GMC Alternator - need help

R

robert m fukumoto

New member
Apr 22, 1998
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I was working on my 76 Palm Beach today trying to get it ready for its
maiden voyage in two weeks. The alternator stopped working. I removed
the alternator and had it tested at the local Checker auto and found that
it was fine. (I put in a new one though, the bearings seemed to be a
little questionable). I tested the #2 lead from the alternator to the
battery junction block and it was fine. I haven't been able to find
where the #1 lead goes to. However, like the manual said, I tested all
three leads from the alternator with the ignition key in the on position.
The #3 lead to the battery showed no voltage. The #1 lead to the dash
light showed 2.4 volts. The #2 lead to the battery junction showed 12.6
volts.

As I read the manual, there is a shutoff of power production in the
alternator if the #1 and #2 leads don't have the proper voltage coming
back into the alternator.

I can use all the help and ideas that anyone can give me on what's
causing the alternator not to put out any voltage.

Bob Fukumoto
 
Bob,

Your voltage readings are probably near normal with the engine stopped.

The GMC maintenance manual makes it confusing by discussing a generic
alternator system rather than a system with a diode isolator.

The diode isolator blocks a voltage indication at the alternator "BAT"
terminal (one with the nut on it) until the engine is running and the
alternator is actually producing.

The diode isolator is like a check valve in that it lets current go from
alternator to battery but not the other way.

To make matters still more confusing, the alternator light will sometimes be
dimly lit when the motor is running and the alternator is working correctly.

This can lead one to think the alternator is defective when it is not. GM
says this abnormal indication is normal.

My quick check is to look for a battery voltage increase when engine is
running. If alternator is charging OK, battery voltage should increase from
about 12 volts (engine stopped) to about 14 volts (engine running).

Don

>I was working on my 76 Palm Beach today trying to get it ready for its
>maiden voyage in two weeks. The alternator stopped working. I removed
>the alternator and had it tested at the local Checker auto and found that
>it was fine. (I put in a new one though, the bearings seemed to be a
>little questionable). I tested the #2 lead from the alternator to the
>battery junction block and it was fine. I haven't been able to find
>where the #1 lead goes to. However, like the manual said, I tested all
>three leads from the alternator with the ignition key in the on position.
> The #3 lead to the battery showed no voltage. The #1 lead to the dash
>light showed 2.4 volts. The #2 lead to the battery junction showed 12.6
>volts.
>
>As I read the manual, there is a shutoff of power production in the
>alternator if the #1 and #2 leads don't have the proper voltage coming
>back into the alternator.
>
>I can use all the help and ideas that anyone can give me on what's
>causing the alternator not to put out any voltage.
>
>Bob Fukumoto
>
 
I had a problem with the alternator when the coach battery was not
charging and all the energy was going to the back battery. The front
one was not charging. If you find water missing from the back battery
that could be why the unit is not charging.

Regards
Al Chernoff
 
Donald Miller:

When running, the alternator is putting out no voltage at all and the
permanent after market voltmeter and my multimeter both show less than 12
volts with the engine revving.

Any ideas on why the alternator circuits are shutting down when the bench
test shows that the alternator is fine?

Bob

>
> Bob,
>
> Your voltage readings are probably near normal with the engine stopped.
>
> The GMC maintenance manual makes it confusing by discussing a generic
> alternator system rather than a system with a diode isolator.
>
> The diode isolator blocks a voltage indication at the alternator "BAT"
> terminal (one with the nut on it) until the engine is running and the
> alternator is actually producing.
>
> The diode isolator is like a check valve in that it lets current go from
> alternator to battery but not the other way.
>
> To make matters still more confusing, the alternator light will sometimes be
> dimly lit when the motor is running and the alternator is working correctly.
>
> This can lead one to think the alternator is defective when it is not. GM
> says this abnormal indication is normal.
>
> My quick check is to look for a battery voltage increase when engine is
> running. If alternator is charging OK, battery voltage should increase from
> about 12 volts (engine stopped) to about 14 volts (engine running).
>
> Don
>
> >I was working on my 76 Palm Beach today trying to get it ready for its
> >maiden voyage in two weeks. The alternator stopped working. I removed
> >the alternator and had it tested at the local Checker auto and found that
> >it was fine. (I put in a new one though, the bearings seemed to be a
> >little questionable). I tested the #2 lead from the alternator to the
> >battery junction block and it was fine. I haven't been able to find
> >where the #1 lead goes to. However, like the manual said, I tested all
> >three leads from the alternator with the ignition key in the on position.
> > The #3 lead to the battery showed no voltage. The #1 lead to the dash
> >light showed 2.4 volts. The #2 lead to the battery junction showed 12.6
> >volts.
> >
> >As I read the manual, there is a shutoff of power production in the
> >alternator if the #1 and #2 leads don't have the proper voltage coming
> >back into the alternator.
> >
> >I can use all the help and ideas that anyone can give me on what's
> >causing the alternator not to put out any voltage.
> >
> >Bob Fukumoto
> >
 
Bob,

FWIW, some considerations.

