Charging system questions Options

[ThinAir]

So this is really about my son's 1975 VW Bus, but it applies to 914s because 914-4s use the same Voltage Regulator and Alternator. I'm hoping someone here can help me figure out what is going on.

The problem is that when the weather gets cold, the battery doesn't have enough charge in the morning to start the engine and my son ends up push-starting it (lives on a hill). The only symptom that I've been able to see is that if the car is idling with the headlights on, turning on the blinker will push the electrical load high enough to cause the alternator light to blink in time with the blinker. The glow of the alternator light is fairly dim, but it's easily seen.

Well the '70 teener isn't going anywhere this week since we have 6" of snow on the ground and more coming so I decided to swap batteries and put the teener battery into the bus. They are both Optimas, the only difference is that one is red-top and one is orange-top. The battery from the teener had not been used in a week, but it had plenty of power to start the bus.

Unfortunately, with the teener's battery in place, the bus still had the same issues with the alternator light. So it's pretty clear to me that this is not a battery problem, but how can I tell which component has the problem? The VR is fairly new AFAIK. There's only 3 components to the charging system - how do I know which one has issues?