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Fixing motorcycle LED turn signal - What size / Type LEDs?

Discussion in 'Troubleshooting and Repair' started by RedMongoose, Aug 30, 2015.

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  1. RedMongoose


    Mar 21, 2015
    Hi all,

    After only a month, my shiny new motorcycle turn signal went dead- much to my dismay. I found out that the trace came up from the board, and must have raked across some LEDs during vibration and blew some LEDs. Here is the board:
    I've soldered a JST connector so I can use an RC battery to test power. Only the LEDs between R1 and R3 come on. I've outlined everything in a crude drawing done on a tablet.

    A quick explanation: Orange is the traces, Grey is what I initially assumed to be the cathode (-), and the yellow with the line is the fat blade "anode" (+). This isn't the case. There's a few resistors in there, of course.

    I went through with a small 1.3v source with resistor and tested each LED, placing a green or red dot on the depending on which one fired:


    As you can see, everything past R4 on the bottom row is fried, so i think that was the path of destruction. The top one that leads down to R1 (361) was my own dumb fault. Plugging it in, only 4 LEDs come on (the one past the broken one on the top row, leading to R3 (301), and down to where it terminates into the ground.

    I desoldered an LED to see if there were any silkscreening behind it, and of course- yes. I've never seen an LED where the fat portion was the positive anode, running to a thin negative cathode. Curious what part I need to replace it with. It's around 6mm high, and 4mm in diameter, and amber in color for turn signals.[​IMG]

    Any idea what replacement LEDs I need here? Ever seen a "fat" positive LED? Am I barking up the right tree assuming that if i replace the busted LEDs, the whole unit should work again?

    Thanks for any help!
  2. Gryd3


    Jun 25, 2014
    Is it actually dead LEDs, or did something else break?
    You can test the LEDs while they are in circuit. Rig up a power supply and resistor, then poke and prod each LED and see what lights up.

    You have 1 resistor in series with a number of LEDs. 4 of these assemblies are then wired in parallel.

    If any component in one of those assemblies goes 'open circuit', then the entire section will fail to light. This could be one or more of the LEDs, and/or the resistor.

    I would double check the LED... they don't make them reversed... perhaps you accidentally flipped it?

    In order to find a suitable LED, simply take a look at the resistors being used.
    Assume the applied voltage is 14.4V. Subtract the 'forward' voltage of each LED connected in series to that resistor, then you can use the left over voltage to calculate current.
    Simply order a new LED with the same current capability and you'll be fine.
    More often then not, the forward voltage will be the same for the same color LED anyway.
  3. dorke


    Jun 20, 2015
    "A shiny new motorcycle"...don't you have any warranty?
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