electrical question re: alternator wiring

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    Marc Lipsius


    I have a question for you electrical gurus about VW alternator wiring.
    The previous owner and his friends have the wiring on my VW all fubared and I’ve been redoing it one or two circuits at a time.
    He had the D+ lead from the alternator connected under the dashboard on a push-pull connector with the other end connected to the + battery terminal and told me, when running the car, to connect those wires otherwise the battery wouldn’t charge, and when the car was off, to disconnect it so the battery wouldn’t discharge. As I understand it, this lead senses 12V so the regulator will allow the alternator to function. All the reading I have done says to connect this D+ lead to an indicator lamp (I have an extra one on the dash, or, the one that was supposed to be there and used), the lamp is supposed to go on when the ignition key is turned on, and go out if the alternator is charging. Am I right so far?
    Also, as we have a wooden dash, there is no ground to this bulb, it is in a plastic case with 2 leads coming off of it, so I originally thought one of the connections to it should be a ground wire to the battery, and the D+ wire connected to the other lamp lead. but there is nothing going through that D+ wire with the engine off so that is not right, it really becomes a ground without the alternator running. I also have read that this wire should be connected to a + terminal on the coil, but this confuses me.
    Can I run this D+ wire to the indicator lamp, and then a wire from an ignition controlled + on one of my fuse blocks to the lamp for this to work? Then the light would go on when the key is switched on, and if the alternator is functioning correctly, then the light would go off?
    Am I making any sense or is there another way to fix this?
    Thanks for any help.



    What year VW is your system? 

    Go to http://www.thesamba.com and click on the Technical Tab (top right) and select wiring. You’ll get a list of color wiring diagrams. Pick Type 1 and scroll through till you find your year. In the diagram you will see a blue and white wire run from the voltage regulator to the light in the speedometer for the Alternator. Follow it out and you’ll do fine. If you get lost, just come back here and ask away.

    Hope this helps.

    Allen Caron
    VW based 53MGTD - "MoneyPenny"
    "If one thing matters, everything matters" - from the book The Shack

    Marc Lipsius


    Thanks. I wished my car looked anything as orderly as that, but it is just as confusing to trace wires. I also don’t know for sure what year my engine is, all I know it’s probably mid 70’s. I don’t have a separate voltage regulator, it’s integral in the alternator.
    The best help from on there was looking at a Baja Bug wiring, which is probably much closer to our cars, and it confirms to me that the D+ wire off the alternator goes to the indicator bulb and that a switched + line goes to the other bulb wire. There is no ground to the bulb, it’s actually the D+ from the alternator if it’s not generating power.
    I’m going to hook it  up that way and keep my fingers crossed as I turn it on and let you know what happens.

    Marc Lipsius


    OK, back real quick, it’s all fixed. A switched hot lead goes to the indicator bulb and the D+ wire from the alternator to the other lead on the bulb.
    The light goes on with the key in the Accessory or On position, goes off after the engine starts indicating that the alternator is charging the battery. The alternator puts out 14 V (battery was depleted by me by accident), so I know that is working.
    The way this works is that the alternator puts out 14 volts, but if there’s 12 volts coming the other way through the bulb at the D+ lead, there’s a 2 V difference and that’s not enough to light up the bulb, so it goes out, and that means the alternator are regulator are working as they should.

    This is from Rob and Dave’s Aircooled VW Pages:

    Necessity for the
    Alternator Warning Light

    It is absolutely essential that the (D+) terminal on the alternator
    be connected to a functioning “Alt” warning light in the instrument
    cluster. If this light is missing or defective, the alternator will
    NOT charge the battery! See my hand-drawn wiring diagram above.
    Also see Speedy
    Jim’s diagram
    , which is much better than mine!

    The system is very simple, but it’s absolutely critical that you
    get it right. The (D+) terminal on the alternator MUST connect to
    a functioning warning light in the instrument cluster. There should
    be just one wire (blue) from the (D+) connector on the alternator
    to the button on the bottom of the normal indicator light in the
    dash. The dash light is also wired from the (D+) (Blue) to ignition
    terminal #15 on the coil, which in turn is connected to the positive
    (+) post on the battery (Black). There is no ground wire on the
    light; the body of the bulb is connected to ground via the light
    holder (which also provides the ground connection for the other
    dash lights as well.) The three bulbs (ALT/OIL/TURN) all have a
    common connection in the socket which goes to ignition terminal
    #15 on the coil (which receives power from the (+) post on the battery
    by way of the ignition switch).

    The alternator must get a feedback current through the “Alt” lamp
    in the instrument cluster so it can sense the battery voltage; it
    uses that as part of the alternator’s internal circuitry needed
    to charge the battery. In other words, with the ignition on but
    engine off, the indicator light sees 12 volts from the battery (via
    ignition terminal #15 on the coil) and glows, but with the engine
    running, it sees 2 volts (14 volts minus 12 volts) running the other
    way, from the alternator. It doesn’t glow (needing more than 2 volts
    to do that), but the alternator still “sees” the connection to the

    So — If the Alternator is charging, the (D+) terminal has 12 volts
    on it; the blue wire from (D+) carries 14 volts to the “Alt” light
    in the instrument cluster. When the engine is running, there is
    12 volts on the *other* side of the lamp from ignition terminal
    #15, so potential difference is only 2 volts and the bulb doesn’t
    light. But, if the alternator dies or the drive belt breaks, there
    is no voltage on (D+) (looks like ground) and current flows from
    #15 thru the lamp to (D+), and the “Alt” lamp comes on to warn the
    driver of a problem.

    An LED light won’t work for this purpose. LED’s are diodes and
    will not allow current to flow in the opposite direction. With the
    LED, it would see the 12 volts, but the reverse flow 2 volts would
    be stopped by the diode nature of the LED, so that wouldn’t work.

    The 12-volt 2-watt indicator bulbs are available at any VW parts
    store. The same bulb is used for the speedometer illumination bulbs
    (two of them) and the other indicator bulbs in the instrument cluster.
    In a pinch you can borrow one of the illumination bulbs to replace
    a blown Alt indicator bulb — the speedometer will be a bit dim
    on one side but can still be seen until you get a replacement bulb.

    * * * * * 

    Rob and Dave’s pages are actually pretty good and are very instructional and easy to read and understand. Here’s the linkk for all sorts of info.

    Maybe this whole thread should get moved over to the VW Based Kits part of the forum, but I don’t know how to do that.




    Glad you got her straightened out. Lots of good info on the Samba if your TD is VW based. If the folks here can’t answer your VW questions (mostly we can) then the Sambanistas can.

    Allen Caron
    VW based 53MGTD - "MoneyPenny"
    "If one thing matters, everything matters" - from the book The Shack

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