Inconsistent charging issue

[E's-30]

What's up guys, I have been trying to chase down a charging issue and need some help. I just replaced my alternator which seems to be working properly most of the time. About 30% of the time when I'm driving my voltage drops to around 12.4 volts and doesn't seem to be charging but most of the time I am 13.5-14v. I have voltage at the excitor wire but my battery light isn't working. I replaced the battery light tonight and the light still isn't working and there is still voltage at the excitor. Does the light have to be on to trigger something with the excitor wire even though it already has voltage there? I haven't checked all of grounds yet because the battery light wasn't coming on but that will likely be my next step. Does anyone know of a wiring diagram for the battery light/instrument cluster so I can chase this down?

Thanks in advance. I still actually need to do an introduction post but have been too busy running down issues on this thing to get it out for some pictures.

Ian
1988 325ic

[e30davie]

the battery light itself is the exciter, it applies a little bit of load to the alternator which gets the ball rolling.

the light will have 12v on one side, and other other side is attached to the alternator. when teh alternator isnt charging you have 12v on one side, and less than 12v on the other. So you get a voltage drop = light goes on.

Once the alternator is charging the voltage on both sides of the light equalizes, no voltage drop across bulb, and thus light turns off. happy days.

Alternators do work without the excitor light attached but you often have to rev the engine to get them going.

Battery light not working at all is concerning.

Do you get the low voltage at idle? or low rpms?

[E's-30]

the battery light itself is the exciter, it applies a little bit of load to the alternator which gets the ball rolling.

the light will have 12v on one side, and other other side is attached to the alternator. when teh alternator isnt charging you have 12v on one side, and less than 12v on the other. So you get a voltage drop = light goes on.

Once the alternator is charging the voltage on both sides of the light equalizes, no voltage drop across bulb, and thus light turns off. happy days.

Alternators do work without the excitor light attached but you often have to rev the engine to get them going.

Battery light not working at all is concerning.

Do you get the low voltage at idle? or low rpms?

My voltage is slightly less at the excitor than across the battery or alternator charging post.

[ForcedFirebird]

Alternators do work without the excitor light attached but you often have to rev the engine to get them going.

On the race cars, the alternator gets excited at 1000RPM. If you start your car and let it idle, the battery will drain. Newer cars automatically go over 1000 after starting. The blue exciter wire doesn't "need" the light, just a 12v source to work. We put a turbo m20 in a 2002. It didn't have any dash instruments/lights at all (save coolant temp) and it charged fine with ignition power, which surprised me at the time.

Most domestic (and most modern) alternators need a light or ~12ohm resistor on the exciter wire.