I thought my Alternator was fixed!!!

B

Bruce Hart

Active member
Oct 18, 2011
1,414
6
38
:cry: Two weeks or so ago I announced that my alternator was working again after changing out the ignition switch which had lost a small spring out
of it. Well I have the same issue not getting a charge out of the alternator.

New development with what is happening. 12+ volts (battery state of charge) at the 12ga. wire. With ignition on I have .2 volts less than battery
charge on resister wire. When I plug into ports 1 and 2 voltage drops to 4 volts on resister wire and no output from alternator when engine is
started.

Had Napa check alternator out and it passed. Replaced alternator in coached with known good alternator, same results. Bowered a never installed
rebuilt alternator with the same results.

Any ideas as to what is causing the voltage drop?

Checked resistance of wire and got 10.4 ohms.

:x :cry: :cry: :x :( :( :(
--
Bruce Hart
1976 Palm Beach
Milliken, Co
 
Which 12GA wire? Where are you metering? Are you sure the isolator is good ?
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
re-enforcing John's comment. Where are you checking the alternator output voltage? The easiest place to check it is at the center terminal of the
isolator to the aluminum plate that the isolator is mounted on for the ground side of your meter.

If the voltage is low or non-existant then you have an alternator or wiring problem. Voltage should read around 14.7 vdc with the engine running and
nothing with the engine off.

Tell us what you are reading there and we can direct you which direction to head from there.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
No voltage at the isolator. I have checked and double checked the isolator.
I am measuring what is going into the alternator at the APC connection.
Back side of plastic housing I can put my meter probe there and get a
reading. That is three alternators that test good, two from Napa and third
was rebuilt with parts kit from Ebay.

Is 4 volts on resistance wire enough to enable alternator to produce a 14.7
charge?

12ga wire going into #2 post of alternator. Resistance wire going into the
#1 post of alternator. Gen light never goes out as well

On Sun, Sep 29, 2019 at 1:11 AM Ken Burton via Gmclist <

> re-enforcing John's comment. Where are you checking the alternator output
> voltage? The easiest place to check it is at the center terminal of the
> isolator to the aluminum plate that the isolator is mounted on for the
> ground side of your meter.
>
> If the voltage is low or non-existant then you have an alternator or
> wiring problem. Voltage should read around 14.7 vdc with the engine
> running and
> nothing with the engine off.
>
> Tell us what you are reading there and we can direct you which direction
> to head from there.
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class
 
With the key on but engine is not running, when I plug the apc into the #1
& #2 ports of the alternator is when I get the voltage drop

> No voltage at the isolator. I have checked and double checked the
> isolator. I am measuring what is going into the alternator at the APC
> connection. Back side of plastic housing I can put my meter probe there and
> get a reading. That is three alternators that test good, two from Napa and
> third was rebuilt with parts kit from Ebay.
>
> Is 4 volts on resistance wire enough to enable alternator to produce a
> 14.7 charge?
>
> 12ga wire going into #2 post of alternator. Resistance wire going into
> the #1 post of alternator. Gen light never goes out as well
>
> On Sun, Sep 29, 2019 at 1:11 AM Ken Burton via Gmclist <

>
>> re-enforcing John's comment. Where are you checking the alternator
>> output voltage? The easiest place to check it is at the center terminal of
>> the
>> isolator to the aluminum plate that the isolator is mounted on for the
>> ground side of your meter.
>>
>> If the voltage is low or non-existant then you have an alternator or
>> wiring problem. Voltage should read around 14.7 vdc with the engine
>> running and
>> nothing with the engine off.
>>
>> Tell us what you are reading there and we can direct you which direction
>> to head from there.
>> --
>> Ken Burton - N9KB
>> 76 Palm Beach
>> Hebron, Indiana
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>
>
> --
> Bruce Hart
> 1976 Palm Beach
> Milliken, Co
> GMC=Got More Class
>
>

--
Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class
 
Possibly a stupid question but it will help everyone know a few things.....do you have a field current to the alternator?
--
Rich Mondor,

Brockville, ON

77 Hughes 2600
 
with the engine running, try measuring the alternator output voltage right at the back of the alternator. You could have a poor/broken connection
between there and the Isolator.

I'm confused how you are measuring the voltages on Pins 1 & 2.

You said in your original posting:
"New development with what is happening. 12+ volts (battery state of charge) at the 12ga. wire. With ignition on I have .2 volts less than battery
charge on resister wire. When I plug into ports 1 and 2 voltage drops to 4 volts on resister wire and no output from alternator when engine is
started."

And in your last post:
"With the key on but engine is not running, when I plug the apc into the #1
& #2 ports of the alternator is when I get the voltage drop"

So it seems to me that you might be measuring the voltage BETWEEN pins 1 & 2. You should be measuring all voltages on the alternator with your
negative lead on GROUND (the case of the alternator or the engine block).

Pin 2 is the battery reference feedback voltage. It should show battery voltage whether or not the key is ON

Pin 1 is the excitation voltage (resistor wire). With key OFF it will be Zero volts. With Key ON you should read something less than battery voltage.
I've never seen a spec for this voltage, nor have I measured it myself, but the GEN light is wired in parallel with the resistor wire so I suspect
there needs to be about an 8 volt drop across the resistor wire to get the GEN lamp to light brightly... so maybe about 4 volts there is right.

You mention the APC. The APC simply adds a diode in series with the resistor wire (pin 1). It also adds an additional pair of connections which can
cause issues. Measure the voltages on both ends of the APC. Pin 2 wire should read battery voltage on both sides of the APC. Pin 1 wire should read
0.7 volts lower on the end plugged into the alternator (key ON). Try removing the APC to eliminate that as the problem.

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
Also, here is a link to a great PDF on the Delco alternator.

http://www.gmcmhphotos.com/photos/data/6522/Delco_Alternator.pdf

Note how it shows the excitation wire having only the GEN light in the circuit and no resistor wire. The excitation current in this case is the lamp
current. The only downside to this is if you blow your GEN light, you also lose your Excitation current and thus likely no output. I suspect the
resistor wire was added in parallel to address that issue and also get the alternator producing power faster at startup with the higher excitation
current.

The resistor wire could be replaced with a #1156 lamp to provide the higher startup current. If something went faulty in the alternator that caused a
high voltage which would overheat the resistor wire, instead the #1156 lamp would just burn brightly, or burn out. It would be an easy fix to the
resistor wire issue.

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
The odds of getting three bad alternators are fairly high. That they test good makes it even higher. I suspect the
alternator is working properly and you have a wiring error - break or swapped wires in a connector someplace. The plug on the rear of the alternator
is the problem most of the time. Take it apart, clean it up, make sure the crimps aren't housing corrosion, and then put it back. Loook also for a
fault in the lamp circuit. A shorted wire might produce the symptom you see.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
Last edited by a moderator:
Let's make that 'the odds AGAINST' :)

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
When you say "post #1 and post # 2" I am not following you. There is 1 output post and a 2 cavity plastic connector that goes to the internal voltage
regulator.
Backing up to basics--
Engine off, meter from isolator B1 to ground you should see 12.6V of the engine battery if it is charged
Engine off, meter from isolator B2 to ground you should see 12.6v of the house battery if it is charged.

Tug on the fusible link wire from the engine side of the boost solenoid to the post on the horn relay to be sure it is solid. All you can also Ohm it
end to end.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
So I just went out the Murray and measured the Key ON voltage on Pin 1 (exciter wire) at 1.00 volts. This makes sense because it is powering the
field winding which is just a copper wire coil with very low resistance. There is also a power resistor on the opposite end to ground along with the
field current regulator transistor. I would expect the transistor to be turned fully ON.

This means the field winding is getting maximum current to develop maximum output... once the field rotor starts turning.

