Charging system trouble

[kennet dejesus]

Hi folks, new(ish) owner and first time poster here.

I've only put about 600~700 miles on our coach since I bought it from Tom Hampton in April. Lately I'd been noticing the "GEN" light had been glowing
dimly. Reviewing this forum I decided I'd try installing an APC (haven't ordered it yet). As it happened, I had the coach back at Tom's shop last
week for shock replacement and while I was there I described the situation to him. I also mentioned that the week prior I had found the engine battery
completely dead. Using the "BAT BOOST" toggle wasn't enough to crank it over either, but you could hear that it was trying to engage the starter at
least. I recharged the engine battery with a trickle charger and that seemed to resolve it.

With a multimeter Tom confirmed alternator output around 13.6V and with engine off, battery was 12.7V so all ok. Then on the drive home the GEN light
came on full bright. I confirmed at the center isolator post that alternator output was only about 1V and the battery was draining below 12V. Strange
timing for this to happen, I though.

I removed the alternator and found it was an AutoZone Duralast reman unit (lifetime warranty). AutoZone gave me a replacement and I installed it last
night. All was great as I compared my new voltage numbers to the ones shown on the flowchart elsewhere on this forum. I drove for about 1/2 hour and
checked again (still good), then left it running with all exterior lights on for about 15 minutes while I adjusted a fog light. When I went back
inside I saw the GEN light was back on again! I quickly checked voltages and saw the alternator was now up to 18V at the center isolator post and
voltage at the engine battery posts was falling (still above 12V). I shut it off and walked away for the night completely disgusted. I'm guessing the
high voltage is from a bad regulator, but isn't that part of the alternator assembly internally? Is there something killing these alternators or
could I just have gotten a "bad" one from AutoZone?

Any wisdom here would be greatly appreciated! I should also mention that I took the engine battery (a newer looking Deka brand) to my local battery
shop and they said it checks out OK. I've also confirmed my negative battery cable grounds to the engine block.
--
1975 Glenbrook 26'

 

[john r. lebetski]

Your symptoms point to a bad isolator. High voltage at center, low at outsides and sense cable adding voltage trim to compensate.
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[Ken H.]

Kennet,

That 18 VDC output from the alternator indicates that its internal voltage
regulator is not getting the required feedback from the chassis battery.
The regulator is supposed have an internal voltage limit at that 18 VDC.
The fact that you saw the 18 VDC at the center post of the isolator and a
voltage more than ~0.6 to 0.7 VDC below that at the chassis battery side
indicates that the isolator is open -- the most common failure for our
isolators.

What you need to do is jumper the center terminal of the isolator to the
chassis battery side and check the voltage there -- it should be back
around 13.6-13.8 VDC. If not, you have a different problem, but if that
corrected the voltage, you need either a new isolator or, my preference, a
combiner.

My speculation is that the isolator failed, causing the alternator to go
into its overvoltage protecting mode, and consequently fail. Your
replacement alternator is probably still good since you caught the problem
quickly.

Let us know how it works out.

Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI & EBL,
Manny Brakes & 1-Ton, etc., etc., etc.
www.gmcwipersetc.com

> Hi folks, new(ish) owner and first time poster here.
>
> I've only put about 600~700 miles on our coach since I bought it from Tom
> Hampton in April. Lately I'd been noticing the "GEN" light had been glowing
> dimly. Reviewing this forum I decided I'd try installing an APC (haven't
> ordered it yet). As it happened, I had the coach back at Tom's shop last
> week for shock replacement and while I was there I described the situation
> to him. I also mentioned that the week prior I had found the engine battery
> completely dead. Using the "BAT BOOST" toggle wasn't enough to crank it
> over either, but you could hear that it was trying to engage the starter at
> least. I recharged the engine battery with a trickle charger and that
> seemed to resolve it.
>
> With a multimeter Tom confirmed alternator output around 13.6V and with
> engine off, battery was 12.7V so all ok. Then on the drive home the GEN
> light
> came on full bright. I confirmed at the center isolator post that
> alternator output was only about 1V and the battery was draining below 12V.
> Strange
> timing for this to happen, I though.
>
> I removed the alternator and found it was an AutoZone Duralast reman unit
> (lifetime warranty). AutoZone gave me a replacement and I installed it last
> night. All was great as I compared my new voltage numbers to the ones
> shown on the flowchart elsewhere on this forum. I drove for about 1/2 hour
> and
> checked again (still good), then left it running with all exterior lights
> on for about 15 minutes while I adjusted a fog light. When I went back
> inside I saw the GEN light was back on again! I quickly checked voltages
> and saw the alternator was now up to 18V at the center isolator post and
> voltage at the engine battery posts was falling (still above 12V). I shut
> it off and walked away for the night completely disgusted. I'm guessing the
> high voltage is from a bad regulator, but isn't that part of the
> alternator assembly internally? Is there something killing these
> alternators or
> could I just have gotten a "bad" one from AutoZone?
>
> Any wisdom here would be greatly appreciated! I should also mention that
> I took the engine battery (a newer looking Deka brand) to my local battery
> shop and they said it checks out OK. I've also confirmed my negative
> battery cable grounds to the engine block.
> --
> 1975 Glenbrook 26'
>

