LED Conversion - Lights Stopped Working

[westonjezek]

Good call. I’m thinking about resistors also. Only problem is the common ground In the headlamp harness.

[Rob Frankham]

Good call. I’m thinking about resistors also. Only problem is the common ground In the headlamp harness.

That isn't a problem... simply take a resistor from each of the indicator feed circuits (i.e. right and left - blue/red and blue/black wires in the original wiring) to ground anywhere in the harness. It doesn't matter where... you could do it in the rear lamp cluster if you wanted to. The important thing is to replicate the resistance value of a 21w filament bulb. You need to calculate the value of the resistor you require using Ohms law.

Take the current used by the one indicator in the headlamp unit (measured by a milliameter - for the sake of the example lets say 0.25 amps) and subtract it from the current drawn by a 21W bulb (say 1.75 amps). 1.75-0.25=1.5 amps.

From this, each of your resistors needs to draw 1.5 amps at 12 volts.

Ohms Law states that resistance equals volts divided by amps. 12/1.5=8 Ohms

It also states that Watts is equal to volts multiplied by amps. 12*1.5=18 Watts

So, to use a LED that draws 0.25 amps, you need to put a 8 Ohm resistor capable of dissipating 18 Watts in patallel with the LED 'bulb'. If it draws more or less than that, use the same equations but just substitute your measured current for the 0.25 amps above.

Note... The 18 Watts will emerge as heat... that sucker is going to get hot so don't put it up against anything plastic.

Of course, you do lose one of the main advantages of a LED bulb, that it uses less power, but you can use a standard flasher relay.

Rob

[westonjezek]

Thanks for the detailed explanation, Rob!

[hal]

Good and rightful explanation Rob, agree that you loose the advantage of LED, but how long do you use your flashers for anyway? I think if the goal is to save electrical power, the headlight is where to look.

Cheers!

[Seth]

Right.
If you are looking to save power, putting resistors in the directionals that burn off electricity as heat seems to not gain anything.
The headlight and tail light are better candidates.
If you really want LED directionals, you’re better off looking at replacing the flasher with one made for low draw bulbs similar to this:

http://www.motoelekt.com/ledKit.jpg
http://www.motoelekt.com/lighting.htm

[Rob Frankham]

Right.
If you are looking to save power, putting resistors in the directionals that burn off electricity as heat seems to not gain anything.
The headlight and tail light are better candidates.
If you really want LED directionals, you’re better off looking at replacing the flasher with one made for low draw bulbs similar to this:

http://www.motoelekt.com/ledKit.jpg
http://www.motoelekt.com/lighting.htm

There are two problems with that approach in this case.
1) We don't know what the current consumption of the LED indicator in this case. The Motoelekt relay is designed to work with specific LED bulbs so we don't know whether it will work in this case.
2) The Motoelekt unit is designed to work with LED bulbs all round. You can't use it with LED at the front and filament bulbs at the rear.

Done properly, the 'resistor' route will means you can use just about any combination of bulbs and LEDs provided you pick the apporopriate resistance value resistor.

Rob