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Old 11-05-2017, 01:03 PM   #1
Chuck Dempsey
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Default Alternator issue

About six years ago I bought a positive-ground 6-volt alternator from Mac's. I've put less that two hours on it now and it developed a minor short. I say 'minor' since touching the wire to the alternator terminal doesn't create a visible spark, but does set the ammeter to 'twitch' discharge. It did it last week and killed my battery overnight. At that point I re-wired the terminal box thinking that was where the short was. That 'fixed' the short but now it is back, and I've traced it to the alternator as described above. So I suspect re-wiring the box didn't really help at all. Any ideas before I remove it and take it to a shop?

Thanks!
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Old 11-05-2017, 02:45 PM   #2
JDupuis
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Default Re: Alternator issue

Sorry Chuck about your alt problem.
Did you rev the motor up first to tickle the alternator? Alot of these alternators come with an oversize pulley. Thus requiring the high rev to excite the alternator.
I know that doesn't explain why the battery would drain over night. But might explain showing discharge while running.
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Old 11-05-2017, 02:53 PM   #3
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Default Re: Alternator issue

Quote:
Originally Posted by Chuck Dempsey View Post
About six years ago I bought a positive-ground 6-volt alternator from Mac's. I've put less that two hours on it now and it developed a minor short. I say 'minor' since touching the wire to the alternator terminal doesn't create a visible spark, but does set the ammeter to 'twitch' discharge. It did it last week and killed my battery overnight. At that point I re-wired the terminal box thinking that was where the short was. That 'fixed' the short but now it is back, and I've traced it to the alternator as described above. So I suspect re-wiring the box didn't really help at all. Any ideas before I remove it and take it to a shop?

Thanks!
Sounds to me like one of the diodes in the internal rectifier is bad, allowing current to flow backwards, thus draining the battery. Should be easy fix.
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Old 11-05-2017, 03:07 PM   #4
Chuck Dempsey
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Default Re: Alternator issue

I have had it running, with a noticeable 'charge' on the ammeter. Even now, when running, the 'charge' overcomes the minor 'discharge'. It's only when not running does this become an issue. Switch off, lights off, but still a minor discharge on the battery. If I undo the belt, should the alternator try to spin if the 'diodes' are bad? I haven't tried that yet.....
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Old 11-05-2017, 03:18 PM   #5
Tom Endy
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Default Re: Alternator issue

A number of years ago I had a 6-volt positive ground alternator that worked just fine, but I noticed when I shut the engine off there was a slight discharge. If I didn't pull the fuse when parked overnight it would run the battery down. I had purchase the alternator years before from Larry Becker, Antique Auto Electric, Norwalk, Ohio.

I removed the alternator and checked it out and found a shorted diode in the rectifier circuit. This is what was causing the current drain. Since I was unable to buy a replacement diode I purchased another alternator from Mel Gross.

A year of so later I ran into Larry Becker at a MAFCA national and asked him if he could repair my defective alternator. "Absolutely he said, send it to me it is under warranty". I had to remind him that I purchased the alternator from him about 10 years ago and did not expect a warranty repair. "Doesn't matter, it is still under warranty". I sent it to him and he sent me a new alternator. All I had to pay for was the shipping. You cannot find better customer service than that.

Larry Becker passed away a few years ago.

I suspect that the problem with Mr. Dempsey's alternator is a shorted diode in the rectifier circuit. I would take it to an auto electric repair service.

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Old 11-05-2017, 03:22 PM   #6
Tom Wesenberg
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Default Re: Alternator issue

No, the alternator will not motor like a generator.
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Old 11-05-2017, 03:35 PM   #7
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Default Re: Alternator issue

Folks often pick on the poor generator but alternators can have problems too. Just sayin'
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Old 11-05-2017, 03:44 PM   #8
Tom Wesenberg
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Default Re: Alternator issue

Quote:
Originally Posted by Flathead View Post
Folks often pick on the poor generator but alternators can have problems too. Just sayin'
Yep, I'll take the generator any day over the alternator for my antique cars.
If you live in a hot climate and add A/C, then you'd want to have the alternator, but the generator will power your lights and horn just fine.
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Old 11-05-2017, 04:35 PM   #9
Chuck Dempsey
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Default Re: Alternator issue

"Folks often pick on the poor generator but alternators can have problems too. Just sayin'" - Seems like you are right. I bought the alternator to avoid all the generator issues I had before. Old issues gone, new issues here......

Thanks all. I'll be hauling in to be looked at tomorrow.
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Old 11-06-2017, 10:48 AM   #10
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Default Re: Alternator issue

I agree with others, you have a shorted diode in the alternator. There are three sets of diodes for each of the three phases coming out of the alternator and you are only getting two thirds of the potential output.
If you have a single wire alternator, 6 or 12 Volt, there is a small current drain in the alternator when you have turned everything off. Just remember to pull the fuse when you stop for a while.
The generator cut out only connected the generator to the battery when the generator was putting out sufficient voltage to pick the relay in the cut out. But you did not have voltage regulation only the third brush to change the charging current. Modern alternators have a built in regulator to control the field current based on the voltage and current required to keep the vehicle going and charging the battery.
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Old 12-06-2017, 12:30 AM   #11
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Default Re: Alternator issue

Alternator or generator, both work and can break. The big issue is voltage regulation to keep from boiling the battery or burning gadgets. I haven’t had the A long and still fixing stuff. Gen was squealing and has lots of end play. Front bearing was shot and bushing had been greased.
Alt has more capacity and a regulator so will get one for now.
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Old 12-06-2017, 10:10 PM   #12
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Default Re: Alternator issue

I would use a good quality ammeter to see EXACTLY what is happening. If you see no spark on the alternator, especially in low light, you could have other places where you have a drain. If it ends up being the alternator, its an easy enough job to replace the diodes. If you bring it in to be checked out, make SURE they understand that it is a 6 volt Positive Ground.
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Old 12-06-2017, 11:41 PM   #13
Drive Shaft Dave
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Default Re: Alternator issue

With a fully charged battery, connected to the car put your hand on the alternator, if you feel it getting warm you have diode shorted to ground.
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Old 12-07-2017, 07:50 AM   #14
Chuck Dempsey
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Default Re: Alternator issue

All - issue was resolved. Bad diode was replaced. Plus the repairman found a couple of brushes were too short to make contact, and they were replaced.

Thanks
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