Identify electric device

[Ken H.]

Peer,

Sounds to me like the chassis battery diode in your isolator is shorted --
an unusual failure on a GMC. Disconnect the chassis battery lead at the
isolator as a preliminary confirmation. Then, without reconnecting to that
terminal, use an ohmmeter to measure between that terminal and the center
one. The resistance should be very low and the same when you reversed the
meter lead connections.

Ken H.

On Thu, Dec 21, 2017 at 5:12 PM, Peer Oliver Schmidt GMC <

> Let me try to clear things up:
>
> When I take the house battery out of the coach, the lights of the house
> still work. Removing the house battery cable from the boost solenoid did
> not turn off the house lights. Removing the house battery cable from the
> isolator caused the lights to go off.
>
> This means to me, that the house battery circuit is receiving energy
> through the isolator from the chassis battery, something that should not
> happen.
>
> Feel free to correct me if I am wrong
>
> --
> Best regards
>
> Peer Oliver Schmidt
> PGP Key ID: 0x83E1C2EA
>
> '76a Eleganza II 26', VA
> '73 Sequoia 23', Germany, SH
>
>
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[Ken B]

> Peer,
>
> This is not any kind of a criticism of of your language capability. (Your English is much better than my German.)
> The blue finned thing is the isolator and the round thing below it is the boost solenoid. Agree?
> (Remember I have another '73 23' and know the issues well.)
>
> Are you saying the the house lights stay on when you remove the cables to the outboard side of the boost solenoid, but go out when you remove the
> wire from the house terminal of the isolator?
>
> Ken wrote: I question your diagnosis in that to have a failure that you are describing, it would take a double shorted diode in the isolator and
> no power being supplied by the house battery.
>
> I have been straining to get this figured out and Ken may be mistaken.... (I never thought I would write that. - Either Ken)
> If the alternator to engine battery went solid (as opposed to open) then this could happen, but it would require some creative rewiring as the
> terminal that is the central to all the 12V house is that post in the boost contactor. This would also make the alternator terminal hot when it
> should not be, but that should cause no other issues.
>
> If the both diodes solid is correct, that is a very strange case. In all my years of 12V electrics, I have only seen one isolator diode go solid.
> These typically fail open when they do fail and that is just not common. And the rarely both fail. (Lightning strikes are another matter.)
>
> I have to believe that that there is something there that should not be.
>
> A simple test with a meter will confirm that the isolator has failed. An ohmmeter reading from the center to either post should give a reading
> one way and nothing the other.
>
> Please tell us if/when you find it.
>
> Matt

Matt, Here is what I was thinking when I wrote that:
He disconnects from the isolator and power goes away. He has no house battery connected at the time
so the power source has to be through the isolator. Now where is that
+12 volts coming from? With the engine running it could come through the
center terminal of the isolator. With the engine not running the center terminal
should be 0 VDC. Second path could be from the the engine side battery. This
would only occur if the isoltaor engine side diode was shorted. I now see what
I think is your point. It would take only ONE SHORTED DIODE on the engine side to
supply power to the center terminal and on through a good (or shorted) house side diode.

What I would to like to see is with the engine not running the voltage reading on the
isolator center terminal (should be 0) and the voltage on the house side isolator terminal.

I'm still thinking that he might have a wiring error somewhere. Metering the three
isolator terminals without the engine running might help isolate the issue. If there is
voltage on the center terminal. I would disconnect the wire on the center terminal
and read the voltage again. There are obviously other ways to accomplish the same thing.

Do not feel bad about calling me out on this. I have made many mistakes in my life
and I would rather have someone correct my errors than give out bad information.
That is why I got married twice. I once worked with a guy that was married 5 times.

Three of those times were to the same woman. Talk about mistakes.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

 

[Matt Colie]

