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Why Don't Rear 3 Ohm Load Resistors Work?

Messages
53
Likes
11
Location
St. Louis
#1
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.
 


brbauer2

Active member
Messages
766
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530
Location
St. Charles
#2
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
 


brbauer2

Active member
Messages
766
Likes
530
Location
St. Charles
#4
I did, but not until after the directionals were sorted.

Sent from my ONEPLUS A3000 using Tapatalk
 


OP
A
Messages
53
Likes
11
Location
St. Louis
Thread Starter #5
Did you have to put a load resistor on the mirror bulb as well?
 


DangerMouse

Senior Member
Messages
812
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334
Location
Rockford
#6
I have led mirror bulbs with no resistors and they work just fine. I do not have led turn signal bulbs though.
 


OP
A
Messages
53
Likes
11
Location
St. Louis
Thread Starter #8
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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