to keep this car kind of original but would rather have reliable instead but it is all mopar. Thanks to everyone for the help
 
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