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Old 06-29-2020, 04:28 PM   #1
TLR
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Default Checking 6 volt neg ground output with multi meter

Can anyone let me know the best way to check a 6 volt neg ground output with multi meter on my 1931 "A" with an alternator. It seems my battery is not charging and getting weaker as time goes on.

Thanks in advance.
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Old 06-29-2020, 04:55 PM   #2
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Default Re: Checking 6 volt neg ground output with multi meter

The easiest way is to measure the voltage of the battery directly with your MM.

Connect - you measure the resting voltage.

Start - you measure the stability / quality of the battery.

Increase RPM - the voltage must rise when the generator supplies power. .
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Old 06-29-2020, 05:32 PM   #3
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Default Re: Checking 6 volt neg ground output with multi meter

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Can anyone let me know the best way to check a 6 volt neg ground output with multi meter on my 1931 "A" with an alternator. It seems my battery is not charging and getting weaker as time goes on.

Thanks in advance.
I gotta ask, Are you sure your car is 6 volts negative ground? Model As are usually 6 volt positive ground.
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Last edited by Ruth; 06-30-2020 at 09:45 AM.
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Old 06-29-2020, 05:39 PM   #4
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Default Re: Checking 6 volt neg ground output with multi meter

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I gotta ask, Are you sure your car is 6 volts usually negative ground? Model As are 6 volt positive ground.
Most alternator conversion regards of voltage are negative ground.
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Old 06-29-2020, 05:42 PM   #5
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Default Re: Checking 6 volt neg ground output with multi meter

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Originally Posted by TLR View Post
Can anyone let me know the best way to check a 6 volt neg ground output with multi meter on my 1931 "A" with an alternator. It seems my battery is not charging and getting weaker as time goes on.

Thanks in advance.



Take take your volt meter and connect it across the battery, take a reading.

Start engine and set to a fast idle and take another reading.

Let us know the results and we can go from there.

What the the vehicles ammeter show ?

The only reason I'm suggesting taking the readings directly from the battery is due to the possibility of poor/dirty connections.
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Old 06-29-2020, 06:26 PM   #6
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Default Re: Checking 6 volt neg ground output with multi meter

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To charge the battery, you will need about 7.2 Volts from the alternator as measured on the battery terminals.
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Old 06-29-2020, 06:35 PM   #7
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Default Re: Checking 6 volt neg ground output with multi meter

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Most alternator conversion regards of voltage are negative ground.

I looked at Snyder's, Bratton's and Berts and their 6 volt alternators
are positive ground.
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Old 06-29-2020, 08:09 PM   #8
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Default Re: Checking 6 volt neg ground output with multi meter

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I looked at Snyder's, Bratton's and Berts and their 6 volt alternators
are positive ground.



Well we'll find out. Maybe thats the problem.
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Old 06-29-2020, 11:13 PM   #9
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Default Re: Checking 6 volt neg ground output with multi meter

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Originally Posted by Ruth View Post
I gotta ask, Are you sure your car is 6 volts negative ground? Model As are usually 6 volt positive ground.
Quote:
Originally Posted by Patrick L. View Post
Most alternator conversion regards of voltage are negative ground.
Quote:
Originally Posted by Bob C View Post
I looked at Snyder's, Bratton's and Berts and their 6 volt alternators
are positive ground.
Thank you, Bob C.
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Old 06-30-2020, 09:33 AM   #10
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Default Re: Checking 6 volt neg ground output with multi meter

I agree with Bob mine has been on for years (+ ground )
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Old 06-30-2020, 10:12 AM   #11
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Default Re: Checking 6 volt neg ground output with multi meter

To insure you have a good battery, here is an inexpensive battery load tester. Does 6 and 12v. A lot of auto parts chains and farm supply store do not have 6v testers. You can do the same thing measuring battery voltage droop running the starter with ignition off.



https://www.harborfreight.com/100-am...ter-61747.html
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Old 06-30-2020, 03:03 PM   #12
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Default Re: Checking 6 volt neg ground output with multi meter

Ah, the 6v alternators from Bratton's and Snyder's are Nu-Rex conversions. That explains it. I was thinking TLR might have a polarity issue. He still may. Maybe we'll find out what the problem is.
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Old 06-30-2020, 03:30 PM   #13
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Default Re: Checking 6 volt neg ground output with multi meter

Sorry it is positive ground..........checked per jrelliott and we are getting 7.1 volts so we should be charging. Just seems to crank slower than it should, maybe the starter is getting tired?

Thanks for the replies
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Old 06-30-2020, 03:48 PM   #14
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Default Re: Checking 6 volt neg ground output with multi meter

Do you have the modification adding a ground cable between where the battery ground attaches to the fame, to the bell housing? And have you cleaned the battery cable connection on the battery and cable frame connection. Also the Starter cable connections on both ends?
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Old 06-30-2020, 05:00 PM   #15
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Default Re: Checking 6 volt neg ground output with multi meter

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Sorry it is positive ground..........checked per jrelliott and we are getting 7.1 volts so we should be charging. Just seems to crank slower than it should, maybe the starter is getting tired?

Thanks for the replies




We still don't know what the ammeter is showing or where the 7.1 v reading was taken from.
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Old 07-01-2020, 09:31 AM   #16
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Default Re: Checking 6 volt neg ground output with multi meter

7.1 is a tad low - 7.2-7.9 is min-max for good charging. Should charge the battery ok, but more slowly. Maybe much more.
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Old 07-02-2020, 02:03 AM   #17
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Default Re: Checking 6 volt neg ground output with multi meter

Make sure spark advance lever is fully retarded when starting. Otherwise it will crank slow and is also hard on the engine.
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Old 07-11-2020, 09:04 AM   #18
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Default Re: Checking 6 volt neg ground output with multi meter

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Originally Posted by TLR View Post
Can anyone let me know the best way to check a 6 volt neg ground output with multi meter on my 1931 "A" with an alternator. It seems my battery is not charging and getting weaker as time goes on.

Thanks in advance.
From your info, you have a negative ground 6 volt alternator installed on a model A Ford that was sold as positive ground. With that said, if the battery is installed now as negative ground, and by measuring using your meter at the battery, at higher rpm with a low battery, you should be reading 7.2 to possibly 8 volts charge voltage. If those numbers are showing up, and you battery has water in it, check that too if it is a battery that takes water, and fill as needed. You might think about pulling off a battery cable and just putting a 6 volt charger on the battery overnight, and connect it up and check again on startup. Make sure all connections are tight and no lost resistance such as grounding exist also. Let us know what you find out!
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Old 07-11-2020, 09:28 PM   #19
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Default Re: Checking 6 volt neg ground output with multi meter

First you should use an Analog multimeter. A digital meter will not read steady unless it has a good filtering circuit. Most 6 volt positive alternators (don't know about 6 volt negative alternators) have internal leakage and drain the battery over a few days. If you have the starter mounted fuse, remove it when your done driving for the day, and reinstalled when your ready the start the Model A the next time you start the car.
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