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Thread: Alternator Activated relay?

  1. #1
    1978 280Z (stock) TomoHawk's Avatar
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    Default Alternator Activated relay?

    Higuys,

    On a 1978 280Z with OE alternator,

    I was wondering if there was a way to have things like driving lamps to get switched off when the engine is not running, and I remembered that the alternator has "a blue wire" that goes to the volt meter for the charge alert light, and I thought that if you can tap that wire to set up something for a relay system that powers things like the driving lamps or headlamps, then when you turn off the engine, the lamps will go off too, saving the battery.

    So is there a terminal on the alternator that goes hot when the engine is running?

    thxZ
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    No more body roll! SteveJ's Avatar
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    No. If you want something that turns off with the engine, You'd probably be better off tapping into a circuit that is energized by the accessory relay.
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    1978 280Z (stock) TomoHawk's Avatar
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    That wouldn't work.

    You need to stay away from anything that is switched by the key, and only allow things to be energized or powered when the engine is running. It is too easy to leave the key in and switched on.
    Last edited by TomoHawk; 08-03-2014 at 11:24 AM.
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    No more body roll! SteveJ's Avatar
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    Actually, you prefer not to have it off the key switch. I don't seem to have a problem about leaving the key in and switched on.

    Energize a relay off of the fuel pump relay. That's a low current draw and won't affect your fuel pump. Then power the lights through the relay.
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    Quote Originally Posted by TomoHawk View Post
    So is there a terminal on the alternator that goes hot when the engine is running?
    Yes, but I'm not sure it would work for what you want to use it for.

    The blue "L" wire off the internally regulated alternators does exactly what you want (goes hot after the alternator spins up through bootstrap), but I'm not sure how heavy of a load you can hang there before you start messing with the regulation.

    And if you hang too light of a load there, you'll get a voltage reading whether the engine is running or not because you'll pull current through the charge lamp.

    Interesting question... I'll give this a little more thought and see if I got an inclusive answer.

  6. #6
    1978 280Z (stock) TomoHawk's Avatar
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    All you'll need is a power feed to a relay coil that is grounded inside the cabin by a switch. From that you can power other more demanding devices.
    Last edited by TomoHawk; 08-03-2014 at 08:40 PM.
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  7. #7
    Boat Anchor Repairman Captain Obvious's Avatar
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    Well I'm not sure I did a good job of explaining why it might not be as simple as you describe above...

    Any time the key is on "ON" or "START", the "L" terminal of the internally regulated alternator will always have "some" voltage present on it. Even if the alternator isn't spinning. This is because that "L" terminal is connected to the battery through the CHARGE lamp.

    At the "L" terminal you've got a voltage divider between the resistance of the CHARGE lamp filament and the resistance of the alternator's rotor coil. Accordingly, the "non-spinning" voltage on the "L" terminal is determined by those two resistance values. Any relay coil that you would hang off the "L" terminal to ground would be in parallel with the rotor coil, and I have no idea what the resistance of that rotor coil is.

    You're going to have to be careful that whatever you connect there has a pull in voltage high enough that it isn't drawn in by that "non-spinning" voltage. In other words, you're going to have to be careful that whatever you connect there has a resistance high enough such that the portion of the current that flows through relay coil isn't enough to energize the relay coil until the alternator has spun up through bootstrap.

    And in addition, you're going to ask the rotor coil to share it's bootstrap current and I'm not sure it's going to like that.

    And if that's not enough, that "L" terminal supplies power to the internal voltage regulator and I'm not sure if there would be any unwanted interactions with voltage regulation due to the additional current draw on that control line. The drawings show it as connected just like any other output from the alternator, but I have no idea how robustly it is built and what it's source resistance is. Since it's really NOT just another alternator output, they probably built it smaller and cheaper than the main output circuit and it might not like the current draw of a relay coil.

    Not saying any of this wouldn't work... Just saying there are lots of things that would need research and testing.

  8. #8
    No more body roll! SteveJ's Avatar
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    This leads to what I posted earlier. If the concern is to shut off when the engine is not running, the best solution may be to come off the fuel pump relay. IIRC, it will drop out if the oil pressure drops too low. Another possible solution is to use a time off relay connected to the ignition circuit. That way if the car dies while running, you don't immediately lose the lights, but as soon as you turn the car off, the timer would start. That would happen regardless of whether or not the key is removed.
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  9. #9
    1978 280Z (stock) TomoHawk's Avatar
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    I was studying this diagram to get an idea why my brake lamp isn't working lately, and it led me to think that you could activate other stuff with the engine. It's on page BE-34 of the FSM.

    Brake Lamp Warning System, 1978 280Z

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  10. #10
    No more body roll! SteveJ's Avatar
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    And that is the gist of what I put in post #8.
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  11. #11
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    Not what you're looking for though. That brake warning relay does the opposite of what you want. It's energized when the ignition is "ON" and the engine is NOT running.

    However, if you were to change the brake warning relay over to a DPDT style, you could get what you want... Use one pole as originally designed for the brake warning lamp and use the other pole to control your power. NC side connected to the switched ignition relay voltage and the common of that pole would be a signal that you could use to do whatever you wanted. Essentially use the NC pole to switch the power from the ignition relay.

    When the engine starts and the alternator reaches regulation, the relay would de-energize. The brake warning pole would open circuit and your "Hot when the engine is running" pole would close.

    Just make sure you use a relay that has a similar coil spec as the original one as far as resistance and pull in is concerned and you should be good.

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