New Voltage
dgc333 said
Alex's original post was that he had an issue that he was seeing 12-13 volts at the fuse panel with the lights on and 14 volts with them off. I am assuming that he had 14 volts when the lights were on or off before adding the relays. That being the case then by moving the light load to the alternator output terminal is some how (bad ground, bad connections, etc. somewhere in the system) causing the voltage regulator to not be driving the alternator enough to ensure there is adequate voltage in the system.
Respectfully Dave, I do not agree with this theory. I believe that what Alex experienced is still a voltage drop in the factory wiring between the alternator and the fuse box. When the headlights are turned on, you also get parking lights, tail lights, side marker lights and instrument panel lights. All these together can add up to several Amps, and these are still being powered through the original wiring.
I made the statement that with a fully charged battery, and no load, the alternator (voltage regulator) should try to maintain approximately 14.2V. However, with a large load such as headlights, I would expect the alternator output to drop to somewhere between 13.4V and 13.8V with the engine idling. Now couple that with a 1V drop in the wire harness to the fuse box, and it is not surprising Alex measured 12-13V there. It would be interesting to see exactly what the voltage was at the alternator under the same conditions.
dgc333 said
Good engineering practice tells you to provide common points for providing power and grounds (that's one reason why you don't see accessories powered from the alternator from the factory). It eliminates all kinds of weird issues that come up from ground loops and multiple connections. If you have a concern that the charging circuit (the issue with inadequate wiring throught the light switch has altready been addressed by the relays) through the fire wall is not adequate to deal with the 9 amp load the head lights add to the system then you would be better served to fix that then put what I would consider a bandaid on it by taking power from the alternator.
In a perfect world, I would agree 100%. But these 30 year old Mopars are not perfect. So yes, taking the power directly from the alternator is a bandaid for the poor factory wiring.
dgc333 said
IMHO, if you have dim lights due to old wiring and you are not add lights that draw additional current then the correct thing to do is fix the wiring. The factory wiring is quite adequate for standard sealed beam bulbs.
I think this depends on your definition of adequate. I have personally measured over 1V drop between the fuse box and the headlamps on my Dart. This does not include any voltage drop in the wiring from the alternator to the fuse box. To me, a >1V drop in a 14V circuit is unacceptable. Maybe my standards are too high.