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Thread: Alternator check

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  1. #1
    Registered User lmo1131's Avatar
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    Alternator check

    I don't get along well with electricalcity issues, so I could use some advice.

    The bike is placed on a float charger every night, but starting can be an issue, especially if I ride to work (only about ten miles) and the bike sits for twelve hours. "Work" is right on the water's edge of the Pacific Ocean and the temperature hovers in the mid-forties, low-fifties most nights.

    On a daily basis, once started it never fails to restart regardless of the distance between "start/stop"; the "state of tune" is excellent and generally lights off in one or two revolutions of the engine.

    With the engine at around 3,500 rpm, my multimeter indicates 12.8 volts with the probes on the + and - terminals of the battery (which is about nine months old).

    So, I (think) I need to check the alternator output...

    With the multimeter set on A/C, and the expectation that the voltage readings (regardless of what they are) should be close, do I alternately touch the probes to:

    W - U
    W - A
    A - U
    A - W
    U - A
    U - W

    Guidance would be appreciated.

    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  2. #2
    Administrator 20774's Avatar
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    Lew -

    Usually the order of checking is:

    - rotor -- what is the resistance across the slip rings
    - voltage regulator -- when bypassed, does the charging improve
    - diode board -- one of the outputs at the side of the board takes the power, runs it through heavy cables connected to the starter and then finally to the battery
    - stator -- usually pretty rock solid, but the brushes need to be checked to ensure they're not getting hung up, resulting in no contact on the slip rings

    I'm a little confused... A/C multimeter setting? U-A-W?? Is that the United Auto Workers?
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    MOA #24991 Pauls1150's Avatar
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    Also: when measuring AC voltage, polarity does not matter - so U to W is identical to W to U, W to A is identical to A to W, and A to U is identical to U to A. Saves half the steps.

    12.8 volts is a bit low. I'd be leery of totally bypassing the regulator - that might allow too much voltage to the rest of the system and smoke something (not a suggestion).

    The rotor, brushes, and an open (or lifted) diode are the typical maladies.
    By a "lifted" diode, I refer to the connection of the diode to the board. Some of these were assembled with little or no fold on the diode's leads where it lays against the board, forcing that branch of current to travel through the solder only. Solder is tin and lead, so that can melt (just like a fuse) or break away from the board.

  4. #4
    Administrator 20774's Avatar
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    Quote Originally Posted by Pauls1150 View Post
    I'd be leery of totally bypassing the regulator - that might allow too much voltage to the rest of the system and smoke something (not a suggestion).
    Actually this is a standard approach and I think Rick Jones writes about it in his Classic Charging booklet...obviously, don't let the system run on it's own for very long, but if the regulator is bypassed and the voltage climbs to 15v or so, then the alternator proves good. The circuit should be able to take that for a short period of time.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  5. #5
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    This article seems appropriate for /5 charging. There is even a link to measured charging voltages.

    http://www.buchanan1.net/charge.shtml



    Edit: if one were to read the above article linked measured voltages - the voltage you measure at 3500 rpm is 0.1 volt higher.

    http://www.buchanan1.net/slash5_charging.shtml
    Last edited by Stan_R80/7; 05-24-2012 at 09:53 PM.
    Stan

    AH# 13238

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