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  1. #1
    polarbearben's Avatar
    polarbearben Guest

    Default I have a 86 Mercruiser 470 wit

    I have a 86 Mercruiser 470 with Alpha 1 outdrive. I recently put on a conversion alternator kit (disconnect original voltage regulator). At idle and medium speed the volts meter is showing 13-14 volts. But when I get it over 3000 rmp the needle jumps all over (between 0 and 14). Is this normal?

    When I drop down below 3000 rpms it goes right back to 13-14 volts.

  2. #2

    Default "I dont know what the problem

    "I dont know what the problem is, but I am pretty certain that it is not normal. With it jumoing in that range it would seem to be a bad connection. 14 would be a normal high for a charged battery, but the 0 suggests a loose connection or open circuit."

  3. #3
    Join Date
    Aug 2006
    Location
    SW Ohio, USA
    Posts
    10,180

    Default "If you don't find a loose

    "If you don't find a loose connection per Jay, check for AC voltage at the DC voltage output.

    Use a digital voltmeter on the AC scale and attach the + lead to the alternator's DC output terminal and the - lead to the alternator case. If the meter displays more than 0.4 AC volts, one of the diodes is shorting out and needs to be replaced. Erratic AC voltage can make a DC analog meter jump around.

    Guy"
    Fix minor things (replace wear items for preventative maintenance.) that cause major problems before they break.

  4. #4
    polarbearben's Avatar
    polarbearben Guest

    Default To Guy G: With the situatio

    To Guy G:

    With the situation you discribed (diode shorting out) would the rpm of the engine be a factor? At idle or medium speed everything is fine until higher rpms.

    I will test it like you said in the morning.

  5. #5
    Join Date
    Aug 2006
    Location
    SW Ohio, USA
    Posts
    10,180

    Default "A diode generates heat as cur

    "A diode generates heat as current is drawn thru it when it converts AC to DC. To maximize the alternator's ability, a full bridge rectifier circuit of 4 diodes are used; two convert the negative portion of the AC sign wave while the other two convert the positive portion of the AC sign wave.

    When a diode is beginning to fail, it will function under a less hot condition, i.e., idle or low RPM. Increase the current demand of charging a battery and more heat is generated to a point that the diode no longer functions properly. They can fail open and reduce charging current or fail as a short and allow AC volts onto the DC volt circuit which can cause all sorts of strange things to happen.

    Something that I just thought of is that the alternator brush assy. can crack and the high RPM will make it vibrate and give a similar cond. If you are not sure of the alternator's operation, take it off and have it tested at one of those discount auto parts stores.

    You did disconnect all of the old wires to disable the old alternator?

    Guy"
    Fix minor things (replace wear items for preventative maintenance.) that cause major problems before they break.

  6. #6
    polarbearben's Avatar
    polarbearben Guest

    Default To Guy G: Yes all the wires

    To Guy G:

    Yes all the wires from the old water cooled voltage regulator have been removed.

    I just put this mercruiser conversion alternator kit on the engine and the alternator was new (less one hour of running time). Is it possible one of the diodes has a problem?

    If I test it tomorrow morning and the AC is under .4 should I still get it tested at a discount auto parts store?

    Thanks you for the help!

  7. #7
    Join Date
    Aug 2006
    Location
    SW Ohio, USA
    Posts
    10,180

    Default "If it is 0.4 volt or less, it

    "If it is 0.4 volt or less, it is considered to be within operating specifications. So the answer is NO. Use the 10 or 20 volt scale when you test it. Not trying to insult you, but when you read the meter, make sure not to confuse a reading of 0.04 with 0.40 which I have done before. You may have to run the boat on the lake using a hard wire connection to get a meter reading at the RPM you are having a problem. You can use the voltmeter in place of your gauge in the DC mode to verify the accuracy of the gauge and then check the alternator in the AC mode.
    Guy"
    Fix minor things (replace wear items for preventative maintenance.) that cause major problems before they break.

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