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Brake light fuse blows intermittently

Discussion in 'Electronic Repair' started by amdx, May 12, 2013.

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  1. amdx

    amdx Guest

    My daughters 97 Avalon blows the brake light fuse about every other day.
    I thought I'd eliminate some wiring as a troubleshooting procedure.
    There is a connector block on the side next to the drivers foot.
    I found the brake light wire at the connector and in the trunk. I cut
    the wire at the connector and ran a new wire from the front to the
    trunk, I also cut the wire in the trunk, and wired the new wire to the
    brake lights. That leaves just a short old wire in the trunk going to
    both the left and the right brake light.
    The fuse opened again a couple days later.

    Is there anything internal to the brake lights that could short to
    cause the fuse to blow?

    There is at least one other circuit run by that fuse, there is a shift
    lever release (solenoid?) mechanism.
    You must push the brake pedal to move the shift lever.

    Any help?
    Thanks, Mikek
  2. Jamie

    Jamie Guest

    Very possible the wires have chafed around the solenoid, it does move a
    little when operated, it's possible the wire harness is moving with it.
    It's also possible the solenoid has come loose and moving excessively,
    which in turn will move the wires.

    And there is also the other possibility, if the solenoid is under the
    car, outside, you may have rot or varmint eaters.

    I would put a tail light bulb inseries with the fuse or use it as a fuse
    and start moving some wires around when the brake is applied, and move
    the shifter etc.. watch for sudden bright glow.. That'll save fuses.

  3. amdx

    amdx Guest

    Thanks Jamie,
    Good suggestions re: the solenoid and series bulb.
    I'll have to figure out the Shifter release, no clue how that operates.
    Everything is so tight, I don't like working under the dash.
    My wife's relative usually does my repair work, but he got in a little
    huf when he said he new how to fix it and I quizzed him on why "THAT"
    part would short to ground. I backed off and let him do what he does. He
    replaced the part and it still blows fuses, I didn't tell him
    his repair failed, because generally I'm happy to have him fix most of
    my auto problems.
    Anyway next time my daughter comes to town, I'll try your suggestions.
    Hmm, while I'm waiting I think I'll look into a resettable circuit breaker.

    PS, I do like the fact that when the brake light fuse blows, the shift
    lever won't move. It gives you notice! However my daughter was creative
    enough to find the mechanical release and drive anyway. I didn't think
    much about it when she told me, until I had a random thought, "I wonder
    if her brake lights work. That's when she started supporting the fuse
  4. Is the fuse the right size?
  5. Guest

    Googling finds wiring diagrams at WIRING DIAGRAMS/12053.pdf
    and WIRING DIAGRAMS/12078.pdf
    ; not sure if those are exactly the right ones for your car but maybe
    they help. There are more at WIRING DIAGRAMS/

    Both diagrams show that the stop fuse should be 10 A. There should
    also be a sticker inside the fuse box (or lid of the fuse box) with the
    correct fuse ratings.

    General advice: In my experience, the problem is more likely to be in a
    switch, lamp socket, or connector, rather than the wire itself. If it
    *is* in the wire, it's often near the end of the wire (where it is more
    exposed to moving parts, humans, etc) rather than buried in the middle
    of the harness somewhere.
    If the lamp socket is really mangled, it's possible for the "hot"
    contact to touch the "ground" contact. You can usually tell by taking
    out the lamp and looking down into the socket.

    It's even possible, but really rare, for something to be rattling around
    inside the bulb (like a piece of a filament support), but I mention it
    because it's usually really easy to check - just take out the lamp, hold
    it up to the light, turn it this way and that and maybe thump it with
    your fingers.

    You might check the lamps to make sure that the right part number is in
    there. The owner's manual and/or website of the friendly local auto
    parts store should help. It should be relatively difficult for somebody
    to install lamps of such high power that they blow the fuse, but it's
    pretty simple to check.

    A single rear brake lamp should draw about 2 to 2.5 A depending on the
    exact part number. The third brake lamp will be less than this,
    probably closer to 1 A. This gives a minimum of about 5 to 6 A; if
    somebody has installed a 5 A fuse for the brake lights, you have just
    found the problem. :)

    Some rear lamp clusters have a short harness (like 8 inches or 20 cm)
    that goes from all of the lamp sockets on the cluster to a single
    connector with maybe 6 pins; this is for ease of assembly and
    replacement. Often this harness is zip-tied or otherwise fastened to
    the back of the cluster; maybe the zip tie cut the wires and shorted
    the brake light wire to ground.

    Sometimes cargo in the trunk can cut the rear light wires, especially
    if the trunk lining is gone or out of place.

    If this car has or ever had a trailer hitch on it, check the trailer
    wiring, or the remains of it. Often this is not properly routed and
    sometimes get pinched against the body. Or, if it had a hitch that
    was later removed, sometimes people just let the wires dangle down
    behind the rear bumper. If it doesn't have a hitch now, get a good
    light and look under the back of the car, towards the bumper, for non-
    factory-looking holes, or cuts in the bumper cover, where a hitch
    might have gone. Also look at the rear lamp clusters from the trunk
    to see if there are any non-factory-looking splices.

    Check the third brake light as well; it's on the same circuit. Usually
    its wire runs on the bottom side of the rear parcel shelf, in the trunk,
    from the light over to one side of the car. The owner's manual should
    tell you how to get the lamp out of it, which will let you inspect the
    lamp and socket.

    You might look at the brake light switch, which is mounted to a bracket
    near the brake pedal arm; it's above your toes if you're stepping on
    the brake. Usually it's not too hard to at least get your eyeballs on
    it, even if working on it is more entertaining. Usually when these
    fail, the plastic switch body cracks and the brake lights just stop
    working. Sometimes the driver catches the wires to the brake light
    switch with the toe of his or her shoe and pulls them around, which
    might make them short out on the brake light switch bracket or other
    nearby metal.

