I have noticed that the engine runs much less smoothly (and more liable to stall) when the headlights are on - or even sometimes the fan. I can understand the drain on the alternator affecting idle speed due to loading but rougher running at higher RPMS while driving? Any educated guesses or experience.

Put a voltmeter on the battery with the lights on & off. If the alternator is going bad, and can't provide enough power with the load, the voltage of the system will drop. The ignition system is powered by the same system, and a lower voltage to the coils will affect the spark intensity.

 

Also, it's an old car. The grounds are probably sub-optimal at this point.

 

-=Russ=-

Put a voltmeter on the battery with the lights on & off. If the alternator is going bad, and can't provide enough power with the load, the voltage of the system will drop. The ignition system is powered by the same system, and a lower voltage to the coils will affect the spark intensity.

 

Also, it's an old car. The grounds are probably sub-optimal at this point.

 

-=Russ=-

 

The only thing is I would have thought a bad alternator would lead to flat battery issues - which has never happened in nearly 2 years.

Put a voltmeter on the battery with the lights on & off. If the alternator is going bad, and can't provide enough power with the load, the voltage of the system will drop. The ignition system is powered by the same system, and a lower voltage to the coils will affect the spark intensity.

 

Also, it's an old car. The grounds are probably sub-optimal at this point.

 

-=Russ=-

 

Yep, check the battery, alternator and all the ground points. I've been reading that adding a few grounds really helps our cars as well.

Along with causing supply lower voltage, poor ground connections can contribute to higher noise levels on the battery system. Especially higher frequency noise, which can cause problems with the electronic systems. Batteries make pretty good low frequency filters but are not so good at higher frequencies where a good ground is needed to help keep unwanted noise out.

I long ago added another ground to the other side of the engine. Anywhere else I should do so to assist with the ignition system?

Since the headlights use a switched ground you may need to ground the dash area better. It would be good to make some checks with a voltmeter to locate the problem. I would tie the common lead of the meter to an extended ground lead from the battery and then check suspected bad ground areas with the various loads on. If there is a bad ground then there will be a voltage of some sort there.