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[GMCnet] Isolator -- bypass? [message #326801] Wed, 06 December 2017 12:47 Go to next message
Dave Stragand is currently offline  Dave Stragand   United States
Messages: 26
Registered: October 2017
Karma: 0
Junior Member
Hi All,



Just started poking at the no-charging issue on our 1978 Transmode
(403).



Thanks to whomever created that great flowchart for troubleshooting
charging issues. I was able to determine that the alternator voltage
(center pole of the isolator) to battery terminal ground was 14.70
volts, so the alternator seems within spec..



However, both of the output poles on the isolator (top and bottom) are
connected directly to the same point - the "vehicle battery positive
pole". On a Transmode that may not have had a second or house battery,
that kind of makes sense to me, but if that were the case, why would GM
bother to install an isolator? In any case, the reading from either
pole on the isolator is a dismal-but-steady 12.02V - and that's too low.
It matches the 12.0 on the cheapo cigarette lighter voltmeter inside.



That leaves me with a few questions, and I was hoping y'all might be
able to help.



1) Is the low voltage on the battery sides of the isolator possibly
caused by the fact that both the upper and lower outputs are connected
together?



2) Considering I don't really have any house circuits at present (it's
a Transmode with a partial homemade interior), can I simply bypass the
alternator for the time being, connecting the alternator wire directly
to the "vehicle battery positive" pole?



3) If my isolator is indeed shot, does anyone have a good used spare
they'd like to sell, or should I just buy a new one from one of our
awesome vendors?



Thanks, All!



-Dave

1978 Transmode (403)

Pittsburgh, PA

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1978 Transmode (403) Pittsburgh, PA
Re: [GMCnet] Isolator -- bypass? [message #326802 is a reply to message #326801] Wed, 06 December 2017 13:04 Go to previous messageGo to next message
Emery Stora is currently offline  Emery Stora   United States
Messages: 675
Registered: January 2011
Karma: 1
Senior Member
Both batteries should NOT be connected to the same isolator post. Keep one that goes to the “vehicle battery positive pole”. Disconnect the other one. It should go to the second isolator post.
Most transmodes have a boost switch on the dash and a second battery (the “house” battery).

If you are certain that you don’t have a second battery (look in the back by the Onan) then you can connect the wire from the alternator (center isolator post) to the vehicle battery positive pole.
However, if you do have a second battery for the Onan or the house circuits that battery would not be charged.

Emery Stora
77 Kingsley
Frederick, CO
> On Dec 6, 2017, at 11:47 AM, Dave Stragand wrote:
>
> Hi All,
>
>
>
> Just started poking at the no-charging issue on our 1978 Transmode
> (403).
>
>
>
> Thanks to whomever created that great flowchart for troubleshooting
> charging issues. I was able to determine that the alternator voltage
> (center pole of the isolator) to battery terminal ground was 14.70
> volts, so the alternator seems within spec..
>
>
>
> However, both of the output poles on the isolator (top and bottom) are
> connected directly to the same point - the "vehicle battery positive
> pole". On a Transmode that may not have had a second or house battery,
> that kind of makes sense to me, but if that were the case, why would GM
> bother to install an isolator? In any case, the reading from either
> pole on the isolator is a dismal-but-steady 12.02V - and that's too low.
> It matches the 12.0 on the cheapo cigarette lighter voltmeter inside.
>
>
>
> That leaves me with a few questions, and I was hoping y'all might be
> able to help.
>
>
>
> 1) Is the low voltage on the battery sides of the isolator possibly
> caused by the fact that both the upper and lower outputs are connected
> together?
>
>
>
> 2) Considering I don't really have any house circuits at present (it's
> a Transmode with a partial homemade interior), can I simply bypass the
> alternator for the time being, connecting the alternator wire directly
> to the "vehicle battery positive" pole?
>
>
>
> 3) If my isolator is indeed shot, does anyone have a good used spare
> they'd like to sell, or should I just buy a new one from one of our
> awesome vendors?
>
>
>
> Thanks, All!
>
>
>
> -Dave
>
> 1978 Transmode (403)
>
> Pittsburgh, PA
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org


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Re: [GMCnet] Isolator -- bypass? [message #326803 is a reply to message #326802] Wed, 06 December 2017 13:27 Go to previous messageGo to next message
James Hupy is currently offline  James Hupy   United States
Messages: 4551
Registered: May 2010
Karma: 33
Senior Member
That "dismal 12.0vdc" is with the engine running, or with it not running?
If not running, then you are reading the battery voltage. 12.0 vdc would
indicate to me that the battery shows a low state of charge.
Under ideal laboratory conditions, a wet cell with a specific gravity
of 1.260???? at 70° farenheit, and humidity of 30.00 in hg, should render a
2.2 volt electrical potential. Multiply that by 6 and ideal state of charge
should "potentially" be 13.2 volts. Now deduct resistance, meter
fluctuation, etc., etc., and your real world state of charge should be 12.8
volts or so. Just sayin'.
Now, if your engine is running and alternator output is normal (whatever
that is) you should see AROUND 14.5 VOLTS on the center tap of the
isolator, about .7 volt less on each leg if your isolator is good. If not,
sumpthin' ain't right.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

On Dec 6, 2017 11:05 AM, "Emery Stora" wrote:

> Both batteries should NOT be connected to the same isolator post. Keep
> one that goes to the “vehicle battery positive pole”. Disconnect the other
> one. It should go to the second isolator post.
> Most transmodes have a boost switch on the dash and a second battery (the
> “house” battery).
>
> If you are certain that you don’t have a second battery (look in the back
> by the Onan) then you can connect the wire from the alternator (center
> isolator post) to the vehicle battery positive pole.
> However, if you do have a second battery for the Onan or the house
> circuits that battery would not be charged.
>
> Emery Stora
> 77 Kingsley
> Frederick, CO
>> On Dec 6, 2017, at 11:47 AM, Dave Stragand dave-DOT-stragand-AT-forwardlook-DOT-net> wrote:
>>
>> Hi All,
>>
>>
>>
>> Just started poking at the no-charging issue on our 1978 Transmode
>> (403).
>>
>>
>>
>> Thanks to whomever created that great flowchart for troubleshooting
>> charging issues. I was able to determine that the alternator voltage
>> (center pole of the isolator) to battery terminal ground was 14.70
>> volts, so the alternator seems within spec..
>>
>>
>>
>> However, both of the output poles on the isolator (top and bottom) are
>> connected directly to the same point - the "vehicle battery positive
>> pole". On a Transmode that may not have had a second or house battery,
>> that kind of makes sense to me, but if that were the case, why would GM
>> bother to install an isolator? In any case, the reading from either
>> pole on the isolator is a dismal-but-steady 12.02V - and that's too low.
>> It matches the 12.0 on the cheapo cigarette lighter voltmeter inside.
>>
>>
>>
>> That leaves me with a few questions, and I was hoping y'all might be
>> able to help.
>>
>>
>>
>> 1) Is the low voltage on the battery sides of the isolator possibly
>> caused by the fact that both the upper and lower outputs are connected
>> together?
>>
>>
>>
>> 2) Considering I don't really have any house circuits at present (it's
>> a Transmode with a partial homemade interior), can I simply bypass the
>> alternator for the time being, connecting the alternator wire directly
>> to the "vehicle battery positive" pole?
>>
>>
>>
>> 3) If my isolator is indeed shot, does anyone have a good used spare
>> they'd like to sell, or should I just buy a new one from one of our
>> awesome vendors?
>>
>>
>>
>> Thanks, All!
>>
>>
>>
>> -Dave
>>
>> 1978 Transmode (403)
>>
>> Pittsburgh, PA
>>
>> _______________________________________________
>> 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
>
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Re: [GMCnet] Isolator -- bypass? [message #326807 is a reply to message #326803] Wed, 06 December 2017 14:37 Go to previous messageGo to next message
Dave Stragand is currently offline  Dave Stragand   United States
Messages: 26
Registered: October 2017
Karma: 0
Junior Member
Thanks Emory & Jim,

12.02 volts is -with- the engine running, so indeed sumpthin' ain't right. I'm betting the isolator is completely open so I'm just reading battery voltage at that point. I should have checked the voltage difference between the input and output on the isolator as well, which would verify that guess.

