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  1. #1
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    Why Don't Rear 3 Ohm Load Resistors Work?

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    Hi,

    I would like to switch my turn signals to LEDs but only use 1 3 ohm resistor in the rear to minimize clutter and unnecessary wire modifications. It worked on my previous car and figured it would work on the Fiesta. When I temporarily wired the resistor in to test with the new LED bulbs, I got a hyperflash and I cannot figure out why. When I put my incandescent bulb back in the front, led in the rear and wired in 6 ohms of resistance on the rear led, it worked fine.

    From digging on the internet:
    Osram 3157A incandescent bulb = 27 watts
    P = I * V, 27 watts = I * 12 volts, I = 2.25 amps; V = I * R, 12 volts = 2.25 amps * R, R = 5.33 ohms

    Osram WY5W incandescent bulb = 5 watts
    P = I * V, 5 watts = I * 12 volts, I = 0.42 amps; V = I * R, 12 volts = 0.42 amps * R, R = 28.8 ohms

    Assuming that the turn signal circuit for each side, 2 3157 incandescent and 1 WY5W incandescent, is a parallel design, total resistance is 2.44 ohms

    iBrightStar 3157 LED bulb = 4 watts
    P = I * V, 4 watts = I * 12 volts, I = 0.33 amps; V = I * R, 12 volts = 0.33 amps * R, R = 36 ohms

    Assuming that the turn signal circuit for each side, 1 3157 incadescent, 1 3157 LED, 1 WY5W incandescent and 1 6 ohm load resistor in the rear, is a parallel design, total resistance is 2.40 ohms

    Assuming that the turn signal circuit for each side, 2 3157 LEDs and 1 WY5W incandescent with 1 3 ohm load resistor in the rear, is a parallel design, total resistance is 2.36 ohm

    I would think being .08 ohms under OEM would be fine for the ECU to tolerate. The ECU tolerates the 2.40 ohms from the 1 LED circuit. Anyone have any insight as to why using 1 3 ohm load resistor in the rear wouldn't work? I can only think that the turn signals front and rear are on different circuits?


    Also:

    Auxito 194 LED bulb = 1.5 watts (from an amazon review)
    P = I * V, 1.5 = I * 12, I = 0.13 A; V = I * R, 12 = 0.13 * R, R = 96 ohm

    Assuming that the turn signal circuit for each side, 2 3157 LEDs and 1 WY5W LED with 1 3 ohm load resistor in the rear, is a parallel design, total resistance is 2.50 ohm. I haven't tried this yet.

  2. #2
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    Lots of numbers so I didn't read....

    ....but I had to run 6ohm resistors on each corner to get rid of hyperflash.

    Sent from my ONEPLUS A3000 using Tapatalk
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    Did you change out your mirror bulbs to LED?

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    Member 246 Rep Points brbauer2's Avatar
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    I did, but not until after the directionals were sorted.

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    Did you have to put a load resistor on the mirror bulb as well?

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    I have led mirror bulbs with no resistors and they work just fine. I do not have led turn signal bulbs though.

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    Member 246 Rep Points brbauer2's Avatar
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    Quote Originally Posted by ajw85 View Post
    Did you have to put a load resistor on the mirror bulb as well?
    Nope, just the bulbs.

    Sent from my ONEPLUS A3000 using Tapatalk
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    Guest-only advertisement. Register or Log In now!
    FWIW:

    I wanted to see if the lighting circuit had all lamps in parallel.

    I rigged up a harness and used a motorcycle battery to apply 12V to the rear turn signal lamp, it did not light the front turn signal or side marker turn signal. Looks like at a minimum the turn signals are on 2 separate circuits, thus requiring a 6 ohm resistor at each bulb. I don't know why Ford would run two separate circuits but whatever man. I'll try to apply 12V to the side mirror and see if the front lamp lights up.

    Also, there are no fuses for the turn signals. I studied the owners manual and tried fuse 10 and 11, which are really for the L and R headlights. I know it's just 2 amps going through the lighting circuit but that's 2 amps that could wreak havoc in an ECU.

 

 

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