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Thread: Auto electric question

  1. #1
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    Auto electric question

    OK, got one of those that has me and the guy that does my electrical work stumped. 91 Cheby PU. When you first start it often times the battery light comes on. It will stay on a while then go out. Sometime off and on several times. Lights dim and brighten with the light so there is a voltage drop, not just an idiot light. There is a voltage gage on the panel too. He checked the alternator and couldn't find anything wrong. Went ahead and put in a regulator just in case. Worked fine for a while and then started again.

    Just now it did it again when I could pull it to the Lab and check it out. Voltage at the battery is reading 12v then it will jump to 13.8 or something close. Then it would drop back to 12.something. So that confirmed there is a problem, not just a bad gage in the dash.

    I checked the voltage at the Alternator output straight to the battery ground. 12.0 volts. Checked the voltage using the Alternator case as the ground same results. I was suspecting a wiring problem. But if I get the same voltage grounded to the case as the battery ground, that should eliminate a wire problem and confirm it is the alternator, correct?

    OR, could a dirty ground from the motor to the battery still cause this? I was thinking that I bypassed that by using the case as the ground but the more I think about it I am not so sure.

    Either way I will clean the ground(s) tomorrow from the motor to the battery and if I find any body to motor grounds. I do think that this eliminated the wire from the alternator to the fuse panel. That was another wire I was suspicious of. Had that go bad once in a VW and it was the devil to find that problem!
    God grant me the senility to forget the people I never liked anyway,
    the good fortune to run into the ones I do,
    and the eyesight to tell the difference.


    Kudzu Craft Lightweight Skin on frame Kayaks.
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  2. #2
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    The first thing I'd do at this point is check all of the grounds. A bad ground can cause you all sorts of headaches! You said that you checked the alternator output and it was 12V. Well that in itself is a problem, it should be about 13.8 to 14V. If it's only putting out 12 it's not working correctly.

    If I can come up with any other suggestions I'll let you know.

    Good luck!

    John
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten"

  3. #3
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    I agree with John. 13.8 would be normal. 12 would not. Check the contacts of the positve wire also. Make sure there is no corrosion. Same at the battery terminals. Maybe double check the regulator where it makes connections as well. Jim.
    Coolmeadow Setters...
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  4. #4
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    Just called a buddy of mine that is a super electrician. Told me a trick I would have never thought of. Said to look for that lost 1.5 volts. Hook my voltmeter to the battery ground and the alternator case, if I saw a reading of 1.5 volt (give or take) when it was acting up, then my problem was wiring and almost certainly a ground issue. I shouldn't see any voltage above say .1 volts between the battery and the case unless there is a wiring problem. I could also hook a jumper from the case to the ground and see if that cured it.

    Will do that in the AM. It always does it when I first start it.
    God grant me the senility to forget the people I never liked anyway,
    the good fortune to run into the ones I do,
    and the eyesight to tell the difference.


    Kudzu Craft Lightweight Skin on frame Kayaks.
    Custom built boats and Kits

  5. #5
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    Jeff, while I wouldn't rule out a ground problem, I think the problem lies within your alternator.

    An alternator generates A/C current, unlike the old generators that generated D/C power. Your vehicle, as well as your battery can't use A/C so within the alternator are a set of diodes that transform A/C into D/C.

    Over the years of having my own auto electric repair shop, where I rebuilt alternators, generators and starters, as well as any other auto electric repairs, I came across more strange problems with alternators than any other problem, especially when A/C current was getting out of the alternator. Second would be ground problems.

    Another thing could be the brushes are worn down and sticking, which would require a rebuilt alternator.

    While I haven't done any auto work in quite a while and haven't kept current with alternator technology, I'm curious as to the regulator change on the alternator. GM alternators have had the regulator built into the alternator itself. GM started that in the late 60's, early 70's as I recall. The only way to change the regulator was to remove and disassemble the alternator.

    After rebuilding alternators for awhile I found, as a general rule, that the brushes in an alternator should be suspect after around 70-80,000 miles. That's not to say that I didn't see a fair share that went well over 100,000 miles.

    If you do decide on replacing the alternator, don't go for the cheapie ones. I replaced more of those on vehilces than I care to remember. And from the customers stand point, it was usually my fault that the alternator that they bought from one of the major discount auto repair joints, had failed. After they spent the money to get it fixed right, I gained a customer.

    There's my $1.398 cents worth.

    Karl
    Last edited by Karl Laustrup; 08-25-2007 at 11:33 AM. Reason: Spelling

  6. #6
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    i`m in agreement that the alt. is out....a diode trio would fix the charge/discharge problem but given the low cost of gm alternators just change it out with one from a reputable parts house....
    i converted my panel truck to an alternator 8-9 yrs ago and bought a "lifetime" unit from the local parts house, about every 2-3 yrs it`ll start doing as you discribe so on the way home i`ll pull into their lot and in 15 min. i`m on the road no out of pocket expense.....
    the diode trio is only about 7 bucks but by the time you figure replacing the brushes 4-5$, the bushings,2$ if the bridge rectifier is gone? 12-15$,and the time to tear it down you`re money ahead to pop for a good rebuilt...tod
    [SIZE="1"] associated with several importers and manufacturers.[/SIZE]

  7. #7
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    I overslept this morning! Get this coffee down and to the truck I go.

    Karl, the alternator is a fairly new one (rebuilt) unit. I have a friend that has an Auto Electric service. He checked the unit and couldn't find anything wrong. He said the only thing that made sense to him was a bad regulator. So he went ahead and put in it while he had it torn down. I always let him rebuild my starters and alternators but he was closed and this is a rebuilt unit I picked up before a trip. I know all about the cheap rebuilt units!

    The more I have thought about this the more I leaning toward a problem with the alternator too. I was thinking ground problem but since I can connect the alternator output to the battery ground that seems to eliminate (by bypassing) the ground as a problem.

    OK coffee is gone, lets go see what I can find.
    God grant me the senility to forget the people I never liked anyway,
    the good fortune to run into the ones I do,
    and the eyesight to tell the difference.


    Kudzu Craft Lightweight Skin on frame Kayaks.
    Custom built boats and Kits

  8. #8
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    Bad alternator. It's Saturday so I am stuck buying another rebuilt unit from the Auto Parts. My buddy isn't open on Saturdays. (Smart man). And he is always covered up on Mondays plus I have work Monday. Oh well.
    God grant me the senility to forget the people I never liked anyway,
    the good fortune to run into the ones I do,
    and the eyesight to tell the difference.


    Kudzu Craft Lightweight Skin on frame Kayaks.
    Custom built boats and Kits

  9. #9
    Been reading and lurking.... IMHO it is the Alternator. Rebuilt Alts are as they are called, rebuilt and when they rebuild them they only replace the parts necessary to get them running again, marginal parts are left to continue to be marginal at best. So the Rebuilt is not as good or reliable as a new. (This info I got from a friend who used to work for a rebuilding outfit. He now has his own business rebuilding Alternators and such, but he does a complete rebuild, but also reminds that the unit is a Used unit, not new.)

  10. #10
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    I am stumped. New alternator and the same thing. Can not find a reason for this either! At least it was replaced under warranty. Guess it goes to my Buddy on Monday. Maybe he can find it. I am stumped. Totally clueless at this point. Run out of things to check!
    God grant me the senility to forget the people I never liked anyway,
    the good fortune to run into the ones I do,
    and the eyesight to tell the difference.


    Kudzu Craft Lightweight Skin on frame Kayaks.
    Custom built boats and Kits

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