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DT 175 MX 12volt conversion

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Part 6 Avoiding the Big bang
How to connect it all up

Changing over to 12volt is hassle free, There is no need to cut any wires of the original loom, so if you don't like it or you're not happy reverting back to 6volt is easy.

Don’t be tempted to install the battery first then turn the lights on! its not a good idea as all your 6volt bulbs won't last very long!
Changing all the bulbs is clearly straight forward and obviously should be done first!
The indicator flasher unit needs to be replaced along with the bulbs,
(I did try seeing what would happen with the 6v one still attached but it flashed far to slowly,) The flasher needs to have the two pins at right angles, and rated at 12v 18w~23w.
A suitable one is a universal type WRE04 and comes with the rubber mount.

The combined regulator/ rectifier used is from a HONDA C90 cub G model
4 pin version suitable for a late 80’s. bike. SH570-12 / 6.7 011
I mounted mine next to the indicator flasher so the wires from the original rectifier attached directly. The C90 regulator has a separate earth tag so does not need to have the body in direct contact with the frame, so for the moment I just secured it with a couple of cable ties.


Once you’ve secured the reg/rec then unplug the old 6v rectifier and plug them into the positions in the diagram. The C90 reg has all 4 Lucar male spade terminals. My old 6v rectifier has a male and a female connector, If you want to keep the option of returning the bike back to 6v, don’t cut the male spade off just make up a short female to female connector to connect the wire from the rectifier to the new one.

Disconnect the old 6v regulator and run a new length of wire from that connector in the loom to the New reg/rect as shown.
Finally connect another length of wire as an earth lead and find a suitable Earthing point.
As I fitted the new reg/rec next to the indicator flasher, the lead can connect to the common earth under the HT coil.

The Main headlight will now run off the generator. The rest of the lights and horn will, as before, run off the battery. Only this time it’s 12volt.

That's it! Simple!

If it doesn't work out for you, you can swap it all back to 6volt. No cut wires.

I'll report back on the success of project "Firefly" soon.
So far everything works perfectly. Not had to externally recharge the battery.

Hope you enjoyed the posting its taken quite a few hours to write it all up.

Yamaha Haynes workshop manual 210 ISBN185010 3003
Mastering Electronics john Watson ISBN 0333 408233
Hillier's Fundamentals of Automotive Electronics V Hillier ISBN 0748726950
Automobile Electrical and Electronic Systems ISBN: 0750662190
Yamaha club forum, Honda C90 website,

Remember this is a 12v conversion for a 1978/79 DT 175 MX UK
2K4 and 2X2 models. I’ve also checked the Haynes wiring Diagrams for UK 4J4 which is compatible.
Yours may be different and may require additional research before converting

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...BuMp... Happy to report that the 12v system is still fully functional after 5+ years 

Yeah, my headlight is about the same, under 2k rpms and it is not very bright. I am using a 60/55w bulb as well. Are you running turn signals? If so, how are they working for you?

Thanks for the fast reply. I took your advice and introduced myself in the new members forum.  I don't remember seeing something like that resistor on my bike but I'll check for it next time. 

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............Hope you enjoyed the posting its taken quite a few hours to write it all up.


Great work NEO very impressive right from research through to quality practical work, Not all that often people contribute so much :thumb:

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:prince: NE0,

I just finished with the conversion! What a great success! Driving at night now is a pleasure and I can venture further after work without the fear of darkness catching up with me. Thank you so very much for the relevant info and I am sure that every DT owner out there would benefit from such a conversion as it is extremely easy and affordable to fit.

Keep up the good posts! :thumb:

P.S. Can this post be "Pinned" in the Yamaha Workshop Forum please?

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  • 2 weeks later...

Just to update you on the success of Project firefly!

It's been a month now since I did the conversion.

No bulbs blown and there has been no need to externally charge the battery.

I ride with the lights ON all the time I've done a total of 190miles, so theres been a constant draw from the battery, including using the indicators all the time (No green laning).

The timing and CDI was untouched.

Everything works fine......... a worthwhile upgrade to 12volts. :D

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  • 3 weeks later...

but what is the point of keeping to a 35w headlamp
(posted earlier)

More news....

Still not had to externally recharge the battery which means its still on the original charge.

This proves the battery is being charged quite happily on the charging circuit.

remember I ride with all the lights on day and night!

Anyway, just to let you know I've sourced a 12v 45w/45w bulb same design as the 6v one with the 3 holes in the flange

from good ol' ebay. Look for APF Bulb 12v 45/45w

its a lot brighter than the 35w, and is at maximum brightness above 3500revs. Not saying it lights up the whole street but an improvement all the same.

I've taken off the fiamm air horn as I think the 6volt horn (i never upgraded that part) sounds much louder with 12v supply...don't worry it can cope with it.


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  • 3 weeks later...

What if I wanted to do this on an older one? I have a 75 DT100, didn't do any research prior as when I got it someone put a 12 volt battery and bulbs in it, replaced all of the bulbs and battery. But as far as I know now it's still a 6 volt system, and it's non cdi, correct?

