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?
 

[Zetsaz]

Is your "new" fusible link from before or after replacing the alternator and having the headlights left on? Fusible links aren't particularly reliable, and there's a reason they are no longer used. Continuity by itself isn't enough to know if it's passing enough amps to do anything.

 

Did you bypass the old external regulator properly? It has been a very long time since I switched to the later internally regulated alternator on my car, so I'm not sure the wiring you're describing in point #2 is related to the bypass.

 

Most of your checks are reasonable and detailed, so that's the only two I can think of off the top of my head. I'm sure someone with an early 240 instead of my early 280 will chime in at some point who would know more.

[jhm]

Just curious....what voltage levels do you have on the system when the engine is running?  (And you may need to take readings at multiple points in the system to help narrow down where the problem(s) is/are.)

[268Z]

Turns out, I had the wires plugged into the wrong sides of the diode (backwards). Doh! I switched them and now the car reads 14.37 volts at idle. Yay! 

 

The new fusible link was replaced after the lights being left on and during diagnostics. I was just trying anything and everything at one point to figure it out.

 

The voltage would start at 12.6 (with a full battery charge) and just slowly drop a tenth every 30 seconds or so with the engine running, which is consistent with the alternator not knowing it should be charging while the diode was not connected correctly.

 

Thanks for the responses!