I believe I'll bypass the isolator and see if I have any luck that way, temporarily.

Battery is new, but I have been topping it off by charging it with my other vehicle until I figure this out. It had run down to a point where cranking was slow, but I charged it off of my car for about 45 minutes or so and it topped off just fine. I'll have to note the voltage at the battery with the engine off as well tomorrow.

The generator does indeed have a separate battery, but it is -not- connected to any house or 12v circuits -- it's just a starter battery. (Well, it would be a starter battery if it was physically there. Or if the generator actually started. But those are issues for another day.)

Forgot to add a link to Ken Burton's great flowchart. It's http://www.gmcmhphotos.com/photos/data/15/GMC_charging_system_checkout.pdf

I got to step #3. Just wondered if the cause was the combined top/bottom leads, but it makes more sense that it's just failed open.

-Dave
1978 Transmode (403)
Pittsburgh, PA

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces-AT-list-DOT-gmcnet-DOT-org] On Behalf Of James Hupy
Sent: Wednesday, December 06, 2017 2:27 PM
To: gmclist-AT-list-DOT-gmcnet-DOT-org
Subject: Re: [GMCnet] Isolator -- bypass?

That "dismal 12.0vdc" is with the engine running, or with it not running?
If not running, then you are reading the battery voltage. 12.0 vdc would indicate to me that the battery shows a low state of charge.
Under ideal laboratory conditions, a wet cell with a specific gravity of 1.260???? at 70° farenheit, and humidity of 30.00 in hg, should render a
2.2 volt electrical potential. Multiply that by 6 and ideal state of charge should "potentially" be 13.2 volts. Now deduct resistance, meter fluctuation, etc., etc., and your real world state of charge should be 12.8 volts or so. Just sayin'.
Now, if your engine is running and alternator output is normal (whatever that is) you should see AROUND 14.5 VOLTS on the center tap of the isolator, about .7 volt less on each leg if your isolator is good. If not, sumpthin' ain't right.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

On Dec 6, 2017 11:05 AM, "Emery Stora" wrote:

> Both batteries should NOT be connected to the same isolator post.
> Keep one that goes to the “vehicle battery positive pole”. Disconnect
> the other one. It should go to the second isolator post.
> Most transmodes have a boost switch on the dash and a second battery
> (the “house” battery).
>
> If you are certain that you don’t have a second battery (look in the
> back by the Onan) then you can connect the wire from the alternator
> (center isolator post) to the vehicle battery positive pole.
> However, if you do have a second battery for the Onan or the house
> circuits that battery would not be charged.
>
> Emery Stora
> 77 Kingsley
> Frederick, CO
>> On Dec 6, 2017, at 11:47 AM, Dave Stragand dave-DOT-stragand-AT-forwardlook-DOT-net> wrote:
>>
>> Hi All,
>>
>>
>>
>> Just started poking at the no-charging issue on our 1978 Transmode
>> (403).
>>
>>
>>
>> Thanks to whomever created that great flowchart for troubleshooting
>> charging issues. I was able to determine that the alternator
>> voltage (center pole of the isolator) to battery terminal ground was
>> 14.70 volts, so the alternator seems within spec..
>>
>>
>>
>> However, both of the output poles on the isolator (top and bottom)
>> are connected directly to the same point - the "vehicle battery
>> positive pole". On a Transmode that may not have had a second or
>> house battery, that kind of makes sense to me, but if that were the
>> case, why would GM bother to install an isolator? In any case, the
>> reading from either pole on the isolator is a dismal-but-steady 12.02V - and that's too low.
>> It matches the 12.0 on the cheapo cigarette lighter voltmeter inside.
>>
>>
>>
>> That leaves me with a few questions, and I was hoping y'all might be
>> able to help.
>>
>>
>>
>> 1) Is the low voltage on the battery sides of the isolator possibly
>> caused by the fact that both the upper and lower outputs are
>> connected together?
>>
>>
>>
>> 2) Considering I don't really have any house circuits at present
>> (it's a Transmode with a partial homemade interior), can I simply
>> bypass the alternator for the time being, connecting the alternator
>> wire directly to the "vehicle battery positive" pole?
>>
>>
>>
>> 3) If my isolator is indeed shot, does anyone have a good used
>> spare they'd like to sell, or should I just buy a new one from one
>> of our awesome vendors?
>>
>>
>>
>> Thanks, All!
>>
>>
>>
>> -Dave
>>
>> 1978 Transmode (403)
>>
>> Pittsburgh, PA
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1978 Transmode (403) Pittsburgh, PA
Re: [GMCnet] Isolator -- bypass? [message #326823 is a reply to message #326803] Thu, 07 December 2017 00:25 Go to previous messageGo to next message
Ken Burton is currently offline  Ken Burton   United States
Messages: 8368
Registered: January 2004
Location: Hebron, Indiana
Karma: 31
Senior Member