> > Peer,
> >
> > This is not any kind of a criticism of of your language capability. (Your English is much better than my German.)
> > The blue finned thing is the isolator and the round thing below it is the boost solenoid. Agree?
> > (Remember I have another '73 23' and know the issues well.)
> >
> > Are you saying the the house lights stay on when you remove the cables to the outboard side of the boost solenoid, but go out when you remove
> > the wire from the house terminal of the isolator?
> >
> > Ken wrote: I question your diagnosis in that to have a failure that you are describing, it would take a double shorted diode in the isolator
> > and no power being supplied by the house battery.
> >
> > I have been straining to get this figured out and Ken may be mistaken.... (I never thought I would write that. - Either Ken)
> > If the alternator to engine battery went solid (as opposed to open) then this could happen, but it would require some creative rewiring as the
> > terminal that is the central to all the 12V house is that post in the boost contactor. This would also make the alternator terminal hot when it
> > should not be, but that should cause no other issues.
> >
> > If the both diodes solid is correct, that is a very strange case. In all my years of 12V electrics, I have only seen one isolator diode go
> > solid. These typically fail open when they do fail and that is just not common. And the rarely both fail. (Lightning strikes are another
> > matter.)
> >
> > I have to believe that that there is something there that should not be.
> >
> > A simple test with a meter will confirm that the isolator has failed. An ohmmeter reading from the center to either post should give a
> > reading one way and nothing the other.
> >
> > Please tell us if/when you find it.
> >
> > Matt
>
>
> Matt, Here is what I was thinking when I wrote that:
> He disconnects from the isolator and power goes away. He has no house battery connected at the time
> so the power source has to be through the isolator. Now where is that
> +12 volts coming from? With the engine running it could come through the
> center terminal of the isolator. With the engine not running the center terminal
> should be 0 VDC. Second path could be from the the engine side battery. This
> would only occur if the isoltaor engine side diode was shorted. I now see what
> I think is your point. It would take only ONE SHORTED DIODE on the engine side to
> supply power to the center terminal and on through a good (or shorted) house side diode.
>
> What I would to like to see is with the engine not running the voltage reading on the
> isolator center terminal (should be 0) and the voltage on the house side isolator terminal.
>
> I'm still thinking that he might have a wiring error somewhere. Metering the three
> isolator terminals without the engine running might help isolate the issue. If there is
> voltage on the center terminal. I would disconnect the wire on the center terminal
> and read the voltage again. There are obviously other ways to accomplish the same thing.
>
> Do not feel bad about calling me out on this. I have made many mistakes in my life
> and I would rather have someone correct my errors than give out bad information.
> That is why I got married twice. I once worked with a guy that was married 5 times.
>
> Three of those times were to the same woman. Talk about mistakes.


Ken,

As is the common case, we agree completely.

My big problem is that in many decades of doing 12V diagnostics, I have only seen one other diode go short. Open? Lots, but Short? I know how that
one happened. It would be difficult to make that happen again, but we won't know until Peer does more diagnostic. At least we have a good hint that
one side is right.

I'll Wait, I want to add this to the "You know what happened once" list of strange things.

Matt

--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

 

[johnny]

They short - usually from enough overcurrent to melt the junction, but not enough to burn it open. I pulled a lot of shorted diodes out of HV supplies
over the years. (There's also a relatively famous circumstance in a specific equipment model wherein resistors short. Never seen that anywhere
else.)

--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

 

[peer oliver schmidt]

Ken Henderson schrieb/wrote
> Peer,
>
> Sounds to me like the chassis battery diode in your isolator is shorted --
> an unusual failure on a GMC. Disconnect the chassis battery lead at the
> isolator as a preliminary confirmation. Then, without reconnecting to that
> terminal, use an ohmmeter to measure between that terminal and the center
> one. The resistance should be very low and the same when you reversed the
> meter lead connections.

Ken,

my findings are very strange. The resistance is infinitive, no matter
what way I select. This is without the motor running, and with
disconnected house and chassis battery from the isolator. But no
problem, I have already made up my mind to take out the isolator and
solenoid and replace it with the doodad I ordered. Will report back how
things work out.

--
Best regards

Peer Oliver Schmidt
PGP Key ID: 0x83E1C2EA

'76a Eleganza II 26', VA
'73 Sequoia 23', Germany, SH

 

[Ken H.]

Peer,

It is a little strange for the bilateral resistance to be infinite. But it
may only be that the voltage of your ohmmeter is not high enough to
overcome the forward voltage breakdown of the diode ('tho' I would expect
there to be no such barrier in a shorted diode),

I'll repeat Ken B's caution: I would NOT depend on the new combiner to
also serve the boost function. If it's current limited somehow (as with
Yandina's extended "Do Not Cut" leads), then it won't supply sufficient
current for the starter. If not limited, its contacts are not likely to
long survive that current overload.

If the diagnoses so far are correct, the old boost relay is probably still
good.

JWID,

​Ken H.​

 

[peer oliver schmidt]

Ken Henderson schrieb/wrote
> If the diagnoses so far are correct, the old boost relay is probably still
> good.

I agree.

--
Best regards

Peer Oliver Schmidt
the internet company
PGP Key ID: 0x83E1C2EA

 

[Keith V]

if you are using a digital volt meter it wont be able to forward bias a diode ( by design ) you have to use the diode test function.

It will show you the voltage drop across the diode

But I am skeptical of infinite, thats pretty high

________________________________
From: Gmclist on behalf of Peer Oliver Schmidt
Sent: Friday, December 22, 2017 10:51 AM
To: gmclist
Subject: Re: [GMCnet] Identify electric device

Ken Henderson schrieb/wrote
> If the diagnoses so far are correct, the old boost relay is probably still
> good.

I agree.

--
Best regards

Peer Oliver Schmidt
the internet company
PGP Key ID: 0x83E1C2EA

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