Originally Posted by willwren
Now we can't just jumper a parallel line, or the Highs would stay on with the lows selected....BUT, we CAN use just a single high-wattage diode. No kidding. I think a single diode will do it. May have to boost the rating of the fuse, and there'* a fusible link I need to check on, too, but I think this can be done for about a dollar. At least on a 93.
I understand how your diode would work, but your high beam circuit would be drawing power for four headlamps instead of two; I'd wonder about overloading some portion of the circuit.
Actually all you really need is a 5-pin automotive relay, available at places like Radio Shack. Current to the high beams would be used to close the relay, which would then light the low beams alongside. The power to actually _light_ the low beams can come from a separate power source, so that you're not doubling the load on the high-beam wiring.
The reason for a 5-pin relay instead of a 4-pin is that when the low beams (only) are in use, power to the low beams would pass through the same relay. The 5th contact, the one in the center of the group, is the normally-closed one, allowing a path through the relay to the output while it is off. When it'* on (e.g. when you switch on your high beams), the relay interrupts the normally-closed circuit, and switches to its other input lead instead. Thus your high beams can switch on your low beams, but your low beams cannot switch on your high beams.
Clear as mud? I thought so!
Anyhow, J.C. Whitney/Warshawsky sells a little kit that does exactly this. Ransack their website and you should be able to find it.