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I have a headlight module made by Signal Dynamics that is suppose to work with the 2014 Honda Grom. It is designed to work with the DC negative ground system which the bike clearly shows to be in the service manual (big thick book that includes everything you would want to know about the bike) The wiring schematic shows the three wires, white (low dim), blue (hi beam) and green (ground) and this can be confirmed with a voltmeter. The part that's strange is that in order to read the voltage from the white and blue wires is by using the negative lead from the voltmeter on those wires and positive lead from the voltmeter to the green ground. It's as if the polarity is backwards which a light bulb won't really care about but a module will. The module won't work and I think it's due to the polarity being backwards. Does anyone have any insight into something I might be missing or maybe someone messed up when wiring the bike? The module is listed for working with the Grom but I am really puzzled about the reverse polarity of the ground and positives. (I am using the harness kit and two different people have triple checked the connections. All is good)

Thanks for any help!
 

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voltage for the headlight is ac. you'll need to convert to dc or alter the harness to source power from the battery instead of the r/r.
 

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TripageLED sells a Grom specific AC/DC conversion module (with a delay ON feature). IT's plug n play, good for HID and LEDs. I will order one for my headlight retrofit.
 

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Discussion Starter · #4 ·
Thanks for the reply's guys. I have a couple of quick questions. I had read somewhere (can't find it now) that the 2014 model was changed over to DC and earlier models that I think were sold outside the U.S. were AC.
2nd question I guess is really for the company that sold the modulator as they state on their site that it will work for the 2014 model. Waiting for a reply as to why they believe it will work with the 2014 Grom.
3rd question. I'm puzzled as to how I can get a steady voltage reading on a DC meter if the current is AC. Is this even possible if it's AC. The only issue was that the positive and negative seemed to be reversed which a light bulb wouldn't care about.

Thanks for any insight.
 

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The headlight power is AC from the factory, we have checked this. Some of the bike is DC and some is AC. When you do the conversion you will make the headlights DC powered and you will should be fine using the modulator then.
 

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Did you ever figure this out? I have a 15 with a RR from a 17 and have issues with the polarity as well- exactly as you had described. Will still light my halogen but will not light my H4 LED. Do I need to switch pins on the plug in order for this to work with the new 2017 DC RR? Its freezing cold in the garage and I'd really appreciate some guidance!
 

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The headlight power is AC from the factory, we have checked this. Some of the bike is DC and some is AC. When you do the conversion you will make the headlights DC powered and you will should be fine using the modulator then.
Did you ever figure this out? I have a 15 with a RR from a 17 and have issues with the polarity as well- exactly as you had described. Will still light my halogen but will not light my H4 LED. Do I need to switch pins on the plug in order for this to work with the new 2017 DC RR? Its freezing cold in the garage and I'd really appreciate some guidance!
 

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Lol this thread. The only AC on the Grom is between the stator and rectifier after that it's all DC. Just two types. People (tripage surprisingly) don't understand how this all works.


The headlight circuit is negative with repsect to ground as the OP stated. If you want your LEDs to work you need to wire them reverse from standard polarity. That's all there is to it.
 

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actually, its ac. It legs off one of the ac windings prior to the rectifier, to the headlight circuit.
 

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actually, it's not AC

it's half wave negative DC, i've been through this before.


this is why the OP was able to measure negative DC volts when he checked, this is also why the LED doesn't work at all when connected forward bias, and works when connected reverse bias. if it were AC in the circuit, the LED would work regardless of how it was connected.


if it were AC the meter would read zero when set to DC volts.


the OP is correct in his thinking, anyone saying it's AC is not.



this is what comes off the rectifier for the lighting circuit





if you don't buy it, go put a scope on it.
 

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The headlight circuit is negative with repsect to ground as the OP stated. If you want your LEDs to work you need to wire them reverse from standard polarity. That's all there is to it.
Thanks for the quick response! It seems like the OG grom is wired for a negative switch headlight like old toyotas (constant power, switching earth between hi and low). My hope was to be able to use the factory H4 harness with my H4 LED headlight, but the only way I can think to do that is to run a couple of relays and switch the polarity that way. The only issue here is there is such limited space between the fork tubes. I've switched the 2016 RR to a 2017 OEM unit and the multimeter is reading ~18v at the oem h4 connector, wired just as the OP had described. The headlight I have is a quality JW speaker unit and I'd like to stick with it to improve night riding with solid light output, so I don't want to dig into the headlight itself to mess with the wiring. I'm open to any and all thoughts at this point! Thanks!


IMG_3957.jpg
 

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the grom isn't switching the negative, it's switching the positive (the ground actually is positive in the negative DC circuit)


you're getting 18v because you're adding the DC negative to the DC positive.


you need to use one or the other DC supplies, not both.


i'm not 100% on the SF rectifier but so far as i can tell it's doing something similar to the OG grom. there is likely to be a half wave negative DC on that bike as well.


what tripage does to simplify everything is eliminate the half wave circuit entirely by de-pinning it from the rectifier harness and applying +12v to it.
 

