I need some ideas. My 2009 Ford F150 pickup uses the same wire for daytime running lights (DRL) and "full" headlights. The stock setup was halogen bulbs with full brightness receiving 12 V and the DRL receiving 7 V. Now I have HID headlights wired through a relay. The relay closes at 7 volts so the headlights are on full power during the day.

I plan to convert to LEDs for the DRLs hooked up to a 12 V. I just need to drop the 7 V on the main headlight wire so the HIDs are not on at all during day. I'm looking for advice on how to do this.

  • 1
    Seems the better choice would be to add circuitry between the existing wire and HID relay. Something that only turns on the coil when V is over 7V. Leave everything else the same. Coils are not precise voltage switching devices. It could be that 3V will be enough to turn it on, or hold it once it is on.
    – mkeith
    Jun 1, 2016 at 4:53
  • 1
    F150? Stock? DRL? HID? Too many TLAs (three-letter acronyms). Your question doesn't read very easily. Think of your audience.
    – Transistor
    Jun 1, 2016 at 6:24
  • F150 = ford pickup, HID = arc lamp, stock = as it came from the factory, DRL = no idea...
    – Tom
    Jun 1, 2016 at 10:28
  • DRL = daytime running lights
    – Dave Tweed
    Jun 1, 2016 at 10:48
  • Since you changed to HID, what will you use for your DRL? Are you planning on not running DRL? Sounds like it would be better to cut the wire (or pull the fuse) for the DRL. Optimally, I would think you would use some other sort of light for the DRL, and run the DRL wire to that light.
    – rpmerf
    Jun 1, 2016 at 15:10

4 Answers 4


The easist thing would be to remove the Daytime Running Lights (DRL) resistor and leave that connection open. Most F150s this simply unplugs as a module. If you ever change your mind you simply plug it back in. But I would be very hesitant to run my HID ignitor modules on 7v all the time.

Without knowing what year of truck, I can't tell you where exactly this resistor is located.

  • It looks like the truck for various years uses a field effect transistor and the body control module to control the headlights
    – Ben
    Jun 2, 2016 at 19:54
  • Now that I know it's a 2009, you are absolutely correct - and it's impossible to get at the FET base to fool it. Buried in the CJB I'm pretty sure, and uses PWM to dim.
    – SteveRacer
    Jun 3, 2016 at 5:51

Thanks all. The daytime running light at 7v and fulllow beam headlight at 12v is run on the same wire. The high beam is a second wire. If I cut the wire I would have no headlights.

I will be using LEDs for the daytime running lights hooked up to a 12v ignition source. I just need to drop tge 7v on the main headlight wire so the HIDs are not on at all during daylight. The truck is a 2009 F150. Thanks

  • Tim if you are the OP associate your accounts and you will be able to edit your original post. Jun 2, 2016 at 16:49

I'd suggest cutting the low beam headlight wire and intercepting the ground signal wire at the switch. So as to bypass the body control module altogether, unless you want to retain autolamps.

  • Ben is right, you will have to rewire the system completely to remove the BCM (actually Central Junction Box "CJB" control... BCM is a General Motors term for the same kind of animal). It's not 7v you are looking at, I'm pretty sure it's a Pulse Width Modulation signal. Which is REALLY something I would not feed expensive HID igniters, even if that signal is not the source of headlight power. Diodes will drop voltage in series in 0.7v increments, but I refuse to recommend that as I'm not sure of how that will affect the HID system -- and it will surely fail at the worse possible moment.
    – SteveRacer
    Jun 3, 2016 at 5:46

I'd suggest doing some searches to see if there is a way to disable the DRLs on your truck. Failing that, the coil current on the relay for the new HiD lights might be low enough that you could get away with putting a variable resistor between the 7/12 V wire and ground and then connecting the HID relay to the wiper (the third, variable connection). The adjust the resistor until the HID relay drops out when the light switch is off, but pulls in when the lights are turned on.

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