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  1. #1
    Cadet
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    Unhappy Is my alternator working?

    I have a brand new battery. I am not running into any problems so far I just was testing to make sure everything was fine before taking the boat out on the water. When I measure volts across my battery with the engine running I get 12.21 volts. But if I go to the alternator and measure from grnd to the only positive terminal coming off the alternator I get like 10.5. that wire goes to some kind of amp looking device with about 5 bolt down wire fasteners on it. the lead to the battery comes off of that amp and so does a wire going to something that looks like a solenoid mounted to the engine block. if I disconnect the lead going to the battery from the amp the terminal on the amp reads the same as the alternator output I am getting. have let the boat run for up to an hour with no problems. However at that time I was not worried about this and did not measure voltage. I am not sure if this should work on a boat like I think it does on a car but i unhooked the negative cable from my battery and the engine died. I thought if the alternator was working it would keep running. Should I unhook the alternator lead and measure current? or should I take the alternator to autozone and have it tested? I looked up re[placement alternators in a mag I have and the cheapest was $500, is this true of all marine alternators>? Thank You in a Advance. Sorry for the long post

  2. #2
    Senior Chief Petty Officer sqbtr's Avatar
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    Default Re: Is my alternator working?

    It would help to know your eng and drive.

    But no your alt. is not working, and never disconnect your batt. from a turning alt.

  3. #3
    Cadet
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    Default Re: Is my alternator working?

    It is a 1972 mercruiser 250, 165 hp with a pre-alpha outdrive

  4. #4

    Default Re: Is my alternator working?

    I've heard that alternators can "partially fail" where it just puts out a lower than normal voltage, which I think just means you have to replace the diodes inside and that just costs about 6 bucks or so if you can do it yourself.

  5. #5
    Petty Officer 3rd Class
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    Default Re: Is my alternator working?

    Remove it from the boat and take it to a shop and have it diagnoised. Usually can get a new one at napa for + or - $150.00
    1984 - S2 Yachts - 23' Tiara Continential - 260 mercrusier - alpha 1 drive

  6. #6
    Seaman Apprentice
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    Cool Re: Is my alternator working?

    Make sure a replacement is for Marine only. Can't use an car one in a boat. BOOM! ! !

  7. #7

    Default Re: Is my alternator working?

    battery voltage test with engine running should be over 13 volts. but sometimes the rpms have to be above 1000.

  8. #8
    Petty Officer 1st Class
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    Default Re: Is my alternator working?

    There is a simple test.

    Measure your voltage with the key on, but engine not running.

    Then, start the engine and run up to 1500rpm. Do not go higher...there is no need.

    If the voltage goes up, your altenator is charging. If it stays the same or goes down, it is not charging.

    If it's not, then there are two posibilities:

    1) Altenator

    2) Connections

    As you have measured from ground to the altenator and found a low voltage, you may have both.

    With a good connection, and good alt, the voltage at the alt should be the same as the battery

    With a poor battery to alt connection, and a good alt I'd expect to see the altenator voltage high, and the battery voltage low

    With a poor connection and a poor alt I'd expect to see what you are getting. No charge from the alt and a difference in voltage to the battery

    But wait! Some altenators need an external +12V feed and ground to run the regulator. The regulator controlls the field of the altenator, and therfore it's output voltage. This +12V source sometimes comes from the battery/altenator connection.

    So, fixing the connection between the battery and the alt may be all you need to do.

    IF the altenator is dead, the most likely culprit is the brushes. Now and again, the diodes will die, but it's less likely.

    Good luck

  9. #9
    Petty Officer 1st Class
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    Default Re: Is my alternator working?

    I reread your post...the solenoid looking thing is just a terminal block.

    Pull the wires off and retest...I bet there is a bad connection there or at the alt or the battery.

  10. #10
    Supreme Mariner bruceb58's Avatar
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    Default Re: Is my alternator working?

    Your alternator has an external regulator more than likely. By pulling the battery lead while running the engine, you proably killed it if it wasn't dead already. My suggestion is replace your alternator/regulator with a one wire alternator. The regulator is built into the alternator.

    And NEVER disconnect the battery while the engine is running.
    1998 Wellcraft Eclipse 24 Cuddy
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    "Common sense is not very common"
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  11. #11
    Petty Officer 1st Class BAproject's Avatar
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    Default Re: Is my alternator working?

    do not use a car alternator on a boat engine!!!!! unless you want to go boom
    -BAproject
    ----------------------------------------------------
    "If it ain't broke then you'll probably have to fix it anyway"
    ----------------------------------------------------
    '91 Bayliner Capri
    4.3L V6 Mercruiser
    Alpha I Outdrive

  12. #12
    Supreme Mariner bruceb58's Avatar
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    Default Re: Is my alternator working?

    Quote Originally Posted by BAproject View Post
    do not use a car alternator on a boat engine!!!!! unless you want to go boom
    yeah...that too!
    1998 Wellcraft Eclipse 24 Cuddy
    Volvo Penta Duo-Prop 7.4L "LK"

    2006 Sun Tracker Party Barge 21
    Mercury 90 4-Stroke FI
    "Common sense is not very common"
    "Failing to prepare is preparing to fail." -- John Wooden

  13. #13
    Cadet
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    Default Re: Is my alternator working?

    Ok, thanks everyone! What is the amplifier looking device that my alternator lead goes into? And I think I already have a one wire alternator with a regulator built in. At least there is only one hot wire coming from the alternator and it goes to the amp thing. the only other wire on the alternator is the grnd. I also have a small rubber cap on the back of the alternator that says do not remove. Is this a 1 wire setup already? I will try to post a pic but I have never done this on this forum before. Thank you

  14. #14
    Cadet
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    Default Re: Is my alternator working?

