See this post
Identifying Chrysler Alternators (1960-1976)
and then scroll up in the thread to see photos of the parts.
That means the engine is running on battery power. The ammeter should be showing discharge.
Then the question to answer is whether the field is getting power. (see the link above for explanation).
Begin answering this question by measuring the voltage at the field terminal with the blue wire. You'll have to backprobe.
> Voltage here should be the same as the rest of the system.
IF it is, then place a jumper wire from the other field terminal to a ground with the engine running at slow idle.
Make a quick check of the system voltage and the ammeter (battery charging).
Do not run the engine at high rpm and do not keep it running it if the ammeter is showing high rate of battery charging.
