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post #1 of 4 (permalink) Old 03-23-2011, 11:49 PM Thread Starter
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Turn Signal turns four way flasher?

Hey, I installed some new turn signals (British Customs) tonight, tested them as I installed each individually and all seemed to be working fine. I put everything back together and now if i switch the indicator left or right, both come on and flash. This does not make any sense to me. Any thoughts?
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post #2 of 4 (permalink) Old 03-24-2011, 02:57 AM
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There are only 4 reasons why all 4 flashers can come on at the same time:

If the "live" side of either the front or rear lamps is shorted or somehow connected to the "live" wire that feeds the other side.

If the warning lamp or its holder are shorted.

A defective or disconnected ground on either the front or rear lamp set.

An internal short in the flasher switch.

My money is on the defective grounding. This is how the flasher circuit is wired:

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post #3 of 4 (permalink) Old 03-24-2011, 03:08 AM
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It's not poor grounding - these are presumably LED turns I guess?
The reason all 4 come on is because of the OEM indicator lamp - this has much lower resistance relative to the now high resistance LEDs - so appears like a virtual 'short' path to the opposite side.

(The most simple test is to merely unplug your instrument connector plug or pull the lamp - if the turns then work normally, you have your confirmation)

The way the 'normal' idiot light works is:
The main current flows through the incandescent turn signals (Whichever side is selected by switch) to ground;
it also flows to a parallel circuit comprising (the indicator lamp (incandescent) in series with the opposite-side turn signals (incandescent)) to ground

The indicator lamp 'lights' because its resistance is much higher relatively (smaller wattage bulb) than the main turn signal bulb;
so proportionately more voltage is dropped across that indicator lamp than the 'opposite' main signal lamp (actually front & rear lamps in parallel) in series with it ;
so the indicator lights and the main lamp doesn't.
Now - when you install LEDs, they have very high resistance compared to the indicator lamp - so now the applied voltage is dropped over the LED turns as opposed to the indicator
- so both sides flash!
i.e. the indicator is now flashing on the opposite side because the higher voltage component in the opposite path is dropped across the LEDs, rather than the indicator bulb.
Hence the requirement to isolate that potential parallel path with diodes.

This is simplified diagram of how original indicator lamp functioned:

The solution is to do the diode mod for the indicator lamp

(note that on the Twins the negative for the high beam lamp is actually Brown I believe)

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post #4 of 4 (permalink) Old 03-26-2011, 12:12 AM Thread Starter
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I think you were right Forchetto. Pulled everything apart, took a little more care insulating the wires. Seems to be working fine now.

Thanks to you both for the insight.
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