Something is not Kosher here.

The alternator putting out 14.7 volts is great with an isolator installed. The alternator monitors the voltage on the engine side of the isolator and raises or lowers it's output voltage trying to maintain around 14.0 volts at the engine battery. You are saing that is is only 12 volts.

So what you are saying is the alternator is doing it's job correctly while the sensing line to the alternator must be low. I am thinking you have a wiring error(s) somewhere.

Are you positive that you have no 12 volt supply to anything on the house side of the coach?

I am looking at a wiring Labeled "78 GMC Transmode" and it shows an auxiliary battery, Boost relay, Isolator, and Onan Generator.

If you send me your email address I'll forward this diagram to you. It looks the same or very similar to a normal (non-Transmode) 26 foot GMC.

If you want to talk about this on the phone, look up my unlisted number on the "Black List" under Indiana. If you get my voice mail when you call, speak SLOWLY and repeat any numbers twice. I have a hearing loss and if I do not understand your message I will not be able to call you back.

Ken Burton


Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
Re: [GMCnet] Isolator -- bypass? [message #326841 is a reply to message #326823] Thu, 07 December 2017 11:00 Go to previous messageGo to next message
Dave Stragand is currently offline  Dave Stragand   United States
Messages: 26
Registered: October 2017
Karma: 0
Junior Member
Hi Ken,

Thanks for the offer! I may take you up on that sometime. I'm afraid
there are differences in my coach between the original as-delivered
Transmode wiring and its current state. It is a true Transmode and was
not upfitted into a motorhome initially. My research so far shows that
while it was originally delivered to Phoenix, it may have spent time as
a ski lodge shuttle in Tennessee.

The original sales invoice from GM shows it was originally delivered
with an Onan 6K generator, though it presently has a 4K Onan BFA
instead. There was a starter attery for the generator installed at the
driver's rear, but it is not connected to any internal 12v systems.
http://www.gmcmhphotos.com/photos/1978-transmode-first-pictures/p64088-i
mg-2680.html

All of the current 12v house interior systems (pretty much just some
lighting) are connected via a fused link connected to the chassis
battery.

There is a large inverter now connected to a secondary battery that runs
the microwave and fridge (120VAC). I don't know how the rest of the
house is wired yet, 120V-wise, but we will be stripping out the interior
down to the shell and starting from scratch. The other known 120VAC
systems I have found in the coach are two roof AC units and some
outlets. I have not found any water heater yet, but I'm hoping there is
one.

And yes, I know that the way things are now is mostly incorrect, but
I'll set it right.

A current pic of the inverter area wiring is here:
http://www.gmcmhphotos.com/photos/1978-transmode-first-pictures/p63984-i
mg-23521.html I will be replacing the inverter next week.

