Alternator charging Question

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ram250098

ram250098

69 Barracuda
Joined
Nov 25, 2007
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Location
Cookstown Ontario Canada
I have a nice spare dual field alt kicking around that has some gremlins. I had a 72 swinger that had a perfectly good working alt but thought I would swap it out for the nice new looking one. When I did it was over charging, into the 18volt range. I took it in and it tested fine {twice}, tried it in the car again and still 18volts. So I replaced it with the old one. Shoot ahead a year and now I have a 73 Cuda with a ugly old alt "that works fine"...yup put the nice shiney one in and still the same problem...18volts. The guy who rebuilds them could not tell me the problem...I am sure it is not the cars wiring since it has been in two cars and still over charges with the wires hooked up the same way.
What it the alts problem???? would be nice to fix it.
Thanks
Al
 
This is easy. All (what you are calling dual field) "isolated field" alternators have a winding for the field, each end is hooked to an insulated brush.

THERE SHOULD BE NO continuity to ground from either brush holder

There are about three possibilities:

ONE A chicken-sh$$ rebuilder grounded one of the brushes to allow it to be used in a 69/ earlier car

TWO One of the brush holders is wrong, damaged, or missing a piece and is shorting to ground

THREE The winding itself is shorted to the frame of the rotor.

Easy to find out. Use your test lamp and a battery, or an ohmeter

Neither brush should have continuity to ground. Try removing one brush at a time (and examine them) and see if the short is removed.
 
Thanks for the info 67DART. Without putting the alt back into the car, what is the best way to bench test it. I put my meter to continuity test setting I get the beep across both field spades then also each field spade to the alt body.
I can't remember what car I got it from but I guess it could have been converted to a single field. The guy I took it to is a very reputable rebuilder and a classic car buff, I am kinda surprised he didn't pic up on it LOL
He told me to bring the car over and swap it out and he would look at it..sounds like a bit of a waste of time when a rebuilder should know what the problem is to begin with!!!!!!or am I out to lunch on this? LOL
AL
 
When the rebuilder checked my alt do they check the fields as well? Could one be grounding out inside the case or do they just check the reading at the stud?
Thanks
AL
 
Don't have an answer. In a previous life I sold auto parts. It used to be way too common to see the old 69/ earlier units "converted" to isolated field, but sometimes they LEFT the original grounded brush installed!!!

I've also seen a couple of the newer isolated field units with broken/ damaged brush holders that effectively grounded on field.
 
That is what I think it has to be. When I told him I have two cars that are charging just fine, but when I swapped this other alternator into either one it over charges, why could a rebuilder not tell me what it is right away, I figured it would be a snap for him.
It has to be an internal problem, so it should be easy to fix it would think.
Thnaks again 67 Dart.
AL
 
Not a surprise. Old school rebuilders are just about gone. I see more guys in their early 30's running the show now.

The old round back alternator went away about 4 decades ago. Current model cars come with 200+ amp alternators. The kids in the auto electric business look at you funny when you mention a 37 amp alternator - "You mean a generator pops?"

Your DMM will beep with a continuity test between either brush & the case if one brush is grounded.

Set the meter on Ohms, and test from field terminal to field terminal. The result is the field resistance. That is all the kid at autozone (or canadian tire) will test, because the book does it that way.

Now, test from either field terminal to case ground. If the alternator is isolated field, there will be infinite resistance. If one terminal reads the field resistance, and the other terminal almost zero - the "zero" brush is grounded.

B.
 
I set my DMM to 200 ohms the one field jumps around 11.1 to 15.6 where as the other terminal reads 00.3.... So the 00.3 side is grounded? Is that the problem? What is it a brush or an isolater that does it?
Thanks B.
AL
 
You are not telling us where you have the probes

You should have continuity between the two brushes. This may vary a little if you turn the rotor due to the brushes moving.

But you should show "infinity" (open) between EITHER brush and the frame of the alternator
 
Sorry about that LOL
I put the red probe to the one field terminal and the black to the case, it read around 11.1 to 15.6, then I did the same to thwe other field terminal and it read 00.3.
Am I doing this right? LOL
I need to greatly expand my trouble shooting skills!!LOL
AL
 
Yup. The field reading .3 is shorted to the case.
 
Great thanks very much, the alt looks like brand new it must have been for a single feed. Funny the rebuilder couldn't tell me that LOL And I took in twice to test it...a quick test... that simple...why wouldn't they just do it *G*
Thanks again at least now I can tell him what it is.
So by that field gounding out it puts the alt into full output?
It's a good day...learned something new..
Maybe I will take the back off and see if it is something I can do!!!

AL
 
If that rebuilder does not understand that simple concept about Mopar alternators, then you need to find a new rebuilder.
 
If that rebuilder does not understand that simple concept about Mopar alternators, then you need to find a new rebuilder.
LOL I guess so *g*
Thanks again for all the help.
AL
 
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