Alternator issues 1976 Palm Beach

Sketch4u

Sketch4u

New member
Sep 9, 2019
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Pittsfield, Illinois (Pike County - Deer Hunting C
I am experiencing a charging issue with the engine battery on a 1976 GMC Palm Beach Motorhome. I had the engine battery to go dead while going down
the highway. I took the alternator off and took it to NAPA parts store for a bench test and was told the alternator was "dead"! I replaced with a
remanufacture alternator. After doing so, I started the engine and the "GEN" light came on. I turned the ignition off and waited a couple of minutes
an re-started the engine.  After re-starting, the "GEN" went off, but it still did not appear to be charging. I checked the battery and am showing 12
volts. I checked the "red" wire to the alternator prior to starting the engine again and I do not have any power. I started the engine and am still
not getting and voltage reading on the red wire to the alternator.

The "yellow" wire coming from the alternator plug is showing 12 volts when I check it against the brown wire next to the yellow wire in the plug.

I have traced the red, yellow, and brown to the from of the hood area looking for an in-line fuse link or a voltage regulator box (Nothing found). All
three wires run to the hood area and after doing so, the red wire is piggy-backed with another red wire running to a blue box {mounted on the back
wall with GMC 706751 labeled) with one red wire going to a solenoid. The yellow wire coming from alternator runs to the hood area and is pig-tailed
with a red wire, which the red wire goes to one of the three post on top of the blue GMC 706751 box.
I did discover today, when I turn the house battery on, the red wire on the alternator is reading 12 volts. Turn house battery off and turn engine
battery on, no voltage reading on the red wire at alternator.

I am trying to find out of the blue GMC box is the problem or would it be a bad solenoid? I have attached a couple of pictures of the blue box and the
solenoid.

Any assistance will be greatly appreciated.

Samuel Ferguson 
 
If a wire shows voltage with the house battery on but not when it's off, that wire is connected in some manner to the house battery. Your description
sounds as if the PO or Os have been playing with the wiring. Have a look at the PROPER schematic for your coach - it's in the back of the maintenance
manual - take your meter, and prove all the 12 volt connections to the isolator and underhood components. There's likely a miswire in there
somewhere.

--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
 
I did find a cable that runs from engine battery compatment to the back of rv (I assume this cable is the battert boost cable) has a lot of coating
worn from it which is possibly shorting out. I unhooked and ordered another alternator. Hoping this is my problem that caused new alternator to burn
up!
 
Replacing alternator again tomorrow. Does anyone have a schematic for the wiring from alternatir and which post the wire goes to on the battery
isolator? Isolator has 3 posts GMC #706751.

Thank You

Samuel
--
Samuel Ferguson
Pittsfield, Illinois
(West Central Illinois)
1976 GMC Palm Beach
 
Go here and you can download the wiring diagram for your coach.

The ground for the alternator is the case of the alternator itself, all voltage measurements are made from the case of the alternator.

The brown wire is the startup excitation wire. It has a resistance in the wire and provides the field current to get the alternator going. The voltage
on this wire when the engine is NOT Running but the key is ON is something less than battery voltage. I've never had a cause to measure it myself, but
I'm guessing maybe 1/2 battery voltage (6 volts).

The wire beside it in the connector (white or red) is the battery voltage sense wire. This wire should have battery voltage on it. It senses the
battery voltage to give the voltage regulator a feedback voltage of what the battery voltage currently is.

The output wire is a larger wire on a bolt stud. When the engine is RUNNING, it should be 0.7 volts higher than the battery voltage. When the engine
is OFF, this wire should be near Zero volts.

hope this helps,

--
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
 
Samuel,

Bruce missed answering just one of your questions: The output of the
alternator should be connected to the center terminal of the isolator. The
batteries are each connected to one of the other terminals.

Ken H.

On Thu, Sep 12, 2019 at 10:56 PM Samuel Ferguson via Gmclist <

> Replacing alternator again tomorrow. Does anyone have a schematic for the
> wiring from alternatir and which post the wire goes to on the battery
> isolator? Isolator has 3 posts GMC #706751.
>
> Thank You
>
> Samuel
> --
> Samuel Ferguson
> Pittsfield, Illinois
> (West Central Illinois)
> 1976 GMC Palm Beach
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
Sam F:
How long have you owned your GMC?
I am surprised we have never seen you at a GMC 6 Wheeler function (within a 200 mile range of Chicago).
We have a rally next week in Pontiac, IL.
Mike/The Corvair a holic
Peoria, IL.
76 GMC Eleganza II 26’ (our best and soon to be for sale).
76 GMC Glenbrook 28’ stretch
78 GMC Glenbrook - project coach

Sent from my iPhone

>
> Replacing alternator again tomorrow. Does anyone have a schematic for the wiring from alternatir and which post the wire goes to on the battery
> isolator? Isolator has 3 posts GMC #706751.
>
> Thank You
>
> Samuel
> --
> Samuel Ferguson
> Pittsfield, Illinois
> (West Central Illinois)
> 1976 GMC Palm Beach
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Sam:
Ahah - a Newbie.
Also sounds like you have mechanical skills - good for you.
The GMC 6 Wheelers is your local GMC club.
We meet (usually the 3rd weekend) of ea month May thru Oct. The Sept. rally will be 9/19 thru 9/22 (Thur. thru Sun. AM) at the Pontiac, IL. 4H facility. Our Oct. rally will be at Geneseo, IL. 10/17 thru 10/20 (Thur. thru Sun. AM).
As a GMC owner you are invited whether you bring your GMC or not and whether you stay a few hours or all weekend!
It’s a good group and we look forward to meeting you.
Visit our website at GMC6wheelers.org at your convenience. Yearly cost to belong is just $10.00 - the best bargain in town.
Mike/The Corvair a holic