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my 2015 had the yellow wire at the rectifier. the wire from the stator is white. the tripage "dc jumper" involved de-pinning the yellow from the rectifier connector and hooking it up to +12v


it's 100% half wave DC





here's the problem. if you measure yellow to green on AC volts, you will get a reading. the AC volts will measure the DC ripple. just because your meter reports an AC voltage does not mean that you are measuring a sinusoidal waveform.

had you set your meter to DC volts, you'd find that it is actually DC. if it were AC your meter would read ZERO when set to DC, but of course it won't because it's not sinusoidal alternating current. it's negative half wave.


the 2014 and 2015 grom share the same rectifier, and the same wiring harness. so i regret to inform you that you have likely just done an improper "test" to come to your conclusions.



if anyone wants to put this to bed it's very easy to do. get someone with a multimeter and a 2014 or 2015 grom.


with the bike running (the negative dc circuit is only energized from the stator, it's isolated from the battery, this was intentional by honda to run the lights ONLY from the stator to reduce battery load) measure DC volts from yellow to green (red lead on green, black lead on yellow), record the DC volts.

now measure from battery positive to battery negative (red lead on batt positive, black on batt negative), record the DC volts

now measure from battery positive to yellow (red lead on batt positive, black lead on yellow)


you'll find that the resulting voltage is the SUM of the previous two readings, because the yellow wire is NEGATIVE DC.



if the yellow wire were AC, none of these tests would produce these results.
 

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After reading what you wrote, I went back and read some, remembered some mention of this from wayyy back in school. And you're right. when the ac is 'open' (not connected to a rectifier) its ac, when its wired into the rectifier, the properties change even on the feed side. Thanks for that. This explained it well, to me, and it may others.

https://www.electronics-tutorials.ws/diode/diode_5.html
 

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Not quite. You're thinking of input harmonics in the current waveform. The voltage still looks identical. The current waveform is modified as current can only flow when the voltage supplied exceeds the output dc voltage.

this is for a single phase full wave rectifier, but it applies to half wave as well. you can see the current doesn't follow the voltage as it can only flow when the voltage exceeds the DC voltage on the rectifier output. of course this current waveform can only exist when there is a stable DC on the other side of the rectifier, supported either by batteries or capacitors (a dc ripple smoothing circuit as shown below). in the case of the grom the current waveform will follow the voltage as there is no sustained voltage on the load side of the rectifier. the output of a single phase halfwave rectifier is simply 1/2 Vpeak * .637. but of course in the case of the grom, the half wave circuit is regulated to achieve 12v on the half wave. at idle this is actually much lower (around 7 volts) because the stator output is not high enough to achieve this voltage.





The yellow wire on the Grom is actually a rectifier output. There is no AC supplied to any Grom components except the rectifier. You seem to believe that the lighting circuits are fed directly from the stator which is not the case. A simple look at the wiring diagram will validate this.

if the headlight were wired directly to the AC from the stator, the applied voltage at the headlight would be high (>40VAC at moderate engine speeds) and would destroy the 12v 35 watt headlight bulb.

I'm a master electrician and have done a ton of bike wiring. I own all the test equipment and know all the theory. If my scope wasn't stolen last fall I'd post a pic of the rectifier output on the negative half wave circuit. I'd also need a functioning grom to do so lol...


Anyway, the info here should help the OP, and others who are confused by what they've been told.
 

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makes sense once i looked at it. I changed my rectifier out years ago to a gojin one so the headlight was getting full dc voltage, so i haven't thought about that in a long time, but i see whats going on now, thanks for the explanation, it is appreciated. Compared to most other bikes i've worked on, that's a weird setup.
 

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It definitely is. I believe the idea is to isolate the headlight from the battery. The headlight doesn't need to be on if the bike isn't running. This method allows it to power directly from the stator without any connection to the battery.

I did the same thing on my rc51 by triggering the headlight circuit via the oil pressure switch. I added a 5 second debounce so it won't come on when cranking.
 

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nice solution. I did soemthing similar with the grom with a delay circuit working a relay on mine when i originally had it wired to switched battery. (there's a post on here somewhere)

some of the really old bikes i have use what could be considered similar in thought, where there's an ac lighting and charging coil. one leg of the winding goes straight to the headlight. the other leg of the winding goes up to a single phase diode/rectifier and charges the battery. (6v usually). the battery then powered the tail, brake, and gauge light. There is also no stand alone regulator on these systems, it just regulates off the battery being charged off one leg of the system. This was pretty common on small bore bikes way back in the day. (also way before my time). The ignition was fired by a separate set of windings and was unrelated to the lighting. mid 60s to early 70s 100 cc and similar bikes. I think the assumption by many (myself included) was this was the same 'type' of setup, and its clearly not.
 
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