 

[Gene Fisher]

Read here
http://www.gmcmhphotos.com/photos/data/15/GMC_charging_system_checkout.pdf

And send me your mailing address
So I can send you an APC
See my tag line

On Sat, Jun 10, 2017 at 7:08 AM John R. Lebetski
wrote:

> Your symptoms point to a bad isolator. High voltage at center, low at
> outsides and sense cable adding voltage trim to compensate.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
>
>
> _______________________________________________
> 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

 

[kennet dejesus]

> Read here
> http://www.gmcmhphotos.com/photos/data/15/GMC_charging_system_checkout.pdf
>
>
> And send me your mailing address
> So I can send you an APC
> See my tag line

Done, PM sent. Thank you!

So I checked again this morning and all was ok again which makes me think that temperature is playing a factor here too. Does that make sense in this
scenario?

Is there a specific brand/model of isolator (or combiner) that folks recommend?

Thanks, everyone, for the prompt responses!

-Kennet
--
1975 Glenbrook 26'

 

[johnny]

Please check the most common cause - and the easiest to fix - make sure the two wire plug going into the alternator is making contact. The pins often
bend such that the don't make good contact. Losing the 12V wire will result in no output. Losing the other wire (which is the sense wire) will
result in the output climbing to clamp which is around 18 volts and will break things fairly quickly.

--johnny
--
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" - ol Andy, paraphrased

 

[a.]

> ...Is there a specific brand/model of isolator (or combiner) that folks recommend?...

http://www.appliedgmc.com/prod.itml/icOid/986
--
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Every day I become more convinced that I am the only person left on the planet that recognizes nonsense for what it is."

 

[kennet dejesus]

I gave those terminals in the coupler a quick polish with a small file when I installed the new alternator, and used some dielectric grease. I
couldn't hurt to double check them though. Thanks for the suggestion.

I'll be away from the coach the rest of this weekend, so further troubleshooting has to wait a couple of days.
--
1975 Glenbrook 26'

 

[kennet dejesus]

Would it be any concern that the isolator in that link says "95AMP" while my alternator is rated at 100 Amps?
--
1975 Glenbrook 26'

 

[terry1]

The 95 amp limit is only of concern if you are using more than that while charging. It's possible with enough accessories running, especially a big
inverter or trying to jump start another vehicle while the engine is running, but not likely under normal conditions.
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.

 

[Ken B]

Take Colonel Ken's suggestion and connect your $1.00 combiner (2 alligator clips and a piece of 16 gauge wire) between the center terminal and the
edge terminal supplying the engine side of the coach.

If it works you can drive it this way until you get a new isolator. Your house side batteries will not charge while doing his.

You did not state what the voltage is on the isolator terminal feeding the house side of system. If the voltage there is sightly less that the
center terminal then the house side of the isolator is still good. You could connect your $1.00 combiner between the two edge terminals instead of
using the center one. This would supply booth sides (engine and house) with charging voltage rather than just the engine side. Be sure to disconnect
the $1.00 combiner when parked to prevent electrical things in the house from discharging the engine battery.

The end result is you will need to replace that isolator eventually and these are just temporary work arounds to keep you on the road until then.


--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

 

[Matt Colie]

> I gave those terminals in the coupler a quick polish with a small file when I installed the new alternator, and used some dielectric grease. I
> couldn't hurt to double check them though. Thanks for the suggestion.
>
> I'll be away from the coach the rest of this weekend, so further troubleshooting has to wait a couple of days.

Kennet,

Welcome to the group, family, cult, asylum...

You will find lots of good information here, you are also in striking range of several that know GMC electrics very well.
It is very rare (in all my boat work, I have seen two) for diode isolators to become intermittent. In any case, they are for the most part durable
and inexpensive. Most often, they go open. There is no means to repair them.

I hope you have marketed your calendar for the week of September 15~21 for the GMC International Convention that will be in Elkhart IN (about 180
miles NW). Do what ever you can to get there. Even if you can only come on the two weekend days, get there.

Please do both us and you a big favor. Build a sigfile (Control Panel/Account Settings scroll down) and fill in your name (we use real names here), a
bit about the coach (year model and major mods - they are not all the same), and last a geographic reference because the people that read this as
e-mail don't get to see the header where it says - Troy OH. This way someone that can help and is even in range can know and he will know if it is
feasible. You can change your alias too - if you choose to, I did many years back.

Now, you have to understand that you have found your way here and have become part of a very special community. These are generous, supportive and
helping people that all want to see you enjoy your coach as much as we all do. The only other community that I know of that is at all like this is
that of the watermen that are my world. For that reason, I have taken to welcoming new owners here much as nay new owner or vessel is welcomed there.
So,

May the Good Lord bless this coach and all those that set forth within here.

Welcome Kennet

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Still Loving OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit