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1975 280z New ZX Alternator Not Charging


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A really good test light is $5 and best of all doesn't need batteries and worry about polarity. It just tells you if 12 volts is present, or if connected to 12 volts, if there is a ground present. It's a very hand diagnostic tool

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I started the car just a minute ago and checked voltage at some areas and after I finished I decided to leave it for a few minutes for it to warm up.

After I got back it was fully warmed up and I tried revving it to 4K rpm and I saw the ammeter jump up!!!

I jumped out the car and checked the battery its getting 13.5v consistently at idle! Even with a bunch of accessories on!

I have no idea why it finally started charging. I guess it needed a jump start.

I tried revving it up before but this time it decided to work.


Thank y'all for helpin'

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"After I got back it was fully warmed up and I tried revving it to 4K rpm and I saw the ammeter jump up!!!"


This is common.  A lot of internally regulated alternators do this.

An alternator has two main parts that generate electricity.  The rotor, and the stator.  The stator stays still, and the rotor rotates an electromagnetic field inside the stator.  This generates an alternating current in the stator, and six diodes are used to turn the AC current into direct current, that the car uses.  The diodes also prevent current from the battery from flowing back in to the alternator.  So the stator can stay hooked up to the battery all the time, and not drain the battery.


The rotor is an electromagnet.  The voltage regulator controls how much current is applied to the rotor, to change the strength of the electromagnet.  The amount of current the alternator generates (outputs) depends on two things.  How strong is the electromagnetic field, and how fast it is moving (RPM).  A strong amount of current in a slow moving rotor (low RPM) is used to generate a lot of current to charge the battery, at low engine speeds.  When the engine is sped up, the current is cut back, to control the output of the alternator.


You cannot always leave the rotor connected to the battery, because it would drain the battery.  The voltage regulator for the alternator has to have an ignition switched source of power.  The internal regulator uses the power from the charge light to supply power to the internal regulator, and it is applied to the rotor.  Once the alternator is spinning, and generating power, it can use its own power to energize the rotor.


There is a tiny bit of magnetism that stays in the rotor, it is a very weak permanent magnet.  If you spin the alternator fast enough, that tiny magnetism is enough to start the alternator to generate power.


That is why the alternator started working when you revved the engine.  


Your 1975 Z does not have a charge light.  It has an ammeter to tell you the alternator is working.  But it did have ignition switched power to the external voltage regulator, to supply power to the alternator rotor. 

On the 77-78 Z, the charge light was put back in, because it is a convenient way to supply to the internal regulator in the alternator.


On a 1970 521, there is a ground wire from the alternator to the inner fender, and one of the bolts that hold the voltage regulator.  This wire is very important.  It connects body sheet metal to the negative battery post.  If this wire is missing, or not hooked up, the body sheet metal tries to ground to the battery through the engine controls, throttle linkage, or some other way.

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The 280ZX has a very different fuse and relay system. I had a 280ZX that would only charge after revving the engine to 3.5k rpm. Eventually I replaced a couple of those factory relays and that fixed the problem for that car. I had already tried replacing the internal regulator in the alt.


Maybe your 280Z has something similar. These are super expensive from Nissan so go to a wrecking yard and get a couple. (if they even exist on the 280Z)





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