Generator Light

B

Bruce Hart

Active member
Oct 18, 2011
1,414
6
38
Need some advice on why the generator light won't come on when I turn the key switch to on position.

Generator light will come on bright when I start up the engine and stays on.

I checked the voltage at the isolator and was charging at 15.7 volts.

This also happened to a spare alternator I had. Both alternators tested good at NAPA
--
Bruce Hart
1976 Palm Beach
Milliken, Co
 
15’u volts seems a bit high unless your battery might be completely dead. After a short time the voltage should drop to perhaps the high 14s. A generator light on all the time is usually associated with the diode trio inside the alternator.
It may have tested right but there still could be a problem.
Can you try putting on another alternator and see if it works properly?

Emery Stora
77 Kingsley
Frederick, CO

>
> Need some advice on why the generator light won't come on when I turn the key switch to on position.
>
> Generator light will come on bright when I start up the engine and stays on.
>
> I checked the voltage at the isolator and was charging at 15.7 volts.
>
> This also happened to a spare alternator I had. Both alternators tested good at NAPA
> --
> Bruce Hart
> 1976 Palm Beach
> Milliken, Co
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Emory is probably correct. However, before you go to the trouble of changing the Alt out, check the instr lights fuse behind the glove box. (It's the
little tiny one) If it is blown the Alt light will come on when running. That happened to me but it was at night and I was checking for the dash
lights. Changed the fuse, the dash light came on and the Alt light went out.

It's worth a check.
--
Patti & Jerry Burt
73 Gmc 26' Canyon Lands -
77 Palm Beach
Members: FMCA - GMCMI - GMCWS
 
15.7 volts measured at which terminal of the isolator?

There are two leads going to the terminals of the alternator. One of them goes through alternator light and provides power to start the alternator
anytime the key is on. The second terminal is a sense line to the alternator to tell it that the voltage in the coach is correct. I would get out
a wiring diagram and start measuring to see if those wires are intact.

When mine went over voltage it was a poor connection on the sense lead where it plugged in to the alternator. I squeezed the terminal with a pair of
pliers for a better connection and took care of the problem.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Emory, it happened with both alternators.
Will check the fuse next,
Ken, checked voltage at center post of isolator. Will follow up with
connector.

Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class
 
> Emory, it happened with both alternators.
> Will check the fuse next,
> Ken, checked voltage at center post of isolator. Will follow up with
> connector.
>
> 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

OK, so you are about 1 volt over expected. That point should be around 14.7 volts give or take a few 1/10s of a volt. The other two terminals on
the isolator should be around 14.0 volts again give or take a few 1/10s.

This is from memory and could be wrong. Thee is a sense lead that is connected to the big terminal on the horn relay. On that same terminal is the
fusible link connected to the isolator. The wire you want is a #12 red one that goes to the alternator pin #2. Along it's route it changes colors
from red to white. I would be reading the voltage at the horn relay and also at the #2 terminal at the alternator. They should be the same reading
and around 14.0 volts with the engine running.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Are you using the Gene Fisher UPC cable attached to the alternator?

Emery Stora

>

>> Emory, it happened with both alternators.
>> Will check the fuse next,
>> Ken, checked voltage at center post of isolator. Will follow up with
>> connector.
>>
>> 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
>
> OK, so you are about 1 volt over expected. That point should be around 14.7 volts give or take a few 1/10s of a volt. The other two terminals on
> the isolator should be around 14.0 volts again give or take a few 1/10s.
>
> This is from memory and could be wrong. Thee is a sense lead that is connected to the big terminal on the horn relay. On that same terminal is the
> fusible link connected to the isolator. The wire you want is a #12 red one that goes to the alternator pin #2. Along it's route it changes colors
> from red to white. I would be reading the voltage at the horn relay and also at the #2 terminal at the alternator. They should be the same reading
> and around 14.0 volts with the engine running.
>
>
> --
> 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
 
Read here
http://www.gmcmhphotos.com/photos/data/15/GMC_charging_system_checkout.pdf

On Wed, Aug 29, 2018 at 7:57 AM Emery Stora via Gmclist <

> Are you using the Gene Fisher UPC cable attached to the alternator?
>
> Emery Stora
>

> >

> >> Emory, it happened with both alternators.
> >> Will check the fuse next,
> >> Ken, checked voltage at center post of isolator. Will follow up with
> >> connector.
> >>
> >> 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
> >
> > OK, so you are about 1 volt over expected. That point should be around
> 14.7 volts give or take a few 1/10s of a volt. The other two terminals on
> > the isolator should be around 14.0 volts again give or take a few
> 1/10s.
> >
> > This is from memory and could be wrong. Thee is a sense lead that is
> connected to the big terminal on the horn relay. On that same terminal is
> the
> > fusible link connected to the isolator. The wire you want is a #12 red
> one that goes to the alternator pin #2. Along it's route it changes colors
> > from red to white. I would be reading the voltage at the horn relay and
> also at the #2 terminal at the alternator. They should be the same reading
> > and around 14.0 volts with the engine running.
> >
> >
> > --
> > 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
>
--
Gene Fisher -- 74-23,77PB/ore/ca
“Give a man a fish; you have fed him for today --- give him a URL and
-------
http://bdub.net/gmcmotorhome.info/
Alternator Protection Cable
http://bdub.net/gmcmotorhome.info/APC.html
 
Checked the connector and all seemed good, removed small fuse and checked
ohm value, That was ok.
Found fuse for gauges was bad and now generator light functions correctly.
Thanks all for the help
Emery, recently received APC from Eugene Fisher and installed. (Did not
correct issue).

> Read here
> http://www.gmcmhphotos.com/photos/data/15/GMC_charging_system_checkout.pdf
>
>
>
> On Wed, Aug 29, 2018 at 7:57 AM Emery Stora via Gmclist <

>
> > Are you using the Gene Fisher UPC cable attached to the alternator?
> >
> > Emery Stora
> >

> > >

> > >> Emory, it happened with both alternators.
> > >> Will check the fuse next,
> > >> Ken, checked voltage at center post of isolator. Will follow up with
> > >> connector.
> > >>
> > >> 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
> > >
> > > OK, so you are about 1 volt over expected. That point should be around
> > 14.7 volts give or take a few 1/10s of a volt. The other two
> terminals on
> > > the isolator should be around 14.0 volts again give or take a few
> > 1/10s.
> > >
> > > This is from memory and could be wrong. Thee is a sense lead that is
> > connected to the big terminal on the horn relay. On that same terminal
> is
> > the
> > > fusible link connected to the isolator. The wire you want is a #12
> red
> > one that goes to the alternator pin #2. Along it's route it changes
> colors
> > > from red to white. I would be reading the voltage at the horn relay
> and
> > also at the #2 terminal at the alternator. They should be the same
> reading
> > > and around 14.0 volts with the engine running.
> > >
> > >
> > > --
> > > 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
> >
> --
> Gene Fisher -- 74-23,77PB/ore/ca
> “Give a man a fish; you have fed him for today --- give him a URL and
> -------
> http://bdub.net/gmcmotorhome.info/
> Alternator Protection Cable
> http://bdub.net/gmcmotorhome.info/APC.html
> _______________________________________________
> 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
 
I am still worried about the high voltage at the isolator (all three terminals). Please check the voltages again.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
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