So check the voltage at Pin 1, it should be about 1 volt or so.

John, Delco refers to the pins on the 2 pin alternator connector as Pin 1 & 2. Pin 1 is the exciter voltage from the resistor wire and Pin 2 is the
remote reference feedback voltage from the battery (taken from the wire going to the Ignition switch near the horn relay power terminal (fusible link
terminal)

See:
http://www.gmcmhphotos.com/photos/data/6522/Delco_Alternator.pdf

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
Connector for Terminal 1 with key off 0 volts
Connector for Terminal 1 with ignition on less than battery voltage.
When connected to Terminal 1 I have 4 volts
With engine running no out put voltage from Batt Terminal.

Just to eliminate the fusible link I replaced it with new.

From some of the first suggestions a month or so ago I have cleaned and
tightened all connectors going to alternator.

On Sun, Sep 29, 2019 at 8:01 AM Bruce Hislop via Gmclist <

> So I just went out the Murray and measured the Key ON voltage on Pin 1
> (exciter wire) at 1.00 volts. This makes sense because it is powering the
> field winding which is just a copper wire coil with very low resistance.
> There is also a power resistor on the opposite end to ground along with the
> field current regulator transistor. I would expect the transistor to be
> turned fully ON.
>
> This means the field winding is getting maximum current to develop maximum
> output... once the field rotor starts turning.
>
> So check the voltage at Pin 1, it should be about 1 volt or so.
>
> John, Delco refers to the pins on the 2 pin alternator connector as Pin 1
> & 2. Pin 1 is the exciter voltage from the resistor wire and Pin 2 is the
> remote reference feedback voltage from the battery (taken from the wire
> going to the Ignition switch near the horn relay power terminal (fusible
> link
> terminal)
>
> See:
> http://www.gmcmhphotos.com/photos/data/6522/Delco_Alternator.pdf
>
>
>
> --
> Bruce Hislop
> ON Canada
> 77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
> My Staff says I never listen to them, or something like that
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class
 
Have you checked to see if your alternator is grounded to the engine block,
and if the transmission to frame ground strap is intact? You might run a
test lead jumper from the alternator frame to a known good ground and see
if that helps.
Jim Hupy
Salem, Oregon

On Sun, Sep 29, 2019, 7:49 AM Bruce Hart via Gmclist <

> Connector for Terminal 1 with key off 0 volts
> Connector for Terminal 1 with ignition on less than battery voltage.
> When connected to Terminal 1 I have 4 volts
> With engine running no out put voltage from Batt Terminal.
>
> Just to eliminate the fusible link I replaced it with new.
>
> From some of the first suggestions a month or so ago I have cleaned and
> tightened all connectors going to alternator.
>
>
> On Sun, Sep 29, 2019 at 8:01 AM Bruce Hislop via Gmclist <

>
> > So I just went out the Murray and measured the Key ON voltage on Pin 1
> > (exciter wire) at 1.00 volts. This makes sense because it is powering
> the
> > field winding which is just a copper wire coil with very low resistance.
> > There is also a power resistor on the opposite end to ground along with
> the
> > field current regulator transistor. I would expect the transistor to be
> > turned fully ON.
> >
> > This means the field winding is getting maximum current to develop
> maximum
> > output... once the field rotor starts turning.
> >
> > So check the voltage at Pin 1, it should be about 1 volt or so.
> >
> > John, Delco refers to the pins on the 2 pin alternator connector as Pin
> 1
> > & 2. Pin 1 is the exciter voltage from the resistor wire and Pin 2 is
> the
> > remote reference feedback voltage from the battery (taken from the wire
> > going to the Ignition switch near the horn relay power terminal (fusible
> > link
> > terminal)
> >
> > See:
> > http://www.gmcmhphotos.com/photos/data/6522/Delco_Alternator.pdf
> >
> >
> >
> > --
> > Bruce Hislop
> > ON Canada
> > 77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
> > http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
> > My Staff says I never listen to them, or something like that
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >
>
>
> --
> Bruce Hart
> 1976 Palm Beach
> Milliken, Co
> GMC=Got More Class
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
Bruce,

I'm still a bit confused... are all these voltages measured to ground?