    I definately cant figure out how to post pics, Sorry I know that would be a big help.

  15. #15
    Supreme Mariner bruceb58's Avatar
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    Default Re: Is my alternator working?

    Quote Originally Posted by Grice7 View Post
    Ok, thanks everyone! What is the amplifier looking device that my alternator lead goes into? And I think I already have a one wire alternator with a regulator built in. At least there is only one hot wire coming from the alternator and it goes to the amp thing. the only other wire on the alternator is the grnd. I also have a small rubber cap on the back of the alternator that says do not remove. Is this a 1 wire setup already? I will try to post a pic but I have never done this on this forum before. Thank you
    It could be an isolator or a noise filter.

    http://forums.iboats.com/non-boating-technical-topics/posting-photos-step-step-illustrations-285637.html
    1998 Wellcraft Eclipse 24 Cuddy
    Volvo Penta Duo-Prop 7.4L "LK"

    2006 Sun Tracker Party Barge 21
    Mercury 90 4-Stroke FI
    "Common sense is not very common"
    "Failing to prepare is preparing to fail." -- John Wooden

  16. #16
    Petty Officer 3rd Class
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    Default Re: Is my alternator working?

    an old scrapyard trick ,to know if charging ....

    aproach a screwdriver to the rear bearing cover,it should stick !(magnetize)
    PROPERFORMANCE engine mods at honest prices. 1985 DORAL 164 mercruiser 140HP ,
    protouring 84 cutlass, 86 CJ jeep lots of mods , hemi magnum awd .....just cant keep anything stock !

  17. #17
    Supreme Mariner bruceb58's Avatar
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    Default Re: Is my alternator working?

    Quote Originally Posted by bigdan1 View Post
    an old scrapyard trick ,to know if charging ....

    aproach a screwdriver to the rear bearing cover,it should stick !(magnetize)
    Or actually measuring the voltage!
    1998 Wellcraft Eclipse 24 Cuddy
    Volvo Penta Duo-Prop 7.4L "LK"

    2006 Sun Tracker Party Barge 21
    Mercury 90 4-Stroke FI
    "Common sense is not very common"
    "Failing to prepare is preparing to fail." -- John Wooden

  18. #18
    Petty Officer 2nd Class Searay87's Avatar
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    Default Re: Is my alternator working?

    I replaced the alternator on my 87 mecrusier 3.0L 130 HP tonight. A new one marine grade cost $179 at a local alternator repair shop. I've seen them on ebay for $100 plus shipping. The local marine service shop wanted $350. I noticed the voltage running low (below 12 V) on my dash board meter and was suspicious because it had always run just under 14 volts. I removed the alternator and had it tested at the repair shop and it was indeed fried. The old slip rings self destructed.

    Like others have said the easiest way to test an alternator is to measure the voltage across the terminals of your battery with the engine running. My voltage with the new alternator was 14.4 Volts. The battery voltage with my dead alternator in the circuit was 12.7 volts on my multi-meter.

    Mine had 4 wires on it. A ground (black), a larger gauge wire (orange) with a spark plug type cover on it that was always hot (re: constant 12 volts) which is obviously connected to the battery positive post, and two smaller gauge wires red and blue. The alternator shop tech said one of these wires came from the starter the other went to the distributor.

    There is a diode bridge or regulator used to rectify the AC voltage output from the alterntor to 12 VDC.

    I agree with the other posters, if the voltage across the terminals of your battery doesn't rise to 14+ volts within a few minutes you have a bad alternator.

  19. #19
    Cadet iconracing's Avatar
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    Default Re: Is my alternator working?

    Quote Originally Posted by sea flower View Post
    Make sure a replacement is for Marine only. Can't use an car one in a boat. BOOM! ! !
    i build alternators for 3 years they either bad or good no in between, you can use a car alternator but it is not recomended take it to a shop and tell them to fix whats wrong with it.
    Wrap it up b!

  20. #20
    Commander Eric 3.7LX Alpha 1's Avatar
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    Default Re: Is my alternator working?

    If you are going to get a new alt, see link:
    http://www.dbelectrical.com/c-7743-mercruiser.aspx

  21. #21
    Cadet
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    Default Re: Is my alternator working?

    Thanks eric, Thats the exact one I ordered yesterday, I really appreciate your guys response. this website is just great

  22. #22
    Commander Eric 3.7LX Alpha 1's Avatar
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    Default Re: Is my alternator working?

    Quote Originally Posted by Grice7 View Post
    Thanks eric, Thats the exact one I ordered yesterday, I really appreciate your guys response. this website is just great
    check your PM

  23. #23

    Default Re: Is my alternator working?

    I have a simular situation and I believe I have an idea of where to go from here, but if I missed something please respond. Voltage on my slave solenoid, starter solenoid, battery all read 12.7 at best. Switched out a new alt. with no change. Only thing I can think to do is to replace wires going from alt. to starter solenoid and/or yellow wires from slave solenoid to starter solenoid.

    PS...how are circuit breakers wired up in the scheme of things. My wires are wrapped really well and I dont want to unwrap them. Does the alt wire go to this circuit breaker first (btw, voltage reads the same here as well) then to the starter solenoid?

  24. #24
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Is my alternator working?

    Blueyach.
    Best to start a new thread with your problem.
    When 2 different people with two similar problems get started, it isn't long till no one knows who is answering who .........
    Don S.


    Please, no PM's (Private Messages) regarding boat/engine problems.
    That is what the forums are for.
    Only forum/moderator issues will be answered in PM's.

  25. #25

    Default Re: Is my alternator working?

    I hear you, will do.

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