Battery Not Charging After Lights Being Left On

[268Z]

My 1972 240Z has me majorly stumped. It seems the alternator is not getting the message that it needs to recharge the battery. I’ll try to not write a novel, but here’s some background. I’ve owned it for just over a year and drove it all last summer with no issues. Then I accidentally left the lights on when parked and had to jump start it. The ammeter swung to the right while the battery was being charged as I drove it and then it started jumping around. After a few days of it jumping around, it returned to center and has not moved since. The car continued to run just fine for a few weeks, then quite charging one day. It will jump start just fine but will not charge the battery on its own. There was never any smell of wires burning and all wires appear to be in excellent condition.
Here’s some pertinent info:
1 - The battery is a new (2024) AGM battery. It ran well before the lights being left on. NAPA has since tested it as being good. I have also tried using two other regular deep cycle batteries.
2 - The alternator that was on the car at the time of lights being left on was (I think) from a 260 or 280Z. A PO had replaced the factory harness plug with a round style connector that Nissan used for the later Z’s. It had an external regulator. It now has a new 280ZX style alternator with an internal regulator from zcardepot.com. NAPA has tested this alternator as good. I’ve replaced the round connector with the correct style connector (vintageconnectons.com) and have installed a resistor plug/connector from MSA. The black/white IGN stripe wire passes through the resistor and leads to the upper terminal of the alternator’s connection. The white 12V wire jumps to the lower terminal on the alternator’s connection.
3 - I have removed the ammeter and joined the two wires together.
4 - I have inspected the fuse box.
5 - I have tested continuity between every two points I can think of, both 12V and ground. Have even test each fuse. Everything tests good.
6 - I have tried running a wire directly from the 12V sensing wire on the alternator to the starter where the battery connects.
7 - The fusible link is new and tests good for continuity.

8 - All the wiring (aside from the alternator’s connection that was once altered) appears to be 100% factory.   
9 - I have added (prior to the lights being left on) a headlight harness with relay and have upgraded the wiper motor to a Honda unit, but both of these items are on an isolated 12V circuit that is not even plugged in right now.
10 - I have searched numerous message boards, blogs, and studied the FSM. There is a lot of great information available, but nothing that I can find that has worked.

None of these things (or any combination of) have made any difference. Here is what is happening when connecting a multimeter to the battery (or any 12V point): The charge holds steady when the engine is not running and the battery does not drain if left for a few days or even weeks but will slowly drop with the engine running. It will only charge if I add a portable battery charger plugged in from the wall – with the engine running or not.

This circuit on the car is quite simple, so I can’t think of anything else to check. The engine runs like normal while it has enough charge so I don’t think the coil or other ignition components are causing the problem. Could they?

 Any ideas on what to look at next?

[Zed Head]

22 minutes ago, 268Z said:

have installed a resistor plug/connector from MSA.

Some versions of this sometimes had a melting problem, I think.  The wire with the diode (not resistor) is the wire that supplies power to energize the windings.  I'd guess that that's the problem.  Diodes do fail.

It would be the L(amp) wire, the vertical leg of the T.  This is a ZX drawing so the wire colors are different.

Edited Monday at 05:52 PM by Zed Head

[268Z]

OK, ya that is something I can double check. The connector on the alternator I have is in a = formation not a T, but I understand what you are referring to. I guess it's not a direct replacement for the 280ZX, but is a 70amp unit https://zcardepot.com/products/alternator-high-amp-80-240z-260z-280z?_pos=1&_sid=0210ca6f4&_ss=r&variant=19280584015985 Sorry about that.

Is a 1N5062 diode (like this https://www.amazon.com/NTE-Electronics-1N5062-Rectifier-Tolerance/dp/B008UFVQMS ) the correct diode to use? I'll try one instead of the MSA unit. 

[Zed Head]

I would check for voltage at the L wire first.  With the key on.  That's the one.

[w3wilkes]

You mention that you now have an internally regulated alternator. Has the voltage regulator been replaced with one of these?

Motorsport Auto - The Z Store - Alternator Upgrade Adaptor, 70-72 240Z

[268Z]

11 minutes ago, Zed Head said:

I would check for voltage at the L wire first.  With the key on.  That's the one.

OK, I'll do that first.

[268Z]

3 minutes ago, w3wilkes said:

You mention that you now have an internally regulated alternator. Has the voltage regulator been replaced with one of these?

Motorsport Auto - The Z Store - Alternator Upgrade Adaptor, 70-72 240Z

Yes, that's what I'm looking at now (when I get home from work).

[268Z]

BINGO!! Turns out, I had the wires plugged into the wrong sides of the diode (backwards). I switched them and now the car reads 14.37 volts at idle. Thank you both for pointing me in the right direction! Yeehaw!