You say:
"Connector for Terminal 1 with key off 0 volts
Connector for Terminal 1 with ignition on less than battery voltage."

Then you say:
"When connected to Terminal 1 I have 4 volts"

So... do you get "less than battery voltage" with the key ON and engine not running

And then you start the engine and you get 4 volts on pin 1 with the engine running??

Or how do you read "less than battery voltage" and then "4 volts" on pin 1. With the key ON and engine not running, is "less than battery voltage"
mean about one volt or so below the battery voltage you see on pin 2, or are you meaning 4 volts?.

You say the GEN light is ON whether running or not with the key ON, so that means there needs to be 6-8 volts across the GEN bulb and that is in
parallel with the resistor wire. So a measurement of 4 volts at Pin 1 would make some sense. But the wording "Less than battery voltage" to me
means maybe a volt or so, which in that case, the GEN lamp would not light up.

If the voltage sense wire (#12 wire on Pin 2) is not seeing battery voltage, then the voltage regulator is suppose to shut-down rather than goto a
very high voltage like older alternators did.

Take out the APC and plug the wiring harness directly into the alternator. If that does not correct the fault then measure the voltages on Pin 1 & 2
at the connector directly on the alternator. This eliminates the APC if it is faulty.

Have you tried removing the APC and plugging the wiring harness directly into the alternator?

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
And what are you using for engine ground on your meter?

I want to re-verify your answer to my question. You stated that there is 0 volts at the CENTER terminal of the isolator WITH THE ENGINE RUNNING and
the negative lead of your meter connected to the aluminum plate that the isolator is mounted on.

Is this correct?

If the above is true, then check the output voltage at the alternator. That is NOT on the plug but rather on the big heavy lead mounted with a nut
and stud and covered with a rubber boot.

IF, with the engine running, there is 0 volts there when measured to ENGINE GROUND, then the alternator is not starting. That is usually caused but
no or improper voltage on on Pin 1 of the 2 conductor plug. I would strongly suggest removing the APC at this time and trying it again after assuring
that the two female connectors are clean and have enough tension on them as Johnny previously suggested.

This is not an alternator grounding issue. The alternator is mounted directly to the engine with 3 bolts and it is impossible to not be grounded.
The ground strap to the frame and body are also not in use for for charging and running stuff on the engine side of things.

I made a flow chart one time for troubleshooting things like this. It was on gene's web site. I will see if I can find it.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Many alternators are well grounded by attachment fasteners, but I have run
into a couple that were not. That is why I had him check his. Something is
amis here, I am with you on the APC. Seen those go bad also.
Jim Hupy
Salem,Oregon

On Sun, Sep 29, 2019, 4:11 PM Ken Burton via Gmclist <

> And what are you using for engine ground on your meter?
>
> I want to re-verify your answer to my question. You stated that there is
> 0 volts at the CENTER terminal of the isolator WITH THE ENGINE RUNNING and
> the negative lead of your meter connected to the aluminum plate that the
> isolator is mounted on.
>
> Is this correct?
>
> If the above is true, then check the output voltage at the alternator.
> That is NOT on the plug but rather on the big heavy lead mounted with a nut
> and stud and covered with a rubber boot.
>
> IF, with the engine running, there is 0 volts there when measured to
> ENGINE GROUND, then the alternator is not starting. That is usually caused
> but
> no or improper voltage on on Pin 1 of the 2 conductor plug. I would
> strongly suggest removing the APC at this time and trying it again after
> assuring
> that the two female connectors are clean and have enough tension on them
> as Johnny previously suggested.
>
> This is not an alternator grounding issue. The alternator is mounted
> directly to the engine with 3 bolts and it is impossible to not be
> grounded.
> The ground strap to the frame and body are also not in use for for
> charging and running stuff on the engine side of things.
>
> I made a flow chart one time for troubleshooting things like this. It was
> on gene's web site. I will see if I can find it.
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
Ken B,
Yes I have 0 volts at the middle terminal on the isolator with my positive
probe on the middle terminal and negative probe on the aluminum plate the
isolator is mounted to. I even have a electric choke that is power by the
Batt connection of the alternator and with engine running it also shows 0
volts, I place the negative probe on a 1/4" pipe plug on the intake
manifold same place when measuring the voltage for #1 and #2 terminals for
the alternator.

