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My alternator has been draining my battery in my Datsun 240z since I swapped the engine (VQ35). It takes around 4 days to drain the battery to the point I can't start it. It charges it fine when the engine is running.

The symptoms are after driving the car letting all the electronics chill out for a minute the battery will drain 0.17A. If I then disconnect the alternator B+ wire it'll drop to about 0.02A at which point I can reconnect it and still only get 0.02A.

I've tried disconnecting all sorts of things but the only thing that helped was the alternator. This is my third alternator since the swap and they're pretty expensive. The first two I swapped because I was pretty sure they had bad diodes - the place I've mounted the alternator gets pretty hot & I've since built a shroud for it - they had slightly different symptoms thou, after reconnecting they'd still drain hard out.

The one I have now has professionally rebuilt and tested and has done this since I put it in.

So have I done something wrong with my wiring? The obvious answer is I have a bad diode, but it seems odd that disconnecting and reconnecting the B+ fixes it.

How can I work around it? I'm almost considering having a switch on my B+ terminal to isolate it. I potentially could have a relay on it so it switches on/off when I start the car. My concern is the amount of amps it'd have to deal with?

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    I think the relay is a great idea. You could have it switched from the ignition and be golden. I don't know how many amps a regular relay can handle, so it is a consideration.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Nov 25, 2014 at 3:06
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    Are you sure its installed correctly? you may have a short thats causing the drain.
    – Mauro
    Commented Nov 25, 2014 at 12:25
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    How long since you had the alt rebuilt professionally? I'm actually think you have a bad diode as well, where it stays in an "open" state after running as long as there is power, then goes to a closed state after power is lost ... I don't know if this is technically possible, but sounds feasible to my caffeine induced brain.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Nov 25, 2014 at 13:35
  • @paulster2 I tried working around the problem with an 80A relay but the wiring couldn't handle the amount of current and melted the wiring insulation. This kinda scared me so I'll avoid that kinda of work around now I think.
    – Mikey
    Commented Oct 11, 2015 at 20:26
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    The regulator controls how much power it needs to use on the magnets right? Recheck to see where your 3rd wire to the alternator is running to. Might be connected to something with constant power in the fuse block not switched.
    – Tobin S
    Commented Feb 12, 2016 at 1:30

3 Answers 3

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I had an alternator that drained the battery .It was fine for years but I had a small Holley carb fire which was put out with the help of someones disposable fire extinglisher .It made a big mess .The alt leaked a rather unstable 100mA .I cleaned and stripped and cleaned and stripped and cleaned and got it down to 10mA .Put it back in my 1965 Stude which doesnt get used often and no problems for 4 years .

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With the engine running, measure the voltage across the battery. If the voltage exceeds 14.5V then you have an alternator regulator fault. The regulator may be available on its own, but because of age you may have too have some kind of alternator rebuild. An outside chance is that an alternator from a later or differant vehicle may have the same fitting dimensions as your original and could be substituted.

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    The alternator is charging fine. You haven't read the question. The issue is that the alternator is draining the battery when the car isn't running.
    – Mikey
    Commented Oct 11, 2015 at 20:22
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The alternator has a grounding cable connected to the frame of the car. If it is disconnected or has a corroded or loose connector, the alternator will not charge the battery. You can test this by connecting a temporary ground wire from one of the alternator mounting bolts to a good metal grounding point away from any moving parts. If the battery charges, you've found your problem.

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    The OP's problem does not have to do with charging, but discharging the battery when not in use. I'm not sure how your statement answers the question?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 27, 2015 at 11:11

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