Home » Public Forums » GMCnet » [GMCnet] Isolator -- bypass?
| Re: [GMCnet] Isolator -- bypass? [message #326807 is a reply to message #326803] |
Wed, 06 December 2017 14:37   |
Dave Stragand
Messages: 307
Registered: October 2017
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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@list.gmcnet.org] On Behalf Of James Hupy
Sent: Wednesday, December 06, 2017 2:27 PM
To: gmclist@list.gmcnet.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.stragand@forwardlook.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
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