John Ha wrote:
TKOPerformance wrote:
By being supplied by the same power source it still acts as resistance in the circuit, whether its directly in the A to B path to the headlight bulbs or not. The current running through the rheostat hits a bottleneck, against which it has to flow, meaning amp draw increases, and thus amperage available for the other portions of the circuit connected to the same feed is now reduced. It would only be inconsequential if the rheostat was in line after the headlight bulbs.
But it's a completely different, parallel circuit. If I applied your reasoning, the lights in my kitchen would come on when I turned on the bath fan, since both are supplied by the same source - the power coming into the house at the breaker panel. All of the electrical "things" in the car are powered by a single source - the battery. But there are numerous separate, parallel circuits supplied by the battery that provide power to different loads (lights, radio, horn, ignition, wipers, starter, etc.). These circuits, when properly designed, do not affect each other. The only impact to them all is when one circuit (such as the starter) draws a large amount of current and reduces the voltage available to all the other circuits.
The rheostat inside the light switch has it's own circuit and only affects the instrumentation lights. This circuit represents a trivial load as far as the battery is concerned and has no impact on the brightness of the headlights.
Actually that's not where following my logic leads. Resistance in a circuit has nothing to do with switching. Multiple switches can, and often do, share the same circuit in a breaker panel. Turning on one switch has no affect on the devices switched by the other switches in the circuit because each switch controls the switch leg for that particular device. The device only comes on when power is applied through the switch, despite the switches, or even the devices directly sharing a common power feed. You could also switch the device via the neutral, which I've also seen, though I believe that's frowned upon. There's about three different ways to wire devices, but the only way a switch intended for one device turns on another is if something is wired wrong.
On topic, upon review of the wiring diagrams, and lacking an actual switch schematic, its hard to say exactly what's going on in that switch. Its clear that Ford used it as a junction as well as a switch. What makes it hard to tell what's going on is that there is a wire incoming from the fuse panel for the interior lights, presumably so its voltage can be modified by the dimmer to dim those lights. There is also a wire from the switch to the fuse panel for the instrument lights, which I must assume has also come through the dimmer to dim those lights. If that wire is simply jumped through the switch from the interior light feed, why is it fused again? I also question why it would be fused after its had its current potentially modified by the dimmer. Strange though it would be, if that is indeed the case, then I must agree that the dimmer has no affect on the other lights because the lights it controls come from a parallel circuit. I suppose anything is possible, but in an era where they used so few circuits I would be flatly astounded that they chose to in essence double fuse one circuit but allow others to go unfused entirely.
It seems (to me anyway) more likely that the instrument lights are actually fed from the same power feed as the switch, and thus the dimmer is connected to the incoming power feed and could affect the output to the headlights. All I can say for certain at this point is that it could be either way. I think I have an old headlight switch that still works. Experimentation seems in order...
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