91 Civic Front Side Markers Both Flash When Operating Turn Signals

Discussion in 'Civic' started by RC, Dec 19, 2010.

  1. RC

    RC Guest

    I think I have a problem with the integrated control unit (ICU) that
    operates the front side markers on this 91 Civic.


    When either the left or right turn signals are operated, _both_ right
    and left side markers flash. Only the same side is supposed to flash.
    This is the problem. it sends mixed signals to other drivers.

    When the parking lights are on, the orange front side markers are
    supposed to be on. They aren't.on with either the parking lights or
    headlights. If the turn signals are operated, both side markers
    flash. The side markers are supposed to be on, and flash alternately
    with the turn signal.

    When the emergency flashers are operated, both sidemarkers flash at
    the same time.

    I have a wiring diagram and it shows the side markers controlled by
    the ICU. The wiring diagram shows the ICU residing in the fuse panel.
    Can anyone shed some light on the ICU? Is it part of the fuse block?
    Is there something else besides the ICU that can cause these syptoms?

    RC, Dec 19, 2010
  2. RC

    jim beam Guest

    before you presume the icu is at fault, eliminate the other more likely
    causes. if two bulbs go on where only one should, it's usually a bad
    ground connection forcing the first bulb to ground through the second,
    hence they both light. same for when one bulb goes out when another
    goes on.

    you should spend some time making sure all the grounding points are
    clean, tight and electrically conductive. if still not working, then
    you should also spend time with a meter checking the lighting circuits.
    finally, if you're not getting signal, then come back to the icu, or
    more specifically, the turn signal relay which is mounted next to it.
    very unlikely it's at fault - it usually either works or it doesn't - it
    doesn't discriminate about what it decides will light and when.
    jim beam, Dec 19, 2010
  3. RC

    RC Guest

    You're right, I should check the grounds first. Anytime in the past
    I've seen bulbs light up that shouldn't, it has been a ground issue.

    When I printed out the wiring diagrams I was expecting to find a
    common circuit that would lead me to the problem ground. The side
    markers share grounds with the front turn signals, which are working
    properly (G201, G301). I'll make sure those grounds are working. The
    ICU and turn signal relay also share a ground G401. I'll check that

    I have this sneaking suspicion it isn't the grounds though.

    RC, Dec 19, 2010
  4. RC

    jim beam Guest

    which are much higher wattage. you can ground a low wattage side marker
    bulb through a high wattage turn signal bulb and be none the wiser.

    i'll bet you a donut.
    jim beam, Dec 19, 2010
  5. RC

    RC Guest

    You're on. In fact, show up for this and I'll make it a dozen. ;-)


    I should have figured it out by then. I've got a year before it has to
    pass inspection again. I can't believe they let it go through this

    RC, Dec 20, 2010
  6. RC

    Tegger Guest

    Headlights OFF, the side markers are supposed to flash together with the
    signals. Headlights ON, they should flash alternately with the signals.

    That's a different circuit; it does not pass through the ICU.

    It's behind the fuse block, on the left-hand side.

    My reading of the EWD says that the Side Marker Relay Circuit is bad. I'm
    not sure this is a plug-in relay like the Turn Signal one. You may need to
    get a replacement ICU from a wreck.
    Tegger, Dec 20, 2010
  7. RC

    Tegger Guest

    If the grounds are indeed at fault, you're looking inside the taillamp
    assemblies for the cause. Any corrosion in the sockets?

    I don't think it's the grounds either. The lights are dead unless power is
    fed to them.
    Tegger, Dec 20, 2010
  8. RC

    Clete Guest

    I had a vehicle awhile ago that had most of the same symtoms and then
    some. It was the turn signal switch in the steering column that caused
    everything. Swapped it out and everything was fixed.
    Clete, Dec 20, 2010
  9. RC

    RC Guest

    In this case the signal switch is way upstream from the various splits
    and convergences of the flasher circuit. It's a simple left-off-right
    switch. There's no way it could cause these symptoms.

    RC, Dec 20, 2010
  10. RC

    RC Guest

    It may take a few days (or weeks) to track down the root cause. This
    vehicle spends a lot of time away from the driveway where I can't get
    to it. I'll post when I know more.

    Meanwhile it's 35F, breezy, and I've got leaking seals to contend with
    on another vehicle. Time to put on the long undies. :)

    RC, Dec 20, 2010
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