Inherited a GMC! Hello and please help me!!!

Looks like you're in (or near) Austin, TX

Don't be afraid to ask for help. I'm sure there are some GMC people nearby who would be happy to look over your shoulder and (maybe) help.

Worked for me,
--
Jim Gunther
www.LotusV6.com

now former owner - ;(

73 GMC-II 2600
by Explorer
 
Just to clairify, there is nothing connected to my isolator. I am trying to determine why the PO did this by testing to see if the isolator is good
or bad.
I tested using my multimeter on the >/ setting. (I don't know the name). The readings from positive on alt post and meg on other posts was stated.

All batteries in coach (5) are reading between 12.6 and 12.9 volts. Coach is currently plugged into shore., and on float charge.
 
If it is not connected he should test it by using the ohms setting.
Put one meter probe on the center post and the other on one side post. It will show either infinite resistance or close to zero. Then reverse the probes. It should show the same readings but reversed.

Is so that means the diode on that side is good.

Then repeat the procedure testing the other side post to the center post.

Let us know what you find.

Emery Stora
77 Kingsley
Frederick, CO

>
> In case you did not notice, in the original posting, the isolator is not physically connected into any circuit. At best, he can only measure for diode
> failure..
> Tom, MS ll
> --
> 1975 GMC Avion
> KA4CSG
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
> ...I am trying to determine why the PO did this by testing to see if the isolator is good or bad. ...
It looks like you were trying to use the diode setting of the meter. I don't have that setting on mine, and it has been so long since I used a meter
with one, I don't know what you are supposed to see or hear so I am going to tell you how to check the isolator using the OHMS setting. Everyone here
is trying to get a reading that makes sense to them, and what you reported is ambiguous.

Set your meter to 200 ohms. Put the red probe on the center post and the black one on of the side posts. Should read a number. Put the black lead on
the other side post. Should read a number. What are those numbers?

Put the black probe on the center post. Put the red probe on a side post. Should be high or infinite impedance, so it should not display any numbers,
or it should blink something at you. Put the red probe on the other side post. Should not display any numbers, or should blink something at you. Is it
high/infinite impedance on both?

High/infinite impedance of one or both posts on the first test, OR low impedance on one or both of the posts on the second test means one or both
diodes are shot --> dead isolator.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."
 
Most current digital multimeters include a diode checking capability.
Select diode check mode and put test leads across center to end isolator terminals.
You will probably get a result (in millivolts) somewhere around 500 to 600 that indicates a good diode. If you get no reading reverse the test leads.

D C "Mac" Macdonald in OKC

Sent from my iPad

>
> If it is not connected he should test it by using the ohms setting.
> Put one meter probe on the center post and the other on one side post. It will show either infinite resistance or close to zero. Then reverse the probes. It should show the same readings but reversed.
>
> Is so that means the diode on that side is good.
>
> Then repeat the procedure testing the other side post to the center post.
>
> Let us know what you find.
>
> Emery Stora
> 77 Kingsley
> Frederick, CO
>

>>
>> In case you did not notice, in the original posting, the isolator is not physically connected into any circuit. At best, he can only measure for diode
>> failure..
>> Tom, MS ll
>> --
>> 1975 GMC Avion
>> KA4CSG
>>
>> _______________________________________________
>> 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
 
Hi Mac-

I did as you just said and got 540-ish on both terminals, and nothing when I reversed them. Would you assume it's good?
 
If that's using the DIODE CHECK you are, indeed, good to go.
I believe isolators are a very low failure item, but it's good to know for sure.

Now it's time to get wiring straightened out. Output from the alternator goes to the center terminal. The other terminals go to engine battery and "house" battery. Get knowledgeable help to do this if you need it!

This arrangement allows BOTH battery systems to be charged by the engine alternator when the engine is running.

Mac in OKC

Sent from my iPad

>
> Hi Mac-
>
> I did as you just said and got 540-ish on both terminals, and nothing when I reversed them. Would you assume it's good?
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
I fully understand the function and reason for a isolator. Does anyone have an idea why PO would bypass it?

I'll fish out the wire from the ALT tonight and see where it goes as is. NOW, to figure out where the house battery wires are!
 
More than likely the isolator has been replaced with combiners of some
type, but this is a guess at this point.
Jim Hupy

> I fully understand the function and reason for a isolator. Does anyone
> have an idea why PO would bypass it?
>
> I'll fish out the wire from the ALT tonight and see where it goes as is.
> NOW, to figure out where the house battery wires are!
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
Adam, Mac, et al,

I personally would not trust a low current test like that provided by a
multimeter to rate an isolator. I've done post-mortems on a couple of
failed isolators. In both cases, I found that the failure mode was failure
of the welded/soldered joint between one of the diodes and the common base
plate between the two.

I suspect that the continual thermal stress and strain from the unequal
expansion and contraction of the diodes and the epoxy potting material
eventually causes a fracture.

While the integrity of the electrical connection is compromised, the
remaining mechanical contact will conduct a little current. The trivial
current required for the multimeter to measure the voltage across the diode
is not sufficient to stress the joint.

I'd test the isolator voltages while passing at least several amps of
current -- preferably after long enough for the junction to warm up and
separate.

JMHO,

Ken H.

On Thu, Oct 13, 2016 at 5:08 PM, D C _Mac_ Macdonald
wrote:

> If that's using the DIODE CHECK you are, indeed, good to go.
> I believe isolators are a very low failure item, but it's good to know for
> sure.
>
> Now it's time to get wiring straightened out. Output from the alternator
> goes to the center terminal. The other terminals go to engine battery and
> "house" battery. Get knowledgeable help to do this if you need it!
>
> This arrangement allows BOTH battery systems to be charged by the engine
> alternator when the engine is running.
>
> Mac in OKC
>
>
> Sent from my iPad
>

> >
> > Hi Mac-
> >
> > I did as you just said and got 540-ish on both terminals, and nothing
> when I reversed them. Would you assume it's good?
> >
> > _______________________________________________
>
 
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