Sent from my iPhone

>
> Just bought it 2 weeks ago. It's been sitting 20 plus years in a barn. Going through, trying to get all the bugs out!!!! Looking forwad to going to
> the get togethers in the future...
>
> Sam
> --
> Samuel Ferguson
> Pittsfield, Illinois
> (West Central Illinois)
> 1976 GMC Palm Beach
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
The isolator is 2 diodes one each going from the center to an outer terminal. That's why you can't read voltage engine off at the alternator terminal
because you can't see the battery in that direction like wpuld br on a passenger car. I would start by metering the diodes. I would disconnect both
battery negative cablofirst not knowing PO wiring mods. The sense wire is after the diode on the engine battery side.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Put new alternator on this afternoon. My meter is reading 36.9 when engine is running on the alternator wire..(Hoping this is a correct reading!!!).
Battery is at 12.4 volts, while I have'nt checked reading on alternator while not running.
Isolator box - I have 12 4 volts to engine battery. 11.2 volts to house battery. Alternator post on isolator reading the 36.9. (Hope this is correct)

Dash AC working fine enough to figure out I need to recharge the freon. Switch on "hi" and she blows somewhat cool air, but not cold air.

Next project - trace the Booster Batter Cable and replace. Cable has the coatin falling off from touch, and several section of exposed wire, which is
obviously grounding somewhere and possibly "fried" the last two alternators. (Fingers crossed this was the problem)
Once cable traced, I plan to run a brand new one from engine compartment to house battery...

I would like to say "Thank You" to all that responded and gave me tips of what to check, look for, and provided a step by step plan to accomplish what
I had got myself into. I am a newbie to this sort of thing. I grew up learning how to do electrical and mechanical troubleshoots and repairs, but as I
age as others do, I am slow to retrieve what information I took in 30 - 40 years ago. And when I do get the problem figured out, I start trying to
patch the hair I have pulled out of the side of my head and try to place it back to the bald spots I gave myself in the last 6 to 10 hours.
All is starting to look good. A few more bugs to work out and hopefully my wife and I can make a roadtrip in the Palm Beach to one of the two meetings
this year before winter makes it way..

Again, Thank You all for your kindness, support and expertise...

Samuel Ferguson
--
Samuel Ferguson
Pittsfield, Illinois
(West Central Illinois)
1976 GMC Palm Beach
 
36.9 volts on the center post of the isolator (and alternator output) is
wrong wrong wrong.

This sounds like both diodes in the isolator are completely open.

Alternator voltage with good diodes should be approximately 0.7 volts
higher than battery voltage; or about 13 volts DC positive with respect
to chassis ground.

Stu

> Put new alternator on this afternoon. My meter is reading 36.9 when engine is running on the alternator wire..(Hoping this is a correct reading!!!).
> Battery is at 12.4 volts, while I have'nt checked reading on alternator while not running.
> Isolator box - I have 12 4 volts to engine battery. 11.2 volts to house battery. Alternator post on isolator reading the 36.9. (Hope this is correct)
>
 
36.9 volts on the center post of the isolator (and alternator output) is
wrong wrong wrong.

This sounds like both diodes in the isolator are completely open.

Alternator voltage with good diodes should be approximately 0.7 volts
higher than battery voltage; or about 13 volts DC positive with respect
to chassis ground.

Stu

> Put new alternator on this afternoon. My meter is reading 36.9 when engine is running on the alternator wire..(Hoping this is a correct reading!!!).
> Battery is at 12.4 volts, while I have'nt checked reading on alternator while not running.
> Isolator box - I have 12 4 volts to engine battery. 11.2 volts to house battery. Alternator post on isolator reading the 36.9. (Hope this is correct)
>
 
Sam
Napa has them

https://www.napaonline.com/en/p/BK_7821769?partTypeName=Battery+Isolator&keywordInput=battery+isolator

On Fri, Sep 13, 2019 at 7:17 PM Samuel Ferguson via Gmclist <

> Stu,
>
> Would you have any info as to where I could order a new isolator? Part #
> on mine is GMC #706751.
>
> Sam
> --
> Samuel Ferguson
> Pittsfield, Illinois
> (West Central Illinois)
> 1976 GMC Palm Beach
>
> _______________________________________________
> 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
 
OK Samuel, you now have three options for a replacement isolator.

You said earlier "Next project - trace the Booster Batter Cable and replace. Cable has the coatin falling off from touch, and several section of
exposed wire, which is obviously grounding somewhere and possibly "fried" the last two alternators. (Fingers crossed this was the problem)"

That may indicate the genesis of your alternator and isolator failures.

As a temporary expedient (and to prevent further damage) you can completely eliminate the isolator and connect the alternator lead to the starting
battery and leave the house batteries completely disconnected while you troubleshoot the house battery issue. Having insulation flaking off the heavy
wire may indicate it shorted and burned up rather a lot of 'stuff' and replacing the isolator might just start the destruction all over again.

Disconnect both the house and paralled 'booster' battery until the wiring is repaired.

JMHO

Stu

Stu
--
Stu Rasmussen W7QJ
Silverton, OR
gutted '74 Eleganza II (for sale now)
'77 Birchaven
 
Last time I needed one they were around #30.00 for a 95 amp one at a local auto parts store. I forget which one I got it from. The local AutoValue
store has them hanging on one of the boards. I probably got it from AZ or Oreilies. Also any RV store or Jim K. will have them.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
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