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someone put a 12 volt battery and bulbs in it, replaced all of the bulbs and battery. But as far as I know now it's still a 6 volt system

Obviously this thread is about converting a DT175MX, however, the principle will be the same for other 6volt machines with similar layout.

The ignition side though, whether CDI or points, is a separate circuit and should be unaffected by any conversion.

Remember, this conversion is about changing ALL the bulbs from 6v to 12v & Replacing the 6v flasher for a 12v one.( no need to change the horn)

The major parts are the 12 volt battery and replacement of the two 6v parts: the rectifier and regulator and exchanging it for an all in one regulator/rectifier.

However, I've had a look at the circuit diagram of the DT100B in my haynes manual and i see there is no regulator and the headlight is run directly off the generator whilst it charges the battery via the rectifier.

Therefore its likely that the LIGHTING coil appears to be of low output and only capable of doing these two tasks. it would need a regulator if it was any more powerful.

As regards to..

as far as I know now it's still a 6 volt system

I think you mean the HEADLIGHT is still a 6v bulb but the rest of the bulbs are 12v and THEY all run off the 12v battery (which is charged by the LIGHTING coil)

If someones gone to the trouble of changing all the bulbs and battery to 12v, In essence YOU have a 12v bike...but without a regulator I doubt you could fit a 12v headlight bulb.

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As I said at the very beginning........

Disclaimer. This is what I did to my own bike, I can't be held responsible if you fry your electrics or your own bike if you try repeating it.
It's up to you to satisfy yourself that the advice, solution, method is right for your bike
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the headlight is a 12volt bulb, i don't know what size originally came on them but it has a 7 inch bucket on it with a sylvania 12 volt sealed beam bulb.

but no regulators were changed, I'll have to take a look at it, perhaps I can rewire it like the newer bikes with a rectifier/regulator like you did. I'll give it a shot.

oh I do have to point out that bulbs kept popping at hire rpms so I hooked up the headlight to the battery. It still gets brighter at higher rpms so I know it is charging the battery, so... I'll i'd have to do is have a 12v rectifier for the charging of the battery and done, would that sound correct? and I should be getting one at the end of the week, off of a 85 cb650. getting that for parts for the 71 cb750.

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By the way I never introduced myself. names Joel from the hot hot state of Arizona, 22, been working on cars most of my life, always liked the looks of 70s bikes and so I picked up this dt100, and started learning what I could.

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  • 5 months later...

Final Update ....9 months on.

I'm pleased to say that I've still not had to externally charge the 12v battery at all these past 9 months.

This shows the generator is man enough and can keep the battery charged up as well as keeping the headlight on.

Indicators, stoplight and horn of course have worked faultlessly off the battery

I've only had to replace 1 speedometer bulb. ..but that was easy it was 12v..........from stock!

Thanks 1976DT400 for the news on your success, If anyone else has made use of this posting please add which bike has benefited from your 12v conversion.......


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  • 3 weeks later...
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Great thread!!! It's even greater since I own this exact bike (US version). I've gone through a couple headlights and taillights now (including two 6V LED taillights that essentially cannot burn out). So I'm pretty sure my voltage regulator is bad. Since I already need a voltage regulator, headlight and taillight (battery too, it's shot)..... I might as well switch to 12V. I have some notes, questions and comments.

First off, I already changed all my gauge lights to 6V LED lights that I got from superbrightleds dot com. I was able to get each LED bulb in the exact color of the various lenses (amber, blue, red, green and white for gauges) and that made a huge difference. Gauges are much brighter and don't flicker, even without a battery. That is especially good for the neutral light, because the instant you hit neutral it comes on bright and clear. I would strongly recommend updating the gauges to LED bulbs even if keeping the bike 6V. I found the EXACT replacements (exact same base, etc) in LED form, so it was a no-brainer to install them. I think they were around $2 a piece.

Now for a question. You have a UK version of the 1978 DT 175, which apparently allows you to control whether the headlight is one or not via the ignition switch (my 1972 Suzuki TC-125 is that way as well). However, the USA version only has a 2-position ignition switch - the headlight is always on when the bike is running (they must have added that requirement in the USA between 1972 and 1978 then). Further, you say that according to the wiring diagram, your headlight is connected to the battery. Does that mean if you turn the switch on and the bike isn't running, your headlight comes on? The USA model doesn't work that way at all, which may mean your conversion process is different for (or might not apply to) the USA version.

Another thing I don't understand (and not having a wiring diagram is probably to blame) is there is only a single regulator, yet there are 2 (not counting ignition) lighting / charging coils. If there is only one regulator then only one of those circuits is regulated (unless the two are simply tied together at some point, which may be the case).

So I hooked my multimeter up to the headlight socket, and it is clearly AC (0 reading on DC), which means the headlight circuit does not run through the rectifier either. I measured 20VAC at 6k RPMs, which doesn't match either of your charts. Obviously the voltage is going far, far above 6V, which can mean only one of two things. The regulator is supposed to be part of the headlight circuit but it's bad, or the regular is not meant to be part of the circuit in the first place.

