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Headlight Switching/Voltage Confusion (connecting DRL)


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Hey guys,

I'm a bit stomped by what I found Yamaha to be doing on their "Light Control Unit" with the LED headlights.

But let me set the scene for you first: I'm in Europe, we're allowed to have Daytime Running Lights as long as it is inversely coupled to the low-beam: low-beam off=DLR on, low-beam on=DLR off. (needs to be automatic)

On my Honda I had 2 wires leading to the low-beam, one for "on" (12V) and one for ground. Simple solution: make a toggle-switch in the 12V line, have it either power the DLR or the low-beam, and take ground from anywhere on the bike.

On my Tracer 900 GT I found things to be a bit more complicated. First there's a LCU (that Light Control Unit) that takes input from the switches on the handlebar and then there's an output-side connector with 4 wires going into the headlight (glued in, can't get that end out).

So I went ahead and measured what the LCU was outputting to the headlight. I didn't have the space in there to measure 2 pins against each other without shorting them so I connected ground to the engine-case (unpainted screw).

With low-beam on, high-beam off I got these measurements:

Pin 1: 12V (blue wire)

Pin 2: 24V (green)

Pin 3: 12V

Pin 4: 12V


Now I see how that would work for the headlight: it sees 12V between blue and green so the low-beam is on, and 0V between the other two so high-beam is off...

But that makes no sense at all for my setup: I would like to take one wire from the LCU (e.g. green) and connect my DLR-ground to any part of the chassis.

But what's with that LCU ground?

Can anybody explain to me why the LCU has a different ground-level than the rest of the bike?

Do I have to use the "12V-ground" from the LCU to operate my DLR?


Thanks anyone who can shed some light on this.



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1 hour ago, norcal616 said:

Why not use the front turn signals DRL wires?? 

Then I'd have no way to turn off the low-beam which I have to do anyway.

There's no question whether I have to use that green wire. Question is: can I use vehicle ground or do I have to use blue.

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Don't mess with the headlight wiring. Others have attempted and failed - do a search of the forum for details (I'm not going to attempt to recall all the specifics).  

Here's some info for you based on my understanding of the urMT-09 Tracer/urFJ-09 - I'm assuming the GT model operates similarly (which could be a dangerous assumption)

-You've already realized that the LED's are operating at 24v, which is "stepped-up" by the LCU.

-The control unit and LED bulb assemblies are expensive to replace.

-At least one other member of this forum wired his aux lights by splicing into the wiring AFTER the ECM and BEFORE the LCU (IIRC). This worked initially but after awhile overloaded the "driver" side of the ECM requiring an (expensive) ECM replacement.

-IIRC I don't believe that the handlebar switch is operating via any kind of CAN wiring (but you should verify), in which case you could theoretically tap into the switch wiring BEFORE the ECM to get signal V and Ground to operate the low side of a relay which could then turn on your lights. YMMV, I haven't tried it this way, but it would probably be the safest.  HOWEVER, if the ECM is "looking" for a clean signal from the headlight relay and you inadvertently introduce the "shocks" from a cheap relay slamming shut into the circuit, this may damage the other side of ECM. Yamaha uses lots of diodes in their circuits to keep things clean and protect components.


The safest way to wire in aux lights is to turn them on with a switch that you install, and wire them directly to the battery. To keep them from draining the battery, tap into another source of power and ground as Norcal suggested to trigger the low side of a relay, so that the lights can't stay on without key-On power.

YMMV and make sure understand what you are getting into, or it may cost you a lot of $ when something burns out. I'm sure this isn't the answer you were looking for, so maybe gets some "I've done this successfully" answers before proceeding.


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