Originally Posted by willwren
The old dual sealed-beams did that. These don't.
I did a mod to keep my fog lights on with the highs, though. Works great and really 'fills the gap'.
You'd have to totally rewire your headlight circuit. A single relay selects between low and high. You'd also need heavier gauge wire and a bigger fuse for the circuit, as the current demand would double.
Well, what you'd need (and places like J.C. Whitney have kits for this) is a relay that'* controlled by your high-beams, which turns on a separate circuit to power your low beams. Thus the low beams will operate as normal using their normal circuit, _plus_ they'll stay on when your high beams activate, via an additional circuit that you wire in.
You can't just jumper your low and high beams together, because then they'll all come on regardless of which circuit is providing the power, plus the electrical load on the circuit will be doubled. So instead, you use a relay that'* controlled by the high-beam circuit, which switches on when the high beams activate. (Switching on the relay takes a neglible amount of power.) The relay in turn controls a new circuit that you install to power the low beams when their usual power feed is not on.
You should have a fuse or circuit breaker on the new circuit that you add. The normal circuit-breaker protection on the existing headlamp wiring will not be affected (the breaker is built into your headlamp switch), and none of the existing wiring will be overloaded.
Although the factory headlamps use a circuit breaker so you're not plunged into permanent darkness at an awkward moment, it'* okay to use a fuse on your new circuit, since blowing that fuse won't affect normal operation of your headlamps otherwise.