Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss

One wire alternator question

13 views
Skip to first unread message

Ivan Vegvary

unread,
May 8, 2023, 11:58:04β€―PM5/8/23
to
1948 Pontiac, 12V battery, one wire alternator (was working when parked 12 years ago).
No amperage reading even at 2000 rpm. Battery voltage 12.6V. Battery voltage with car running 13.8V, however amp meter only reads 0 or negative with headlights on. Placed an after market ammeter between the alternator and the battery. Same reading as ammeter in the dash.
How could this be? Alternator supplies voltage but no current.
After idle for 12 years maybe residual magnetism is gone. Needs to be excited. Can't find a YouTube that shows me which terminals to zap.
All help is appreciated. I can certainly buy a new unit, but possibly have other problems.

Thank you

Michael Trew

unread,
May 9, 2023, 12:45:08β€―AM5/9/23
to
That's a tough one... if it's been sitting for 12 years, you've got to
start with the basics. Have you cleaned all of the grounds? Is the
terminal for the wire on the alternator clean? In 1948, it would have
been a 6 volt positive ground system. I'm assuming that someone has
since converted the car to 12 volt negative ground.

😎 Mighty Wannabe βœ…

unread,
May 9, 2023, 9:36:32β€―AM5/9/23
to
If you get 13.8V when the car is running, that means the alternator is
providing output.

The alternator output is AC. You need to use AC ammeter to measure the
AC current output from the alternator.


Xeno

unread,
May 9, 2023, 10:40:58β€―AM5/9/23
to
Correction, the alternator output is DC. The 6 internal power diodes
provide the necessary rectification. The 3 exciter diodes are meant to
excite the field coils and set the generation process into gear.

--
Xeno


Nothing astonishes Noddy so much as common sense and plain dealing.
(with apologies to Ralph Waldo Emerson)

😎 Mighty Wannabe βœ…

unread,
May 9, 2023, 1:05:32β€―PM5/9/23
to
Are you sure that's true for 1948 Pontiac?

Maybe that alternator had been modified to use external rectifiers and
regulator.



Paul in Houston TX

unread,
May 9, 2023, 6:45:10β€―PM5/9/23
to
There must be current... but not a whole lot.
Possible corrosion... some where.
the alternator wire at alt or batt.
alt slip rings corroded.
alt brushes stuck.
internal corrosion at the diode plates.
bad alt ground.

Ivan Vegvary

unread,
May 9, 2023, 9:45:28β€―PM5/9/23
to
Thanks all of you for your thoughtful responses. Will check and clean all connections.

Xeno

unread,
May 10, 2023, 12:55:19β€―AM5/10/23
to
Then it wouldn't be a single wire alternator, would it? Think about that
for a moment. Alternators are, typically, 3 phase. That means 3 +ve
power diodes, 3 -ve power diodes and 3 exciter diodes. If there is only
one output wire, then all *rectification* and control has to be on-board.

😎 Mighty Wannabe βœ…

unread,
May 10, 2023, 9:39:11β€―AM5/10/23
to
If the alternator really has DC output, then the car's ammeter and
external ammeter should register DC current.

I believe somehow the alternator burnt the internal diodes, and the
owner rewired it as single phase output and used external rectifier
after the car's dashboard ammeter. That means the dashboard ammeter has
not been working properly ever since. A germanium diode was used so some
reverse current is flowing back into the alternator when the engine is
not running but the key is on auxiliary. That's when and why the
dashboard ammeter registers negative.



😎 Mighty Wannabe βœ…

unread,
May 10, 2023, 8:35:28β€―PM5/10/23
to
> If the alternator really has DC output, then the car's [DC] ammeter
> and external [DC] ammeter should register DC current.
>
> I believe somehow the alternator burnt the internal diodes, and the
> owner rewired it as single phase [AC] output and used external [bridge
> rectifier] after the car's dashboard [DC] ammeter. That means the
> dashboard [DC] ammeter has not been working properly ever since
> [because it was AC coming from the alternator]. A germanium [bridge
> rectifier] was used so some reverse current is flowing back into the
> alternator when the engine is not running but the key is on auxiliary.
> That's when and why the dashboard ammeter registers negative.
>


I've some made minor corrections in square brackets [Β Β Β  ] above to
clarify my points.

I think I have the correct line of thinking.


Scott Dorsey

unread,
May 11, 2023, 12:08:59β€―PM5/11/23
to
Xeno <xeno...@optusnet.com.au> wrote:
>
>Then it wouldn't be a single wire alternator, would it? Think about that
>for a moment. Alternators are, typically, 3 phase. That means 3 +ve
>power diodes, 3 -ve power diodes and 3 exciter diodes. If there is only
>one output wire, then all *rectification* and control has to be on-board.

When someone says "GM 1-wire alternator" they are talking about a series
of alternators introduced in 1970 or so, starting with the Delco 10SI and
including some more modern follow-ons. These alternators use internal
rectification and regulation (really just regulating pulses to the field
coil) and are very, very foolproof.

Because of this they are frequently retrofitted into older cars. I see
them on all sorts of older British cars as part of a conversion to a
modern 12V electrical system.

There isn't much to go wrong other than having the alternator go bad.
Mind you it is possible that the original poster's problem is that they
have a three-wire alernator or an alternator with external mechanical
voltage regulator and none of the other wiring is connected.

But if it is, breaking the connection between the alternator and the
battery and putting an ammeter in there should certainly show some
current. If not... maybe the alternator is not actually connected to
the battery but is going through an open shunt for the meter that isn't
working or something like that.

With cars that have been modified like this you don't get a proper wiring
diagram so you are going to need the continuity tester to see what is
going where. And yes checking the frame strap is always the first thing
to do (and make sure the frame strap isn't bolted to a solid block of bondo
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Snag

unread,
May 11, 2023, 8:48:47β€―PM5/11/23
to
It must be quite difficult to blow smoke up somebody's ass with your
head so far up your own . Ivan , ignore everything this fool said . Pull
the alternator and take it to an auto parts store where that can test it
. Usually free , in hopes you'll buy a new one .
--
Snag
"You can lead a dummy to facts
but you can't make him think."

😎 Mighty Wannabe βœ…

unread,
May 11, 2023, 10:49:57β€―PM5/11/23
to
Do you have a better explanation for a DC ammeter not registering any
amperage at the alternator other than the alternator is outputting AC?

The OP should try using AC ammeter to confirm that the alternator had
been rewired to output AC, and external germanium bridge rectifier is used.



Snag

unread,
May 11, 2023, 11:11:22β€―PM5/11/23
to
The simplest explanation is that the slip ring brush isn't making the
circuit to excite the rotor windings . That alone would explain the
voltage reading 12.8V while running - and slightly higher when stopped
and there is no current draw for stuff like ignition and instruments .
The next simplest would be that the internal regulator is bad , also
resulting in a lack of charging .

😎 Mighty Wannabe βœ…

unread,
May 11, 2023, 11:37:15β€―PM5/11/23
to
Your explanation cannot explain the phenomena the OP had observed:

"No amperage reading even at 2000 rpm.Β  Battery voltage 12.6V. Battery
voltage with car running 13.8V, however amp meter only reads 0 or
negative with headlights on. Placed an after market ammeter between the
alternator and the battery.Β  Same reading as ammeter in the dash.
How could this be?Β  Alternator supplies voltage but no current."

Apparently the alternator is charging the battery. My explanation,
albeit convoluted, can explain exactly how it is possible. The previous
owner didn't want to buy a new alternator and used this convoluted way
to get by. The previous owner wired the alternator to output AC, and
used external germanium bridge rectifier to get DC. My thought
experiment can explain all the observed phenomena in this case.




Xeno

unread,
May 13, 2023, 5:33:58β€―AM5/13/23
to
Ah, a thought experiment! That explains why you don't get it.

😎 Mighty Wannabe βœ…

unread,
May 13, 2023, 6:39:43β€―AM5/13/23
to
Then how do you explain the alternator can charge the battery with no
apparently DC current output?


Xeno

unread,
May 13, 2023, 7:06:05β€―AM5/13/23
to
I can explain it but the issue is with you understanding it. A WOFTAM.

Xeno

unread,
May 13, 2023, 7:11:57β€―AM5/13/23
to
On 12/5/2023 2:08 am, Scott Dorsey wrote:
> Xeno <xeno...@optusnet.com.au> wrote:
>>
>> Then it wouldn't be a single wire alternator, would it? Think about that
>> for a moment. Alternators are, typically, 3 phase. That means 3 +ve
>> power diodes, 3 -ve power diodes and 3 exciter diodes. If there is only
>> one output wire, then all *rectification* and control has to be on-board.
>
> When someone says "GM 1-wire alternator" they are talking about a series
> of alternators introduced in 1970 or so, starting with the Delco 10SI and
> including some more modern follow-ons. These alternators use internal
> rectification and regulation (really just regulating pulses to the field
> coil) and are very, very foolproof.
>
> Because of this they are frequently retrofitted into older cars. I see
> them on all sorts of older British cars as part of a conversion to a
> modern 12V electrical system.
>
> There isn't much to go wrong other than having the alternator go bad.