    The wiring diagram linked above shows a "light failure fixture" which
    lives on the left rear lamp cluster. This monitors the current to the
    brake and tail lights and turns on a "lamp failed" indicator on the dash
    if anything isn't drawing enough current. This could conceivably be
    screwing up and shorting the brake light wire to ground. If it's not
    obviously mechanically beat up, I would assume it's OK on the first
    pass. If you start to suspect it, the test is probably to bypass it and
    see if it stops blowing fuses. The bypass would be to unplug it and
    splice the green-white (brake feed), green-red (regular brake lights),
    and green-orange (third brake light) wires together; and separately
    splice the green (tail feed) and light green (tail lights) wires
    together. The other three wires can be left open. If you bypass it,
    the "lamp failed" indicator on the dash won't work anymore.
    This is a solenoid that usually lives somewhere near the bottom of the
    shift lever. You may be able to hear it "thunk" if you put your ear by
    the shift lever while pushing on the brake pedal.

    The wiring diagram above shows that there is a shift lock ECU that has
    a wire from the brake light switch, as well as direct wires from two
    other fuses. I think the brake light signal is probably an input to
    the shift lock ECU, rather than a power source for it, but a short on
    the brake light signal wire would still blow the brake light fuse. If
    the car has a console shift, sometimes coins, soda, etc falls in around
    the shift lever; usually it just makes a mess but something metal might
    eventually wear through a wire.

    I hope this helps!

    Matt Roberds
  6. amdx

    amdx Guest

    Yes, it is.
  7. amdx

    amdx Guest

    Very helpful, The first diagram let me learn that I have bypassed the
    "light failure fixture" with my new wire. The "light failure fixture"
    had been replaced by the relative that said he knew how to repair the
    problem (he didn't).

    There are more at
    Yes, 10 amp fuse, and I warned my daughter to replace it only with
    another 10 amp fuse.

    My wire replacement seems to confirm your experience :)

    Nothing has been damaged externally, next check I'll open up the rear
    lights and check their condition.

    I'll include that in my inspection.
    I'll keep that in mind, I got no feedback that any lamps had been
    replaced, but one should not assume, especially with a problem that is

    That's not the situation.

    That's where I started, inspecting the wiring in the trunk, even opened
    the spiral wrap looking for damage. No damage found.
    No trailer hitch ever.

    Already disassembled the third brake light assembly, also ran a new wire.
    I'd rather work on the easy stuff! Oh just kidding, I have climbed
    under the dash and looked over the switch, didn't notice any problems.
    But I'm sure I'll be under there again.

    As I said before the "light Failure Fixture" has been replace and
    didn't solve the problem. I have also bypassed it.
    Ideally I would like to separate the two circuits as close to the
    fuse as possible. She can drive the car when the Shift Lever solenoid is
    not functioning. Then if the fuse still blows I have eliminated the
    the Shift Lever Solenoid circuit as the problem.
    It is a column shifter.

    Great information, thanks for the input.
  8. Jamie

    Jamie Guest

    my 2011 Jeep Cherokee had an addendum in the glove on how to release the
    shift lock, you need to move the rubber that goes around the shift and
    you are suppose to be able to reach the solenoid actuator so you can
    release it.

  9. Jamie

    Jamie Guest

    good question, could also be a slow blow type.

  10. Jamie

    Jamie Guest

    if you have a DC clamp meter, it is very possible the solenoid may
    have a few shorted turns and it exerting more current than it should.

  11. Guest

    Sites like the above tend to come and go, so it might be a good idea to
    grab the PDFs now and stash them. Those PDFs are from the Mitchell
    repair manuals; some large public libraries subscribe to them and keep
    them in the reference section. Sometimes the library has electronic
    copies that you can print, but print a test page or two before you go
    crazy; some of them are scanned from the paper manuals at relatively low

    The official Toyota wiring diagram is a slim book by itself. It doesn't
    have much theory of operation or troubleshooting info (those are in the
    thick volumes of the factory service manual) but it does give you a nice
    color wiring diagram. The dealership may be able to get you a new one
    for $30 or $40; sometimes you can snag one used for $20 or so.

    Mitchell adapts the wiring diagrams from the different manufacturers
    into their "house" format. This is probably easier to follow for an
    independent mechanic who works on lots of makes, but if you just have
    one car, sometimes it's easier to follow the factory wiring diagram.
    It looks like the split happens inside the "driver's side j/b"; that's
    where the switched wire from the brake light switch forks. Often these
    junction boxes live on the back side of a fuse panel or otherwise close
    to the fuses. Usually the junction box is kind of a flat plastic thing
    with connectors around the edge; the inside is sometimes zillions of
    flat stamped metal strips, arranged in layers, insulated by the plastic
    housing material. It might be hard to get at the circuit inside the
    junction box; doing it at the junction box connectors is probably

    Matt Roberds
  12. Sparky

    Sparky Guest

    My daughters 97 Avalon blows the brake light fuse about every other day.
    Modern cars have lots of things connected to the brake circuit: cruise
    control computer; engine management computer (strange, but true); "burned out
    bulb" sensor circuit; as well as the shift lock-out solenoid.

    You might unplug one section at a time from the harness for a week and see if
    it still blows. "Long term" troubleshooting...

    Good luck!
  13. amdx

    amdx Guest

    Yes very long term, my daughter comes to town once a month or less.
    Last report, she said she pulled a defective fuse and some little pieces
    of wire came with it. After replacement it has lasted 3 days, more time
    than any fuse in a month. This didn't really make sense to me, but I can
    hope that was a cause to the problem.
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