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I've just replaced the old tail lights on my 1993 Triton ute with LED ones. Now I have the issue where the indicators flash at such a high speed it may as well not be flashing at all. If I put the hazards on, they all flash, although at a faster speed than usual. I've come to the conclusion that the flasher circuit in the old truck must somehow use the resistance of all the lights to determine flash speed and as there is still some of the old bulbs at the front, when they're all on, it works... but when it's just one side, the resistance isn't enough.

Does anyone know if I am correct in thinking that and if so. How would I go about fixing it?

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Don't speed when indicating. ;-) –  GreaseMonkey Oct 12 '13 at 0:22

3 Answers 3

You're exactly correct in that thinking (I replaced the bulbs in my old Volvo once and it caused it to think they were out because the bulb was slightly different (these weren't even LED's; just a different filament or connector or something, I forget; no tolerance at all on that circuit).

I don't know if there's a standard way to fix this (other than use LED's that were designed for it), but it should be easy enough to measure the resistance across the original bulbs with a multimeter and then go out and get some resistors of the correct size. If you're not using LED's designed for this, or aren't putting a resistor between the LED's and your power source anyways you're probably just going to burn the LED's out rather quickly.

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Huh? There's no reason to think you should need resistors between the LEDs and the power source to avoid burning out the LEDs. The correct solution is either to replace the flasher unit with one made for use with LEDs or replace the LED unit with one that goes out of its way to waste power and heat up the flasher unit like a normal incandescent bulb would do. –  R.. Oct 12 '13 at 13:13
    
You may be right; my old car with similar problems put out way too much voltage for normal LED's, but that may have been yet another electrical problem. –  Sam Whited Oct 12 '13 at 17:03
    
The voltage in a car is always 12-15 volts. Any LED unit made for replacing automotive lights is going to expect this voltage, and I would guess they use a DC-DC converter to get a stable output voltage for the LEDs, though other approaches would also be possible. –  R.. Oct 12 '13 at 17:58
    
In that case, definitely another electrical fault in my old car. It put out ~24 volts. It was so regular though that I assumed it was how it was supposed to be. Either way, a resistor (of the appropriate resistance) will probably make it work just so long as it doesn't drop the voltage enough to turn the LED's off. –  Sam Whited Oct 12 '13 at 18:36
    
What kind of car was that, and are you sure? It's possible there are some really old, or non-mainstream-consumer vehicles which use 24v instead of 12v, but 24v output from the alternator would boil off a normal car battery in no time... –  R.. Oct 12 '13 at 21:13

I believe it can be looked at like this: When you have a turn signal bulb go out it causes the other to blink almost twice as fast. Since LED's use much less power, then it stands to reason that by switching to LED's you are not using the expected amount of amps to light the LED's hence, the system see's this as a burnt out bulb.

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There are LED bulbs with ballast, or you can purchase ballast separately to keep flasher units happy.

http://www.superbrightleds.com/carbulb_notes.php#turnsignal

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