New aftermarket headlights on my daughter's truck have separate lights for low and high beams. I would like to wire them together so the low beam will still work independently, but they both will come on when she hits her brights. Thought about wiring a jump wire with a 12v 20 amp diode between the two positive leads so the low beam would work without powering the bright light, and both lights would be powered with bright switch. Concerned about heat buildup at diode and I'm not sure if I'm missing something. Open to help and other options. Want to stick with factory switch setup. Thanks.


2 Answers 2


I would use a two relay setup for this. The relays shall be parallel and one pin or the coil goes to the low beam switch and the one of the other relay to the high beam switch. This way no diode is needed.

But are you really sure you want to enable low beam with high beam? Driving at night has the low beam always on. In the day the high beam is only for signalling, therefore I see no need for low beam.


A diode will work but will drop some voltage. A Schottky diode drops less than a silicon diode but may still be noticeable (or not).

As Solar Mike says, a relay will work, and is liable to be cheaper than using a diode as automotive relays are a very well developed technology with an immense market.

Any automotive SPST (single pole single throw) relay with 12V coil (assuming your'daughter's truck is not 6V), 12 V DC contact rating (as they all will have) and contacts rated substantially higher than the total running low beam current. Filament lamps have a very substantially higher "inrush current" when cold and relay contacts not rated to deal with this will not last as long. if the lights are LED based there is no inrush current and a relay rated to comfortably handle the beam current will suffice.

Using a single pole single throw relay.

  1. Connect one side of coil to ground.
  2. Connect other side of coil to high-beam wire.
  3. Connect one side of contact to low beam wire.
  4. Connect other side of contact to high beam wire.

  5. Alternative - Connect other side of contact to Battery +ve. This is very slightly less desirable from a safety point of view (always live) but allows slightly less voltage drop as the high beam wiring does not have to handle twice the usual current.

  6. enables the relay when high beam is on.

  7. & 4. connect high and low beam wiring when high beam is on.

Existing relays?

Many vehicles use relays to operate lights. If relays are fitted then a diode from the high beam relay activation line to the low beam relay activation line will do what you want.


Example only relays on Amazon here

eg Hella 12V 50a - prices look OK. BE SURE they are genuine Hella.

eg these at 5 for $12 are an utter bargain if they are as good as the Bosch brand they claimed to be in the listing that lead me to this one. In fact they are dnf brand - which may be fine and may not. Why anyone would brand their product D.N.F. I know not :-).

  • Very appreciative of your advice. Will switch to a relay setup. The desire to have both lights function while on brights is because we live on a cattle ranch in the country. She drives on country roads with deer, hogs, etc. jumping out everywhere and then long roads through the ranch with wildlife, cattle, horses, buffalo, etc. I just want her as safe as possible. Thank you again. Mar 14, 2019 at 18:13
  • @RancherBill - And then there's Australia :-). I live "across the ditch" in NZ - about 2000 miles across the Tasman but get to visit occasionally. The "best" time is around dawn and dusk when the kangaroos abound (pun noted but unintended :-)) and all their mates crawl out to help. A kangaroo will stand by the roadside until you just about reach it and the flee. Fleeing consists of hopping either away from the road OR across the road and which they are going to do is like trying to predict a soccer penalty kick. ... Mar 16, 2019 at 11:28
  • @RancherBill ... I have a nice photo sequence of a near miss with my wife applying full brakes to our hired camper van and me taking photos :-). "Roo bars" are a thing. || One must NEVER swerve as it does not decrease chance of hitting a random motion roo but does add to rollover chances. | We met two men silly enough to ride a hired Harley in the outback at dusk. They arrived unscathed but shattered from the near misses. Mar 16, 2019 at 11:30

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