What used to happen to them back in the 70s was that heat and vibration
would damage the regulator - even with its encapsulation. A separate
regulator mounted on the inner guard or firewall and in the underhood
airflow was a much better arrangement.

> Mind you it is possible that the original poster's problem is that they
> have a three-wire alernator or an alternator with external mechanical
> voltage regulator and none of the other wiring is connected.

Not enough detail in the OPs post to really know what's what.
>
> But if it is, breaking the connection between the alternator and the
> battery and putting an ammeter in there should certainly show some
> current. If not... maybe the alternator is not actually connected to
> the battery but is going through an open shunt for the meter that isn't
> working or something like that.
>
> With cars that have been modified like this you don't get a proper wiring
> diagram so you are going to need the continuity tester to see what is
> going where. And yes checking the frame strap is always the first thing
> to do (and make sure the frame strap isn't bolted to a solid block of bondo
> --scott
>

--

😎 Mighty Wannabe βœ…

unread,
May 13, 2023, 7:19:09β€―AM5/13/23
to
It is very simple that you are full of shit. If the alternator has DC
output then inserting a DC ammeter like the OP did should register DC
current. You simply cannot think outside the box.

Come on. Make my day. Explain why there is no apparent DC current coming
out of the alternator but it can charge the battery.

Take you time, here =====>



😎 Mighty Wannabe βœ…

unread,
May 13, 2023, 7:26:13β€―AM5/13/23
to
On 5/13/2023 7:09 AM, Xeno wrote:
> On 12/5/2023 2:08 am, Scott Dorsey wrote:
>> XenoΒ  <xeno...@optusnet.com.au> wrote:
>>>
>>> Then it wouldn't be a single wire alternator, would it? Think about
>>> that
>>> for a moment. Alternators are, typically, 3 phase. That means 3 +ve
>>> power diodes, 3 -ve power diodes and 3 exciter diodes. If there is only
>>> one output wire, then all *rectification* and control has to be
>>> on-board.
>>
>> When someone says "GM 1-wire alternator" they are talking about a series
>> of alternators introduced in 1970 or so, starting with the Delco 10SI
>> and
>> including some more modern follow-ons.Β  These alternators use internal
>> rectification and regulation (really just regulating pulses to the field
>> coil) and are very, very foolproof.
>>
>> Because of this they are frequently retrofitted into older cars.Β  I see
>> them on all sorts of older British cars as part of a conversion to a
>> modern 12V electrical system.
>>
>> There isn't much to go wrong other than having the alternator go bad.
>
> What used to happen to them back in the 70s was that heat and
> vibration would damage the regulator - even with its encapsulation. A
> separate regulator mounted on the inner guard or firewall and in the
> underhood airflow was a much better arrangement.
>

The OP said it can charge the battery to 13.8V, can't you read?

>> Mind you it is possible that the original poster's problem is that they
>> have a three-wire alernator or an alternator with external mechanical
>> voltage regulator and none of the other wiring is connected.
>
> Not enough detail in the OPs post to really know what's what.

I have already provided full explanation. You are too boneheaded to
understand.

Xeno

unread,
May 13, 2023, 7:45:12β€―AM5/13/23
to
The voltage can get to 13.8 *without* pumping much current. Voltage is
*pressure* whereas *current is flow*.
>
>>> Mind you it is possible that the original poster's problem is that they
>>> have a three-wire alernator or an alternator with external mechanical
>>> voltage regulator and none of the other wiring is connected.
>>
>> Not enough detail in the OPs post to really know what's what.
>
> I have already provided full explanation. You are too boneheaded to
> understand.

You have provided proof that you haven't a clue. HTH
>
>>>
>>> But if it is, breaking the connection between the alternator and the
>>> battery and putting an ammeter in there should certainly show some
>>> current.Β  If not... maybe the alternator is not actually connected to
>>> the battery but is going through an open shunt for the meter that isn't
>>> working or something like that.
>>>
>>> With cars that have been modified like this you don't get a proper
>>> wiring
>>> diagram so you are going to need the continuity tester to see what is
>>> going where.Β  And yes checking the frame strap is always the first thing
>>> to do (and make sure the frame strap isn't bolted to a solid block of
>>> bondo
>>> --scott
>>>
>>
>

😎 Mighty Wannabe βœ…

unread,
May 13, 2023, 8:11:12β€―AM5/13/23
to
You don't have the education or practical knowledge to analyze this
abnormal situation. You still cannot answer the question why the OP says
there is no apparent DC current from the alternator but it can charge
the battery to 13.8V when the engine is running.

I don't think you can think. Pity.


