Bosch Alternator issues on MD2B

Discussion in 'OnBoard Electronics & Controls' started by Youngsalt, Jan 14, 2012.

  1. Youngsalt
    Joined: Apr 2010
    Posts: 6
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    Location: QLD Australia

    Youngsalt Junior Member

    Hello All,

    I know how you guys like it so i'll give a detailed description of my problem

    Boats a 34frp 1971 roberts, she's called Pog btw :D... and she's my baby.
    engines a 1971 volvo penta md2b

    the two batteries are just together in parallel and are used as house and starter and have always worked fine.

    We just got an engel fridge and it drained the batteries so i hand cranked the old green machine expecting her to charge up the batteries and she didnt.... i assumed it was the alternator and replaced it.

    It's possible the alternator has never worked and the panels have always kept me sufficiently topped up. Never noticed before we got the fridge... seems cold beer comes at a hefty price.

    THE PROBLEM:

    the new alternator's not working. there's current flowing through it but it doesn't seem to be generating anything.

    It has three connections on the back
    one is a bolt which two +'s connect to. they both seem to go in to the starter, i'm assuming one makes it's way back to the batteries

    the other connector is for the dash light.

    the other connection is for a tacho which i don't have.
    ---
    The dash light doesn't come on when it's connected to the dash connection on the alternator.. it does come on however when the wire touches the case of the alternator. does this mean the alternator is not earthed?

    using a multimeter set on v 20 i can read the batteries voltage off of the + wires on the back of the alternator and using the black probe at the - or earth on the engine... I'm confused.

    should i run one of the wires directly back to the batteries?
    should i run a wire from the engine to a bolt running through the hull... hopefully earthling the engine?
    should i run a earth wire directly from the alternator?

    ---

    Please forgive my lack of electrical terminology.. I'm just a young salt and have much to learn.

    thank you in advance for your reply's and i accept your criticism with open arms so long as it borders on constructive :p
     
  2. CDK
    Joined: Aug 2007
    Posts: 3,324
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    Location: Adriatic sea

    CDK retired engineer

    Process of elimination:
    - The starter motor works, so there is no earthing problem on the engine.
    - The dash light comes on from touching the alternator case, so no ground fault there.
    - There is battery voltage on the D+ terminal, so no need for extra wiring.

    Possible causes:
    - You wired the dash light to the tacho terminal instead of DF.
    - There is no voltage regulator in the alternator or it is defective.

    The regulator in the alternator also has two carbon brushes connecting it to the rotor winding. One is wired to ground, the other connects to the dash light terminal. The path from the dash light terminal to ground is just a few ohms. If it is open -which seems to be the case- one or both of the carbon brushes is/are too short to reach the collector ring. There are several versions of regulators, all looking alike, but with different heights.
     
  3. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    Quote two carbon brushes connecting it to the rotor winding. One is wired to ground,

    Depends if the regualtor is in the live or earth
     
  4. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 145, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    That's true Pistnbroke, Ducellier and Paris-Rhone did that. The French also used red for ground wires.
    Bosch, as far as I can remember, always pulls DF to B+.
     
  5. Youngsalt
    Joined: Apr 2010
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: QLD Australia

    Youngsalt Junior Member

    Hi CDK

    Thankyou for your advice.
    After another day of sitting over the damn thing i have come to the same conclusion as you which is that the alternator is defective. I find it hard to believe because it's a brand new one but i have gone through the process of elimination:

    - The starter motor works, so there is no earthing problem on the engine.
    - The dash light comes on from touching the alternator case, so no ground fault there.
    - There is battery voltage on the D+ terminal, so no need for extra wiring.

    The only answer left is that despite it being brand new the alternator is ineed defective. Call me bright eyed and bushy tailed but i find it hard to accept that a new product from a reputable shop would just happen to be defective.


    So the plan is.. tommorow morning i'm going to take the new alternator to the shop... get them to bench test it... if it's defective i'm going to kick some *** and if it works i'm going to get them to fit it and show me where i am going wrong.

    Pistnbroke, I also live in Noosa. Pog is anchored up in woods bay next to the other wrecks and relics.... although we are planning on leaving for a while as soon as i get this alternator sorted.

    If you are around and would like to come and give some advice or help.. show me where i'm going wrong i would greatly appreciate it.

    I'll give an update on any progress i make tommorow.

    thanks again for the advice
     
  6. Youngsalt
    Joined: Apr 2010
    Posts: 6
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    Location: QLD Australia

    Youngsalt Junior Member

    Took the alternator in to the auto electric shop thismorning and said ' I know it seems highly unlikely but this alternator is broken... i tried everything'

    they bench tested it... what do you know... broken. Someone on the bosch production line had forgotten to solder the voltage reg to the rest of the alternator.. they fixed it straight away and i fitted it back on to the engine and she worked a treat.

    problem solved
     
  7. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Now go to the local RV store and purchase a merge solenoid , under $20. US

    Wire the solenoid to the ACC terminal of the key switch.

    Seperiate the batts with the solenoid so the starter sees only a starting batt and the house loads see the (hopefully) deep cycle house battery.

    Now you will have a hot start batt , after the house batt is depleated

    As soon as the engine is started the ACC terminlal will close the solenoid , and you are now charging the house automatically.

    FF
     

  8. pistnbroke
    Joined: Jan 2009
    Posts: 1,405
    Likes: 33, Points: 48, Legacy Rep: 404
    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    yes FF and the starter battery is discharging into the house battery too..ok for vehicle applications but not for live aboard boats
     
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