Thus I was curious, since the headlight power is not clamped to 6V on my bike, how well a 12V 35W headlight would do right off the bat. The local auto parts store had exactly what I was looking for - exact same size headlight as the 6V, but in 12V 35W. I hooked it up and fired up the bike, and it is noticeably orange tinted, and certainly not as bright as the 6V (my 6V bulb is 30W and the high beam still works so I can compare). So if I'm already getting unregulated power to my headlight, then that's as good as it can get - regulating the system to 12V will could only make the problem worse, not better. So that makes me wonder if your 12V battery is simply averaging things out - it charges when you're at higher RPMs, and is draining at lower RPMs, keeping the headlight bright across the spectrum. If that's the case then, again, that won't work on a USA bike because the battery is not part of the headlight circuit.

Anyway, I'd really like to do the 12V conversion, as I believe my regulator is bad anyway, but I'd like to know for certain that a 12V 35W headlight will be as bright as the 6V light.


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Now that it's finally dark out I swapped in the 12V 35W headlight to see how well it works, and it's a no-go like this. Way too dim and orangish looking. As I mentioned above, I get 20V AC at 6000 RPMs at the headlight socket, and with the 12V bulb it's still too dim. I put the 6V headlight back in and it's nice and bright. I've never had a problem with the 6V headlight not being bright enough, but they are expensive and cannot be found locally.

So I'm still not quite sure what the differences are between the UK and USA version of the bikes, but there are obviously wiring differences with the headlights as you can turn yours off and on, and as you said, based on the switch position different windings of the coil are being connected to the voltage regulator and headlight.

When you turn on the headlight, are the white and yellow coil wires BOTH flowing to the headlight and battery? If so I bet that's my problem, since the headlight stays on all the time on my model and is not connected to the battery, it's a completely separate circuit running off of a single coil only. If the two coils are combined and so there's a single unified electrical system (not counting ignition) then I bet there is enough amperage at the higher voltage to operate the 12V headlight.

That makes a lot of sense, because the one coil just charges the battery and runs the indicator lights, turn signals, tail light and horn. Most of that isn't even drawing power 99% of the time, so there would be a lot of excess current available to help with the headlight.

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Hi oisact,

Wecome to the club, normally we ask that you post in the new members section before we give out a detailed reply. its forum etiquette. You can still do that and you'd be very welcome.

On the other hand Glad to see this now old thread is still helping people.

I've read through your two posts and i think you've answered your own questions but nonetheless I'll still reply........

Well done for moving over to LEDs, it's not something I did. I assume they have bulb holder bases? or do they need to be modified?

you say that according to the wiring diagram, your headlight is connected to the battery.

I never stated the headlight was.

The conversion is the 12v battery runs the tail-light, indicators,brake lamp, dashbulbs and horn.

The charging coil.......charges the battery.

The lighting coil runs the headlight ONLY.

In Part6 it states

The Main headlight will now run off the generator. The rest of the lights and horn will, as before, run off the battery. Only this time it’s 12volt.

Re reading my own posts you might be referring to the earlier post Part3b the regulator where I mentioned

The tail light and pilot/headlight are both on drawing current from the battery

The bulb I'm referring to is the Pilot in the headlight! or the parking bulb in USA???

I'll amend the sentence.

The British Ignition switch, allows the bike to run (position1) charging the battery, with the headlight OFF.

In postion 2, the Battery continues to be charged but ALL the lights mentioned above are ON from the battery and the headlight bulb is ON powered by the lighting coil.

The regulator is SWITCHED in the British ignition switch.

In position 1 on the ignition switch, (engine run on lights off) the white wire from the generator goes to the switch and is connected to the yellow/white wire to the regulator. In essence soaking up the excess volts which would fry the battery.

In position 2 (engine run, lights on) the yellow wire from the generator is switched to the yellow/white wire to the regulator.

In postion 2 the white wire is disconnected

Yes you're right a circuit diagram IS and would be very useful for you.

The 12v 35W headlight is dim at low engine revs in this conversion. At tick over the light glows orange.

But you don't ride along the highway on tick over!

Once moving along the highway, revs above 3000 will make the bulb brighter.

The coil outputs are correct for my bike, others in other countries have been just as succesful, your coil MAY not be the same.

BUT if you're getting 20v at 6000revs then you are getting more than I do on the lighting coil. so i see no reason why it would not work.

On a later update I also mentioned I upgraded the headlight bulb to 12v 45W and the bulb is a lot brighter than the 35w one BUT it still glows orange at low revs but is noticably brighter than the 35w one.

This I'm afraid is a problem......a 12v bulb is an INCANDESCENT bulb and has a filament which glows and gets brighter depending on the power going through it.

Plus all my results are based on the fact I'm using the C90 regulator, which one are you using to get your results? you don't mention it. If you're testing with original 6v one then you won't get the same results.

i.e what voltage you are producing will be clipped down to 6v regardless of how much the bulb wants!

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With regards to the update of the DT175.Many moons ago I was the proud owner of one and "Motorcycle Mechanics", (a blast from the past)did cover this particular subject.I think I still have the magazine tucked away in the garage if you were interested,I can check and email you/or send to the forum.Let me know,Mick.

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