Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need help diagnosing headlight issue on my vehicle.

  1. Nov 20, 2014 #1
    Driver-side low beam was out so I went to the auto parts store to pick up a new one. Well it still won't light up:(

    Using the ohm setting on a multimeter, you can check standard household light bulbs to see whether or not they are defective. Good light bulb = high resistance, since the filament needs to be of high resistance for it to glow. I know this doesn't work for all types of lights (like CFL), but will it work for my light bulbs in my vehicle? They are standard or "OEM" (not HID's).

    I'm almost certain I didn't purchase a defective bulb. So now what? Can I check the actual harness in my vehicle? These light bulbs operate on DC, correct? Would I test Volts or Current? Will it matter? I will check the opposing harness for reference, i.e. I'm getting 10mA on my passenger side low beam harness so I should theoretically be getting 10mA on my driver side low beam harness.
  2. jcsd
  3. Nov 20, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You want to check voltage on the socket. Good incandescent bulb is indicated by low resistance --- high is an indication of a broken filament (open circuit).
  4. Nov 21, 2014 #3
    Will it make a difference whether or not the vehicle is on or off? Perhaps I can just turn the key to accessory position?

    I started the vehicle and checked the socket with my multimeter. It gave me a reading of -10.13 Volts. I had the polarity wrong, but I don't think that matters. I noticed the socket was black in color and had shorted out on the chassis of the vehicle. I will call the dealership in the morning for a new socket.

    My sibling asked me why it made a difference whether or not I measured the voltage across the socket or measured the current through the socket and I was unable to answer his question. Suppose my previous reading of 10.13 Volts is valid. If resistance is fixed, then there's only one value for current, hence E=IR. I could have measured such value on the opposing (passenger-side) functional socket for reference. If the value was significantly off, then I would have known the voltage was off too ...without ever measuring it.

    Is this correct?

    And here I thought I had the light bulb figured out, lol. Same applies to electric heaters? Or is this where high resistance comes into play?

    If an incandescent light bulb manufacturer made their filaments with greater resistance, then wouldn't they last longer? I believe so, but they wouldn't be as bright, correct? What kind of ratio (longitivity/brightness) do light bulb manufacturers shoot for?
  5. Nov 21, 2014 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Whatever it takes to get the other headlight lit.

    Should be getting ~ 12

    The circuit is going to be battery through various accessory switches, fuses, headlight relay, headlight itself (via socket), and to vehicle ground (other battery terminal). Inserting ammeter across that is a short circuit and very high current; all you want to know is whether there is a conducting path, so you're looking at voltage.

    Ohm's Law, E=IR, yeah. Power is what counts for incandescent bulbs, electric heaters, and is EI, I2R or E2/R. Raise the voltage over a constant resistance and you raise the current, or raise current (by raising voltage), or decrease resistance which increases current at constant voltage, to increase power.

    Automotive systems run at 12 volts, so higher resistance is lower power, brightness. Longevity? It's more a function of how well the filament and envelope seal resist vibration, road shock, and aging. Target life? Anybody's guess.
  6. Nov 21, 2014 #5


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    If there is a high resistance contact somewhere in the circuit, you can sometimes find that a meter will show you 12V at the socket with the bulb not there but when you connect the bulb, the volts drop to a low value. (the bulb is acting almost like a short circuit compared with that other resistance in the circuit) If the bulb lights, then you have current (of course) so there isn't much point in measuring it (haha) but, of course, there are other devices that may not perform so obviously as a light bulb and current measurement is sometimes necessary, to reveal a high resistance contact.

    BTW, if, as you say, there has been a real 'short' to the chassis, you may well have blown a fuse and you'll need to replace that before your new lamp will work.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook