Is there a way to make both the high beam and low beam lights on when turning on high beams? My car has two separate bulbs that switch back and forth. I would really like the extra coverage.

  • Drive the low beam relay from the high beam relay using a diode.
    – Solar Mike
    Jan 21, 2020 at 5:21
  • @SolarMike - That's a good answer. You'd need a heavy duty one to put up with the current flow, but it would. You should make an answer out of that. Jan 21, 2020 at 13:55
  • If it's an older bike, you might be able to just add a second mechanical switch. On my '83 Nighthawk, I could balance the switch between high and low, and they would both be on. The stator on that bike could barely handle the load though, so it only worked when I was riding at over 4k rpm. Your experience may vary. Jun 19, 2020 at 17:59

1 Answer 1


You can drive the low beam relay from the high beam relay using a diode.

I would use a small diode to go from the coil supply on the high beam relay to the coil supply on the low beam - this only requires that the current increases by the amount to energize the coil - combined that will be less than 1A.

If you do this on the output side of the relay then you will have the low beam relay supplying all 4 bulbs ie about 20A instead of 10A...

Do make sure that the relays are supplied separately, otherwise you have the same current problem on the supply side. If that is the case then I would run new fused supplies to the relays.

  • Note: the bulbs are NOT designed to work that way. Expect them to burnout very quickly from the excessive heat. This is not clever. Cheaper, easier to purchase better bulbs then have the headlights carefully aimed, so the cutoff is just below horizontal.
    – zipzit
    Jan 21, 2020 at 16:00
  • 2
    @zipzit is that because you have dual filament bulbs? I have 4 headlights - two each side, each one has its own bulb so 4 separate bulbs... And they most definitely are designed to work that way.
    – Solar Mike
    Jan 21, 2020 at 16:11

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