1. Since most everything is built into your new alternator, it may have had
a short life span.

2. Make sure #2 white wire has around 12 volts present with key on and when
engine is running.

3. Your diode isolator might be bad.

If you understand how the diode isolator works, check it out with your ohm
meter.

An alternate diode isolator check is to put a jumper wire between it's three
terminals. If both engine and house batteries charge properly with engine
running and jumper in place, the isolator is the problem.

4. Look for corrosion at the terminals and connectors.

5. Check wire continuity from alternator output to diode isolator input.

6. Check two wires for continuity from isolator output to aux battery relay
terminals.

Be careful, if the alternator does not have a load, when engine is running
its output voltage can be high.
High enough to kill you and high enough to short out the alternator internal
diodes.

Don

>I was working on my 76 Palm Beach today trying to get it ready for its
>maiden voyage in two weeks. The alternator stopped working. I removed
>the alternator and had it tested at the local Checker auto and found that
>it was fine. (I put in a new one though, the bearings seemed to be a
>little questionable). I tested the #2 lead from the alternator to the
>battery junction block and it was fine. I haven't been able to find
>where the #1 lead goes to. However, like the manual said, I tested all
>three leads from the alternator with the ignition key in the on position.
> The #3 lead to the battery showed no voltage. The #1 lead to the dash
>light showed 2.4 volts. The #2 lead to the battery junction showed 12.6
>volts.
>
>As I read the manual, there is a shutoff of power production in the
>alternator if the #1 and #2 leads don't have the proper voltage coming
>back into the alternator.
>
>I can use all the help and ideas that anyone can give me on what's
>causing the alternator not to put out any voltage.
>
>Bob Fukumoto
>
 
I had the alternator light never go out when I first bought my coach. After
replacing the Alternator, playing with the belt, etc NO IMPROVEMENT!!---->
Then, I discovered by accident that the main battery was bad. Put in a new
battery - VIOLA- no alternator(GEN) light! Put on old Alternator, it was
good! I am now tracking alternator operation with an Ammeter gauge. Hope
this helps...

>I was working on my 76 Palm Beach today trying to get it ready for its
>maiden voyage in two weeks. The alternator stopped working. I removed
>the alternator and had it tested at the local Checker auto and found that
>it was fine. (I put in a new one though, the bearings seemed to be a
>little questionable). I tested the #2 lead from the alternator to the
>battery junction block and it was fine. I haven't been able to find
>where the #1 lead goes to. However, like the manual said, I tested all
>three leads from the alternator with the ignition key in the on position.
> The #3 lead to the battery showed no voltage. The #1 lead to the dash
>light showed 2.4 volts. The #2 lead to the battery junction showed 12.6
>volts.
>
>As I read the manual, there is a shutoff of power production in the
>alternator if the #1 and #2 leads don't have the proper voltage coming
>back into the alternator.
>
>I can use all the help and ideas that anyone can give me on what's
>causing the alternator not to put out any voltage.
>
>Bob Fukumoto
>
 
There is an internal voltage regulator which is easy to replace

- ----------
> From: Thom Hole
> To: gmcmotorhome
> Subject: Re: GMC: GMC Alternator - need help
> Date: Monday, July 13, 1998 1:04 PM
>
> I had the alternator light never go out when I first bought my coach.
After
> replacing the Alternator, playing with the belt, etc NO
IMPROVEMENT!!---->
> Then, I discovered by accident that the main battery was bad. Put in a
new
> battery - VIOLA- no alternator(GEN) light! Put on old Alternator, it was
> good! I am now tracking alternator operation with an Ammeter gauge.
Hope
> this helps...
>

> >I was working on my 76 Palm Beach today trying to get it ready for its
> >maiden voyage in two weeks. The alternator stopped working. I removed
> >the alternator and had it tested at the local Checker auto and found
that
> >it was fine. (I put in a new one though, the bearings seemed to be a
> >little questionable). I tested the #2 lead from the alternator to the
> >battery junction block and it was fine. I haven't been able to find
> >where the #1 lead goes to. However, like the manual said, I tested all
> >three leads from the alternator with the ignition key in the on
position.
> > The #3 lead to the battery showed no voltage. The #1 lead to the dash
> >light showed 2.4 volts. The #2 lead to the battery junction showed 12.6

> >volts.
> >
> >As I read the manual, there is a shutoff of power production in the
> >alternator if the #1 and #2 leads don't have the proper voltage coming
> >back into the alternator.
> >
> >I can use all the help and ideas that anyone can give me on what's
> >causing the alternator not to put out any voltage.
> >
> >Bob Fukumoto
 
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