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Alternator help needed

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  • #235907
    Leo
    Participant

    @ldj1002

    I have a Allison MGTD replica on a 74 VW chassis.
    It has a Motorola alternator with internal VR. I turn on ignition and
    alt. light comes on, I start the engine and light goes out but it isn’t
    charging. I checked at battery with meter to be sure. When I check with
    meter with all wires removed from alt. I get continuity between B+ and
    case. I would think that isn’t right. I have read about the alt. light
    has to be run right or it won’t charge battery. It isn’t charging but
    why does alt light go out? Any way I can check alt. with meter to tell
    if bad. It also says correct bulb has to be in alt. light or it won’t
    charge battery. It calls for 12v 2 watt, my bulb says 12-14v 2 watts, Is
    that OK? Any help appreciated

    #266053
    Royal
    Participant

    @royal

    If the battery starts the engine, what makes you think that the alternator is not working exactly like it’s supposed to?  I’m not sure that you have a problem. 

    If the battery is already fully charged, the alternator is not supposed to be sending current to it else it would boil all the electrolyte out and your battery would be toast. 

    #266054
    Leo
    Participant

    @ldj1002

    All alternators I have saw will put out more volts than battery voltage. I can turn lights on and everything electrical on and battery voltage will go down, it should not, alt should kick in to keep it up.

    #266055
    Steve Struchen
    Participant

    @mustang_evets

    Check your belt tension and belt for wear. It sounds like it might be loose.

    #266056
    edward ericson
    Participant

    @edsnova

    My volts meter has been going up and down all season with no ill effect. Sometimes it says 12. Sometimes (when I hit the brakes) it drops a little lower than 12. Other times–like today, as I took a leisurely two-hour cruise around the county for no particular reason–the volts gauge stayed between 13 and 14 the whole time. Except when I was stepping ont he brake pedal, when it dropped down to about 12.3

    Yes, I know, I know. Time to fix the grounds on my brake pedal. I’ll get to it, don’t worry.
    But, as to the volt meter: it’s vexed me all this time, for several years. But the battery never dies, the car always starts, and I run the headlights pretty much all the time.
    You might be OK.

    edsnova2015-11-01 21:53:19

    #266057
    Bob
    Participant

    @lrh

    ldj1002 wrote:
    —>8—continuity between B+ and
    case—8<—

    12v 2 watt, my bulb says 12-14v 2 watts, Is
    that OK?
    Yes, bulb is right.
    Don’t know what “case” is, sorry. Battery itself or the compartment holding it, or bulb case? Battery positive shouldn’t have continuity except to ground or along wiring circuit.
    Guessing you meant bulb holder; if so, yes there would be continuity as to a ‘ground’ anywhere else too. At least I believe that would be true in this “case” for these cars. ;):-)
    Power light is meant to be on until engine is running, or enough output from alternator. Staying out while under way is a good thing.
    I have seen mine glow dimly (in darkness) if I have all lights (including fog) and defroster on. Not sure if belt tension was right at that time or not, haven’t rechecked for that since readjusting for 3/8 to 1/2″ deflection and using 43 ft-lbs torque on pulley nut.
    #266058
    Leo
    Participant

    @ldj1002

    Continuity between B+ and case. Case is the alt. it’s self, the outside of it which would be ground. I should have said between B+ and ground. I see how I confused you, sorry.

    #266059
    Bob
    Participant

    @lrh

    Ah yes, thanks for that. Seems all good then.

    #266060
    Leo
    Participant

    @ldj1002

    Shouldn’t I be getting 14+/- volts at battery with engine running say 1500 rpm and above? Volt meter in dash and multimeter across battery neither show more volts than battery volts even with battery low enough to barely start engine. I have never driven this car except around back yard. I don’t see how it could possible charge the battery with my volt meters showing under 12v when running.

    #266061
    Bob
    Participant

    @lrh

    oops, I see now, missed your 2nd message about it failing to charge. And I made a huge mistake by thinking your “B+” was short for the battery post itself, not alternator output! (slap on head).

    So let me get this right please. Was voltage checked at the alternator B+ to ground found to be low, or battery equivalent of around 12.6 Volts or so?
    If it weren’t getting the full voltage at battery I would say check wiring for too much resistance. Looks like you need more alternator checks as of now, wish I knew more about the subject.

    LRH2015-11-02 02:21:51

    #266062
    Leo
    Participant

    @ldj1002

    I didn’t check voltage at alt. B+ to ground, only checked across + and – on battery posts and observed volt meter in dash, both read same.

    #266063
    Bob
    Participant

    @lrh

    Ah, ok, you probably need to do Ohm (resistance) check on the wires if alternator shows 14+ at the output wire to ground.

    If not good output it might be the built-in VR.
    Or as others said already the belt could be loose but you’ve no doubt looked at that by now.
    #266064
    Leo
    Participant

    @ldj1002

    Ahhh thanks, Yesterday I decided to take a brake from electrical and start on complete brake job including lines and hoses. Won’t get back to electrical until I finish brakes which will be a few weeks. At that time I’ll get back on electrical. I have other gremlins there also. Sure would be nice to have a schematic. If anyone has or knows where I can get a schematic for the Allison it would be appreciated.

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