Sat here drawing a wiring diagram including sealed beam units...and I was wondering...

Should it be a "changeover" between high and low coils...

Or is 'high beam' both low and high coils?

Blond answer - hi and lo beams are separate , not both on at the same time

As above. You need a 5-blade changeover relay which switches the current to one or the other

linky

dips to 87a
main beam on 87

Is there anything inherently wrong with having them both on at the same time?

May have to allow for increased current draw, but aside from bigger wire, anything?

dave

I suspect the bulb will run hotter and so shorten its life. Twin filament headlamp bulbs are never wired to run both filaments together (except for momentary flash) and I presume this is the reason.

Interesting (well interesting if your a sad-o like me) point is that if you have your headlights on (dipped) and use the flash hi-beam circuit as opposed to actually on hi-beam both filaments are on at the same time....doesn't seem to cause a problem in any of the production cars.....Oli.

Hold the flash on and see how long the bulb lasts, then please yourself. The beam pattern is not designed to have both on together. If you plan on driving after dark please buy some proper headlamps with H4 bulbs though, sealed beams are pathetic.

I concur. Sealed beams lights are rubbish!!!
http://www.youtube.com/watch?v=eZA031V5tI0

the only reason you would want a sealed beam is if you go wading in an off-roader.

otherwise they are pants as far as brightness goes.