Xeno

unread,
May 13, 2023, 9:05:23β€―AM5/13/23
to
It has already been mentioned. It's either high resistance or a faulty
regulator. I'd be checking connections for resistance and continuity
before I condemn the regulator. Testing current with a load, ie.
headlights on, is the usual practice. I used to use a large dummy load,
to test alternator and generator output. I'd also give it a big rev up
since the field coils lose their residual magnetism through long periods
of idleness and a big rev up *usually* provides sufficient self
excitation to kick it off. I have seen the regulators on these fail 50
years ago in the 60s & 70s, as mentioned in another post, when I was
working in the area these things were designed for - farm machinery. I
personally prefer the two wire units as the 2nd wire provides external
field excitation - and you can see that it is doing so by the charge
light on the dash. Lights up with Ign On, Eng Off, goes out when Eng Run
since the light get a +ve at both ends and current through the lamp
ceases. FWIW, the regulators on these typically fail when starting or
stopping the engine so it's quite feasible that it was Ok 12 years ago
when parked. Just thinking about that, it's probably been some 30 or 40
years since I have even seen one of those single wire alternators.

😎 Mighty Wannabe βœ…

unread,
May 13, 2023, 9:28:13β€―AM5/13/23
to
If it has high resistance, and connecting after market [DC] ammeter to
the alternator registers cannot detect [DC] current, then it won't be
able to charge the battery to 13.8V and next time the battery won't have
enough juice to crank the motor. Do you have no brain?


> or a faulty regulator. I'd be checking connections for resistance and
> continuity before I condemn the regulator. Testing current with a
> load, ie. headlights on, is the usual practice.

The OP already said there is no detectable [DC] current from the
alternative when the headlights are on. Can't you read?

> I used to use a large dummy load, to test alternator and generator
> output. I'd also give it a big rev

The OP already said "No amperage reading even at 2000 rpm". Can't you read?

Remainder of your bullshit snipped. Just answer to above question and
you will realize that you are stupid.



Scott Dorsey

unread,
May 13, 2023, 4:03:38β€―PM5/13/23
to
Xeno <xeno...@optusnet.com.au> wrote:
>On 12/5/2023 2:08 am, Scott Dorsey wrote:
>> There isn't much to go wrong other than having the alternator go bad.
>
>What used to happen to them back in the 70s was that heat and vibration
>would damage the regulator - even with its encapsulation. A separate
>regulator mounted on the inner guard or firewall and in the underhood
>airflow was a much better arrangement.

Right, and modern silicon can be made a whole lot more tolerant. The
original design used SCRs for switching because transistors were too
fragile, but now we have better transistors and even IGBTs for abuse at
high temperatures.

>> Mind you it is possible that the original poster's problem is that they
>> have a three-wire alernator or an alternator with external mechanical
>> voltage regulator and none of the other wiring is connected.
>
>Not enough detail in the OPs post to really know what's what.

This is precisely the issue, yes.

Scott Dorsey

unread,
May 13, 2023, 4:04:19β€―PM5/13/23
to
=?UTF-8?B?8J+YjiBNaWdodHkgV2FubmFiZSDinIU=?= <@.> wrote:
>
>You don't have the education or practical knowledge to analyze this
>abnormal situation. You still cannot answer the question why the OP says
>there is no apparent DC current from the alternator but it can charge
>the battery to 13.8V when the engine is running.

I think we have all agreed that the original poster is not measuring
something right, and it's probably the current, but details are scanty.

Paul in Houston TX

unread,
May 13, 2023, 5:53:02β€―PM5/13/23
to
The op never said that the alt would charge the batt to 13.8v.
The op never said what kind of ammeter or voltmeter.
A $10 ammeter would likely not show milliamps.

Xeno

unread,
May 13, 2023, 7:27:07β€―PM5/13/23
to
Too many unknowns to be definitive. It takes very little current to keep a
battery charged but if the alternator is outputting sufficient current to
cover standard parasitic loads plus a wee tad more to provide a trickle
charge, the battery won’t go flat. Loading the system with, say,
headlights, would tell a different story.

____
Xeno

😎 Mighty Wannabe βœ…

unread,
May 13, 2023, 9:50:04β€―PM5/13/23
to
Then find your reading glasses and read again.


>
> The op never said what kind of ammeter or voltmeter.


The OP said the external ammeter corroborates with the car's dashboard
ammeter. Can't you read?


>
> A $10 ammeter would likely not show milliamps.
>

NO car ammeter deals with milliamp. Do you have a brain?


