Mike:
Turn up the idle speed. The alternator is putting a bigger load on the engine
when the lights are on. The alternator has to provide the power for the lights,
the battery doesn't.
Many cars idle speed will drop when the lights are turned on. If the engine
can't handle the additional load, I'd start with a good engine tuneup.
If the battery is fully charged, the horn should still sound strongly, lights
should be quite bright, etc. Put a battery charger on the car and see if the
lights and horn improve. If they do, then the charging system needs attention.
You could also check the battery with a hydrometer. Besides a poor alternator,
you could have bad grounds on the alternator, battery, appliances, etc.
Blake
<<<snip>>>
Could a lot of my electrical faults be found in a sick alternator.
Weak horn, once-in-awhile drop of current, slow death when a lot of
electrical stuff is brought on line. Weak dash lights.
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