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Head Lights draining battery?


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Was wondering if anyone could give me an idea of whats up with my car,

So the issue is that every time i turn on my headlights and i come to a stop the car turns off after a while, but the lights are still on.

So i was wondering if anyone knew what this means?




I have 1972 Datsun 510 sedan

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If the battery stays charged and it re-starts and runs normally otherwise I would expect some problem with the ignition.


It should fire the plugs at idle with a reduced charge from the alternator caused by low RPMs and a drain like having the headlamps on. The battery should store enough power to keep it running while idling with headlamps on and even with the alternator not charging.


Get a volt meter and check across the battery posts when not running, when running at idle and when running and the headlamps turned on. This may tell us something.


In the mean time, check all ignition connections are clean and tight. Points and clean and gaped properly, Rotor, distributor, plugs and plug wires in good shape.

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I have seen worn brushes in the alternator cut the alternator output, but it will still charge some, but not enough.  An alternator with one bad diode will also charge some, but with lower output.


All alternators increase output with an increase of engine speed.   The voltage regulator cuts back the current going to the rotor in the alternator to control the output, as the engine speeds up.

If the brushes are bad, or a diode is bad, the alternator running at a slow speed will not put out enough to keep up with electrical demands, at an idle.

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  • 3 weeks later...
  • 3 weeks later...

Sorry, i was dealing with a different problem on the car, but i used the volt meter on it and it read 14 on the on position and

with the car all the way on engine running and all it read 12 and with the lights on it still read 12  volts


Not understanding  on and then all the way on.




Measure across the battery posts. What do you get?...... 1

Pull headlights on. What do you get?............................... 2


Lights off.


Now start the engine. What do you get?........................... 3

Pull headlights on. What do you get?............................... 4

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its charging it enough during the driving, to keep you starting, but the alt is on its way, i would recommend taking the stock alternator to a alt /starter specific shop to have them clean and rebuild/replace the broken part. or buy a good used one, or upgrading to a KA alt   



please for your own sanity stay away from autozone and o'reilly starters and alts.

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I have a O riley alt and its seems OK .

Nissan wants to much money for their starters, alt now. the price like tripled  I got my spares when they were still like 50$ apc


soembody said a 85 subaru or soemthing like that will fit. plus rockauto has new ones Not rebuilt

Not sure the bolt hole size for the mount. I assume its the later L18/20 size about 3/8th.  as the L16 is smaller holes.


says 100% new but who really knows


WPS brand Power select 14742N

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It was mentioned before, but I cannot stress the importance of good grounds.  A lot of, if not most "unusual" or wacky electrical problems are caused by bad grounds.

Good connections are also necessary on the positive side of the electrical system, but those problems are usually more obvious to to diagnose, because they tend to only affect one thing.


Here is an important concept to keep in mind.  As electricity travels through wires, you lose voltage.  The smaller the wire, the more voltage you lose.  Poor connections also cause a voltage loss.


Charging battery voltage should be about 13.5 to 14.5 volts.  You know that.  The voltage regulator is connected to the fuse box, and measures the battery voltage there.  If the connection between the battery positive and the fuse box is not good, the regulator thinks the battery is getting high enough voltage, but it is not.  If the voltage regulator thinks the voltage is high enough at the fuse box, it cuts back on the alternator output.  But the battery is not getting the voltage it needs.  If there is resistance in the wires from the positive alternator output, it also limits the amount of output from the alternator, available to charge the battery.


What about the battery negative?  On a Datsun, battery negative is connected to the cylinder head.  The lug that connects to the cylinder head also has a pigtail that goes down the the alternator frame, or body.  At the alternator frame, there is a second ground wire, that goes into the wiring harness, and this wire goes to one of the bolts that hold the voltage regulator to the inner fender.  If these ground connections are missing, or not good, the voltage regulator again is not measuring the actual battery voltage, and is measuring a higher voltage somewhere else in the electrical system.  A lot of people think that just because the alternator is bolted to the engine, it is grounded.  But the regulator is not bolted to the engine, it is bolted to the inner fender, and if the ground between the inner fender and the engine is not perfect, again the voltage regulator does not know what the battery voltage is.


If you have a digital volt meter, you can check the positive and ground connections between the battery and the alternator easily.  Lets do the positive first.  Put a red voltage test on the positive alternator output.  Put the black voltmeter on the center of the positive battery post.  You should measure less than .200 volts, that is ideal.  If you have a higher number, up to .500 volts, that is OK.  More than that half a volt, that is not good.

Do the same thing with the ground side.  Put the black voltmeter test lead on the alternator frame.  Put the red voltmeter lead on the center of the negative battery post, and measure that voltage.  Less than .200, ideal. less than .500 OK, more is not.


The voltage drops tests obviously need the engine running.  Be careful around moving parts.  Check with headlights, the heater, and the wipers on. The engine may need to be ran to about 1,000 to 1,500 RPM with everything on.

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