Brian:
The light wire essentially "energizes" the field windings through the light
circuit. If the alternator isn't charging, then the light is at ground and
+12V applied to the other side lights it up, indicating there's no charging
going on. The way to connect it is the alternator side to the light, and the
other side of the light to a swtiched +12V. Once the alternator is running
and the field "energized", you can often remove the +12V from the field, and
the alternator will continue to charge. In this case the field is "self
energizing". The light won't light as you've disconnected the current path
for the light, but the alternator is still charging and loading the
engine...so putting a switch in that light won't stop it once its started.
If you start the engine without the field circuit energized however, then the
alternator will not charge, and thus not load the engine, other than the
rotating mass/bearing load.
The best way to tell if the alternator is charging or not is just use a
voltmeter (or ammeter in an older car). When charging, you should see 13 -
14.5 volts on the system. If its not charging, then you'll see 11.5 - 12.5
volts.
Hope this made some sense.
REgards,
Myles H. Kitchen
1965 Lotus Cortina Mk1 #128
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