On Sun, Sep 29, 2019 at 7:15 PM James Hupy via Gmclist <

> Many alternators are well grounded by attachment fasteners, but I have run
> into a couple that were not. That is why I had him check his. Something is
> amis here, I am with you on the APC. Seen those go bad also.
> Jim Hupy
> Salem,Oregon
>
> On Sun, Sep 29, 2019, 4:11 PM Ken Burton via Gmclist <

>
> > And what are you using for engine ground on your meter?
> >
> > I want to re-verify your answer to my question. You stated that there is
> > 0 volts at the CENTER terminal of the isolator WITH THE ENGINE RUNNING
> and
> > the negative lead of your meter connected to the aluminum plate that the
> > isolator is mounted on.
> >
> > Is this correct?
> >
> > If the above is true, then check the output voltage at the alternator.
> > That is NOT on the plug but rather on the big heavy lead mounted with a
> nut
> > and stud and covered with a rubber boot.
> >
> > IF, with the engine running, there is 0 volts there when measured to
> > ENGINE GROUND, then the alternator is not starting. That is usually
> caused
> > but
> > no or improper voltage on on Pin 1 of the 2 conductor plug. I would
> > strongly suggest removing the APC at this time and trying it again after
> > assuring
> > that the two female connectors are clean and have enough tension on them
> > as Johnny previously suggested.
> >
> > This is not an alternator grounding issue. The alternator is mounted
> > directly to the engine with 3 bolts and it is impossible to not be
> > grounded.
> > The ground strap to the frame and body are also not in use for for
> > charging and running stuff on the engine side of things.
> >
> > I made a flow chart one time for troubleshooting things like this. It
> was
> > on gene's web site. I will see if I can find it.
> > --
> > Ken Burton - N9KB
> > 76 Palm Beach
> > Hebron, Indiana
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class
 
Following this since I also have alternator issues. Although my issue is very different (high output of voltage from alt), there has been some
suggestions on this topic, I surely can use in troubleshooting my problem at hand.

Samuel Ferguson
1976 Palm Beach
--
Samuel Ferguson
Pittsfield, Illinois
(West Central Illinois)
1976 GMC Palm Beach (A newbie who will not allow this thing to kick my butt!!)
 
And the engine was running at the time the center terminal of the isolator was read???

If so then move your meter to the big (stud with nut) terminal that is covered with a rubber boot on the Alternator. Because of your failure, I
expect to see 0 volts there WITH THE ENGINE RUNNING but it should be around 14.7. I am just confirming that there is no output right at the
alternator.

If the above is at 0 volts with the engine running and you have tried several alternators, then we must assume that you are not getting the start
voltage at the plug terminal #1. Please make sure that you have removed the APC and the the #1 female connector inside the alternator plug is clean
and very tight. The #2 terminal will not cause this failure. (#2 will cause it to go over voltage, not under voltage or fail to start.)

If that does not fix your problem, you need to trace the circuit into the coach to see why that wire does not have the correct voltage reading.

All of the above readings must be taken with the engine running and the connector plugged into the alternator.

I am convinced that this problem is a loose or corroded / dirty connection somewhere or a broken / burned wire.


--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
You must log in or register to reply here.
GMCMI Spring 2026 Rally
Applied GMC

Share this page

Share this page
Top Bottom
Back