Xeno

unread,
May 14, 2023, 5:53:15β€―AM5/14/23
to
This is what the OP actually said;

>>>> Battery voltage 12.6V. Battery voltage with car running 13.8V

A battery at 12.6V is *not* a battery that has just been charged.
Typically a battery that has just come off charge will have a voltage
*higher* than 12.6V because of surface charge. Once that surface charge
has has dissipated, a *good* battery should remain at ~12.6V.

If the engine is running, you will *not* be reading *battery voltage*.
You will be reading *system voltage* which, typically, should be between
13.8V-14.2V. IOW, what you are seeing is the alternator output voltage.
FWIW, cars do not run 12 volt electrical systems. They actually run 14
volt systems. The battery is, in general, a load for the electrical
system and provides a means of starting the engine.
>
>>
>> The op never said what kind of ammeter or voltmeter.
>
>
> The OP said the external ammeter corroborates with the car's dashboard
> ammeter. Can't you read?
>
Not the issue.
>
>>
>> A $10 ammeter would likely not show milliamps.
>>
>
> NO car ammeter deals with milliamp. Do you have a brain?

No need when the typical alternator of this era was capable of 30-40
Amps output. Output at 1 amp would be a *float charge*.

😎 Mighty Wannabe βœ…

unread,
May 14, 2023, 7:31:51β€―AM5/14/23
to
Apparently I am conversing with a dumb arse. I have to keep repeating
myself and you still cannot think outside the box.

If it is simple then the OP wouldn't be confounded and have to post here
to find answers. Somebody had done something to the alternator that he
did know.

The OP definitely said there is no apparent current from the alternator,
but the battery voltages act normal like a battery voltages in a normal
car would.

The only way to explain it, is that the alternator was modified to
output AC, and a germanium bridge rectifier was used external to the
alternator.

My explanation can explain all the abnormal phenomena like no DC current
from the alternator detected by the ammeter, and reverse DC current
detected when the engine is not running but the car key is on.

I am sick and tired of repeating myself to a dumb arse who cannot think
outside the box. You should read the OP's first post carefully and read
my replies to his post carefully.

Goodbye and please don't reply again.



Xeno

unread,
May 14, 2023, 8:18:08β€―AM5/14/23
to
Promise????

😎 Mighty Wannabe βœ…

unread,
May 14, 2023, 8:35:00β€―AM5/14/23
to
Good. And stay stupid.


Xeno

unread,
May 14, 2023, 8:45:40β€―PM5/14/23
to
You lied

____
Xeno


Paul in Houston TX

unread,
May 14, 2023, 9:44:13β€―PM5/14/23
to
You obviously have no knowledge of basic electricity or electronics and
are just a troll.
By the way, there are no germanium power rectifiers.

😎 Mighty Wannabe βœ…

unread,
May 14, 2023, 10:35:09β€―PM5/14/23
to
Before silicon there were only germanium rectifiers. It was an old car
so most likely the person who modified the car used germanium bridge
rectifier. A germanium rectifier has lower forward voltage, so in high
current application germanium rectifier will consume less power and
generate less heat. That makes it more likely to be used in cars because
of the high current and low 12V battery. Germanium rectifier has higher
reverse leakage current than silicon rectifiers. In this case it
explains perfectly why the OP says the ammeter has negative current flow
when the engine is not running but the key is on.

Apparently you have no imagination to diagnose abnormal situation like
this. The OP says there is no apparent [DC] current from the alternator
but the battery behaves like normal with a rest voltage and a running
voltage. And there is reverse [DC] current when the car is not running
but the key is on.

The reverse current is the current from the battery leaking through the
germanium bridge rectifier back into the alternator coil.

As I have explained in detail in my previous posts in this thread, long
time ago somebody did not fix the alternator problem, and simply used
the AC from the alternator to connect to a germanium bridge rectifier
hidden somewhere inside the dashboard.

By the way, my education background is electrical engineering in university.


Xeno

unread,
May 14, 2023, 11:44:07β€―PM5/14/23
to
I hate to break this to you but *imagination* plays no role in
diagnosis. We mechanics deal with the *facts* of any situation.


this. The OP says there is no apparent [DC] current from the alternator
> but the battery behaves like normal with a rest voltage and a running
> voltage. And there is reverse [DC] current when the car is not running
> but the key is on. >
> The reverse current is the current from the battery leaking through the
> germanium bridge rectifier back into the alternator coil.
>
> As I have explained in detail in my previous posts in this thread, long
> time ago somebody did not fix the alternator problem, and simply used
> the AC from the alternator to connect to a germanium bridge rectifier
> hidden somewhere inside the dashboard.
>
> By the way, my education background is electrical engineering in
> university.

You wasted the money!
0 new messages