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces-AT-list-DOT-gmcnet-DOT-org] On Behalf Of Ken
Burton
Sent: Thursday, December 07, 2017 1:25 AM
To: gmclist-AT-list-DOT-gmcnet-DOT-org
Subject: Re: [GMCnet] Isolator -- bypass?

Something is not Kosher here.

The alternator putting out 14.7 volts is great with an isolator
installed. The alternator monitors the voltage on the engine side of
the isolator and raises or lowers it's output voltage trying to maintain
around 14.0 volts at the engine battery. You are saing that is is only
12 volts.

So what you are saying is the alternator is doing it's job correctly
while the sensing line to the alternator must be low. I am thinking you
have a wiring error(s) somewhere.

Are you positive that you have no 12 volt supply to anything on the
house side of the coach?

I am looking at a wiring Labeled "78 GMC Transmode" and it shows an
auxiliary battery, Boost relay, Isolator, and Onan Generator.

If you send me your email address I'll forward this diagram to you. It
looks the same or very similar to a normal (non-Transmode) 26 foot GMC.

If you want to talk about this on the phone, look up my unlisted number
on the "Black List" under Indiana. If you get my voice mail when you
call,
speak SLOWLY and repeat any numbers twice. I have a hearing loss and if
I do not understand your message I will not be able to call you back.


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

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1978 Transmode (403) Pittsburgh, PA
Re: [GMCnet] Isolator -- bypass? [message #326850 is a reply to message #326841] Thu, 07 December 2017 13:37 Go to previous messageGo to next message
Ken Burton is currently offline  Ken Burton   United States
Messages: 8368
Registered: January 2004
Location: Hebron, Indiana
Karma: 31
Senior Member

Wow

The picture is a great help. I can not follow every wire in the picture but some are definitely missing or moved.

1. There appears to be a wire missing from the boost relay left side to the lower terminal of the isolator. This supplies the house side of things.

2. The wire from the vehicle positive to the isolator goes on the top terminal of the isolator. Yours has been moved to the bottom.

3. It appears that the wire from the alternator that is suppose to go to the center terminal of the isolator has been moved to the bottom terminal. This effectively bypasses the isolator for power from the alternator to the hose side, EXCEPT since the wire from the engine side has been moved to the bottom, so it is bypassing the isolator for power to the engine side.

4. The heavy wire tat goes from the boost solenoid to the rear for the Onan battery and house power circuits ids still installed and could / should be reused to supply alternator power to the house systems.

5. I see a CB mounted near the boost relay. It has probably a 10 ga. wire going somewhere. It is non-standard and I can not guess where it goes.

I am thinking that I would try to get the system back to OEM so we can more easily try to help you. When it is non-standard it becomes very difficult to remotely assist.

Again from what I can see, this Transmode appears to have come out of the factory wired the same as any GM upfitted GMC. So just about any GMC Chassis wiring diagram ought to work for you or I can send you an electronic copy of the one that I have


Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
Re: [GMCnet] Isolator -- bypass? [message #326853 is a reply to message #326841] Thu, 07 December 2017 13:55 Go to previous messageGo to next message
Dave Stragand is currently offline  Dave Stragand   United States
Messages: 26
Registered: October 2017
Karma: 0
Junior Member
I pretty much only get time to work on this coach during my lunch hour,
but hopefully all of the little 30-minute blips will amount to
something.

Today I placed a jumper from the alternator post to the lower post on
the inverter, and immediately started seeing 13.8 - 14.0 V inside on the
cigarette-lighter voltmeter. There was a noticeable difference in the
brightness of the inside lights, etc.

All in all, I'm pretty sure that the inverter is shot. I have a 95-amp
replacement inverter on the way, which should prove it definitively.

