light fault light on 240

Discussion in 'Volvo 240' started by Ed, Nov 18, 2008.

  1. Ed

    Ed Guest

    Anyone have any ideas why the orange "light out" indicator is on, even
    though all the lights are operating? I had a lamp go out in the rear
    light cluster, replaced it and no joy....

    ed
     
    Ed, Nov 18, 2008
    #1
  2. Ed

    Andrea1717 Guest

    I had a similar problem due to a false contact on the rear lamps....try
    to push the brake pedal an watch what happens. If the brightness of the
    lamps change when you push the pedal there is a bad contact.

    Andrea..
     
    Andrea1717, Nov 18, 2008
    #2
  3. Ed

    BakersT Guest

    The "bad lamp" indicator works by detecting the difference in current
    being drawn between "left" and "right" side bulbs (brake, headlamps,
    etc). If a (single) bulb is replaced with one which draws a bit more (or
    less) current than the one on the opposite side, the indicator will come
    on. Best solution: replace in pairs. (It's also possible, but less
    likely, that the device which performs the current-sensing and drives
    the indicator is faulty, but if the problem appeared immediately after a
    bulb replacement this is unlikely.)
     
    BakersT, Nov 18, 2008
    #3
  4. The previous post was an excellent explanation. One way to troubleshoot
    this is to remove wiring harnesses from pairs of lamp holders. The
    problem can either be a mismatch in bulbs or a mismatch in socket
    resistance or both

    First do the obvious,

    1) if on in headlight position or off/running light position and not
    parking light mode then it is a headlight mismatch.

    2) otherwise it is a parking light mismatch, either a front or rear
    parking light mismatch.

    You can troubleshoot this by unplugging the connectors to both of the
    two front parking light assemblies.

    3) if it goes out then it is a front parking light mismatch.

    4) if not it is a rear parking light mismatch.

    Once you zero in on the mismatch, then you have to determine if it is
    the sockets or the lamps. Usually it is the lamps but on older cars,
    especially some 240s it was the sockets. They were not designed right
    and there was a recall. They got too hot and corroded.

    Once you fix one mismatch, there still may be another. When putting the
    connectors back on check each step of the way to make sure the problem
    doesn't come back.
     
    Stephen Henning, Nov 18, 2008
    #4
  5. Ed

    James Sweet Guest

    Or just remove the bulbs, that's usually easier than unplugging wiring,
    aside from the headlamps. Most of the time this problem is caused by
    either mismatched lamps, or corrosion in the socket.

    The sensor is actually a very clever bit of engineering. It consists of
    a number of reed switches which consist of a pair of tiny flexible steel
    contacts sealed within a glass tube. When exposed to a magnetic field,
    the steel reeds attract one another and close the circuit to the lamp
    failure indicator. Around each reed switch are two coils wound together,
    one in series with each set of lamps, with one wired so current flows in
    one direction while the other is opposite. So long as the current in
    each coil is equal, the magnetic field cancels out, but an imbalance
    allows a magnetic field which activates the reed switch. They are really
    a bit too sensitive, I suspect with a turn or two removed from each coil
    they would work better but I haven't attempted to modify one. Still, an
    ingenious bit of engineering, something that could have been built with
    technology of the 1800s that still works well.
     
    James Sweet, Nov 18, 2008
    #5
  6. Ed

    Ed Guest

    Is that why lamps come in "two packs"? I replaced the opposite number
    lamp with it's mate from the pack and presto-- no warning lamp. That's
    interesting how they engineered that switch with the coils and the
    reed switch. It reminds of what someone might say of Tesla's
    continuing relevance in electric fields, a classically trained
    electrical engineer must have come up with that one. So thanks all,
    problem solved!

    Ed
     
    Ed, Nov 18, 2008
    #6
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