I also believe that the current alternator is a 100-amp version, due to
the extra mounting 'ear' on the back side of the case. Can anyone verify
by this photo:
http://www.gmcmhphotos.com/photos/member-galleries/p64089-alternator.htm
l If it is 100 amp, I could imagine that the high-amp alternator killed
the 80-amp isolator at some point

-Dave
1978 Transmode (403)
Pittsburgh, PA

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1978 Transmode (403) Pittsburgh, PA
Re: [GMCnet] Isolator -- bypass? [message #326857 is a reply to message #326850] Thu, 07 December 2017 15:04 Go to previous messageGo to next message
Dave Stragand is currently offline  Dave Stragand   United States
Messages: 26
Registered: October 2017
Karma: 0
Junior Member
Thank, Ken! Very helpful. I am indeed attempting to move it back to a
standard configuration, so this is all very helpful.

It's hard to see from the pictures, but the alternator wire does go
directly to the center post of the isolator. Both the top and bottom
wires on the isolator are presently connected to the 'vehicle battery
positive' post, but I realize that the lower wire -should- be going to
the left side of the boost solenoid. That will be corrected with the
new isolator install.

The larger wire on the house side of the boost solenoid goes to an old
battery I had lying around, and the smaller to the non-standard circuit
breaker, which then goes to somewhere unknown inside.

There are more pictures that can help listed below.

http://www.gmcmhphotos.com/photos/member-galleries/p64090-img-2862.html
http://www.gmcmhphotos.com/photos/member-galleries/p64091-img-2863.html
http://www.gmcmhphotos.com/photos/member-galleries/p64092-img-28641.html
http://www.gmcmhphotos.com/photos/member-galleries/p64093-img-2865.html
http://www.gmcmhphotos.com/photos/member-galleries/p64094-img-28661.html

Here's the battery layout, which at present is very confusing:
I'm calling the rear battery the house, and the front battery the
chassis. Rear battery negative goes to a ground on the frame rail.
Rear battery positive connected to left side of boost solenoid. Front
battery top post negative goes to a ground on the engine. Front battery
side post positive goes to the right side of the boost solenoid. Front
battery top post positive goes to the inverter, and has a couple of
fused wires attached that seem to be for the stereo/amp, and possibly
other internal systems that are unknown at this point. Why they
connected them directly to the battery terminal is unknown, especially
considering there are so many more convenient points to do so in the
immediate area.

-Dave
1978 Transmode (403)
Pittsburgh, PA

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces-AT-list-DOT-gmcnet-DOT-org] On Behalf Of Ken
Burton
Sent: Thursday, December 07, 2017 2:37 PM
To: gmclist-AT-list-DOT-gmcnet-DOT-org
Subject: Re: [GMCnet] Isolator -- bypass?

Wow

The picture is a great help. I can not follow every wire in the picture
but some are definitely missing or moved.

1. There appears to be a wire missing from the boost relay left side to
the lower terminal of the isolator. This supplies the house side of
things.

2. The wire from the vehicle positive to the isolator goes on the top
terminal of the isolator. Yours has been moved to the bottom.

3. It appears that the wire from the alternator that is suppose to go
to the center terminal of the isolator has been moved to the bottom
terminal.
This effectively bypasses the isolator for power from the alternator to
the hose side, EXCEPT since the wire from the engine side has been moved
to
the bottom, so it is bypassing the isolator for power to the engine
side.

4. The heavy wire tat goes from the boost solenoid to the rear for the
Onan battery and house power circuits ids still installed and could /
should be reused to supply alternator power to the house systems.

5. I see a CB mounted near the boost relay. It has probably a 10 ga.
wire going somewhere. It is non-standard and I can not guess where it
goes.

I am thinking that I would try to get the system back to OEM so we can
more easily try to help you. When it is non-standard it becomes very
difficult to remotely assist.

Again from what I can see, this Transmode appears to have come out of
the factory wired the same as any GM upfitted GMC. So just about any
GMC Chassis wiring diagram ought to work for you or I can send you an
electronic copy of the one that I have
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

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1978 Transmode (403) Pittsburgh, PA
Re: [GMCnet] Isolator -- bypass? [message #326858 is a reply to message #326853] Thu, 07 December 2017 15:11 Go to previous messageGo to next message
Dave Stragand is currently offline  Dave Stragand   United States
Messages: 26
Registered: October 2017
Karma: 0
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Link correction:
http://www.gmcmhphotos.com/photos/member-galleries/p64089-alternator.htm
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1978 Transmode (403) Pittsburgh, PA
Re: [GMCnet] Isolator -- bypass? [message #326876 is a reply to message #326858] Thu, 07 December 2017 21:58 Go to previous messageGo to next message
Ken Burton is currently offline  Ken Burton   United States
Messages: 8368
Registered: January 2004
Location: Hebron, Indiana
Karma: 31
Senior Member

I looked at all of the pictures. I still do not see where the alternator wire is coming out of the wiring harness. The alternator wire usually comes out near the top horizontally from the vertical split loom tubing. I'll take your observation and assume The alternator wire is attached correctly.

As far as the isolsator goes I see no advantage in attaching the house and the engine / chassis to the same isolator terminal. I would change it back when you install the new isolator. I would not worry about the 80 amp isolator and 100 amp alternator combination because that is 80 amps per port. That said a 100 or 125 amp isolator is the same price as an 80 so why not go for the heavier one. I saw a 3 battery isolator rated at 125. That might be a good solution if you are buying a new one. You do not have to use the 3rd port now but you could use it in the future.

I'm working on a 78 GMC right now. It needs a new converter. I'm waiting for the owner to get a replacement Progressive Dynamics one. In the interim I attached a 12.5 amp switching type power supply and adjusted it to 13.2 volts. Worked great. Then tonight I disconnected the 120 VAC before disconnecting the 12 volt side. The thing went up in smoke. My whole hangar now smells like an electrical fire. It really smoked.

Good Luck. It sounds like you are on top of your problem(s).

Ken B.


Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
Re: [GMCnet] Isolator -- bypass? [message #326892 is a reply to message #326876] Fri, 08 December 2017 08:15 Go to previous messageGo to next message
Ken Henderson is currently offline  Ken Henderson   United States
Messages: 7318
Registered: March 2004
Location: Americus, GA
Karma: 41
Senior Member
https://docs.google.com/viewer?url=http%3A%2F%2Fwww.gmcmotorhomemarketplace.com%2FGMC_Motorhome_Wiring_Diagrams.pdf

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

On Thu, Dec 7, 2017 at 10:58 PM, Ken Burton wrote:

> I looked at all of the pictures. I still do not see where the alternator
> wire is coming out of the wiring harness. The alternator wire usually comes
> out near the top horizontally from the vertical split loom tubing. I'll
> take your observation and assume The alternator wire is attached correctly.
>
>
> As far as the isolsator goes I see no advantage in attaching the house and
> the engine / chassis to the same isolator terminal. I would change it back
> when you install the new isolator. I would not worry about the 80 amp
> isolator and 100 amp alternator combination because that is 80 amps per
> port.
> That said a 100 or 125 amp isolator is the same price as an 80 so why not
> go for the heavier one. I saw a 3 battery isolator rated at 125. That
> might be a good solution if you are buying a new one. You do not have to
> use the 3rd port now but you could use it in the future.
>
> I'm working on a 78 GMC right now. It needs a new converter. I'm waiting
> for the owner to get a replacement Progressive Dynamics one. In the interim
> I attached a 12.5 amp switching type power supply and adjusted it to 13.2
> volts. Worked great. Then tonight I disconnected the 120 VAC before
> disconnecting the 12 volt side. The thing went up in smoke. My whole
> hangar now smells like an electrical fire. It really smoked.
>
> Good Luck. It sounds like you are on top of your problem(s).
>
> Ken B.
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
>
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> GMCnet mailing list
> Unsubscribe or Change List Options:
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
Re: [GMCnet] Isolator -- bypass? [message #326898 is a reply to message #326801] Fri, 08 December 2017 09:56 Go to previous message
JohnL455 is currently offline  JohnL455   United States
Messages: 2994
Registered: October 2006
Location: Woodstock, IL
Karma: 16
Senior Member
If someone has not already mentioned, you can get a simple out of circuit Go/No Go diode test on the isolator with any decent VOM.

John Lebetski
Woodstock, IL
77 Eleganza II
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