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1974 dt175 head/tail rewire


lotty
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My headlight and tail light bulbs blow within a minute. I don't know much, but do know some. It wasn't the earth. These two lights get their electricity straight from the magneto, not via the battery. So I reckon it is basically that the voltage regulator isn't working right, and that is what is blowing them. I think it is under the tank.

I'm sorting a little problem with my magneto at the moment, so i know the lighting coils are good and so is the condenser, so i would like to just continue a bit further and get the headlight and tail light working. I suppose the best thing would be to replace the parts like the regulator and whatever. I wouldn't be able to test them, and i can't keep blowing bulbs to see. 

So, to do this job, what should I order?  I'll try myself, for certain, but if anyone can help with some advice i would welcome that very much. Doug

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Hi Doug,

Yes it certainly points to the regulator not working. If you disconnect the yellow wire from the magneto and put a Meter on the end of it you'll get results like this...

With the engine running and the generator disconnected from the rest of the bike, the output is as follows: Read the scale of the meter for AC volts.
Yellow wire (lighting circuit)
2000 revs = 6v
4000 revs = 11.5v
5000 revs = 14v
6000 revs= 16.5v

(This is my DT175MX lighting coil your result may be different)

but the principle is the same.....now you can see why your 6v bulbs blow!!!

the purpose of the regulator is to 'clip' back the ever rising output to a steady 6volts to run the lights.  The regulator will get hot as it 'soaks up' the excess voltage, which is why it has the cooling fins on it. As you can see at 6000 revs it's soaking up over 10 volts! just to keep the output at the steady 6volts.

 

 

You'll need a 6 volt regulator, presumably your bike is a DT175 1G1 model?

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You say you wouldn't be able to test the Regulator, but the regulator might actually be ok!, ...it relies on the metal body being in contact with the frame to complete the circuit, so in the first instance clean up the 'Earth connection' and check that the wire to the regulator is intact from the Ignition/light switch.

We already know the wire from the magneto to the light switch is intact. (because the excess volts is blowing your bulbs!) but within the ignition/lightswitch, there are internal contacts which diverts the excess voltage via the yellow/white wire to the regulator.

Disconnect the regulator and test the yellow/white wire which goes to it. You should get tha same rising voltage you see from the magneto, BEFORE it gets clipped by the regulator. This test would prove that the voltage is at least getting to the regulator!

If it is getting as far as the regulator, clean up the earth connection, by removing the body and taking the frame back to bright metal to make sure of a good earth connection.

If it isn't, (no volts at the wire) you need to check the wire for breaks all the way back to the Ignition/light switch. If the wire is intact (and still no voltage) then check the ignition/light switch, the internal contacts might be preventing those volts going to the regulator!!

 

It might turn out to be new ignition switch is needed!

P.S, if you've not got a Test meter, use a 12volt bulb instead, at least it will give you a visual indicator that the volts are getting there! (just don't rev the bike to over 4000v otherwise that bulb will blow too!😂)

Edited by NE0
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Thanks, just the advice i need to begin the journey. The magneto is coming tomorrow. I'll get it all working and get familiar with the wiring and then try what you wrote and post how it has gone or is going so far. A great little winter job. It's a superb bike, great nick, untouched. I'll get my girlfriend to work out how i can show photos of the bike.

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  • 3 weeks later...

Just an update. I bought a bike stand, the scissor type, so can get the flywheel to eye height, sat on stool. Superb, on castors, too. I have forensically cleaned the whole thing. No I am psyching myself up for the light electrics.

I'm rubbish at posting pictures, but this youtube is totally identical in EVERY way, though mine has an 18inch rear wheel and 21inch front; this one looks like they are both 18inch, but probably the photo, so may as well post it. This bike looks in good condition, but cannot be better than mine. You would struggle to find a square mm of rust anywhere, and it is ALL original totally. It runs totally wick, like this one. I can't wait. It's been six years since i last rode it. It has done 6.5k genuine miles (I had to change the original perished tyres). 

Ps, my headlight is cream, and my chain guards and back brake bar are orange.

 

Edited by lotty
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Another update. I began to get my wiring head on by reading the manual and just looking at the loom and the components. What a mess. looming tape falling off, odd wires here n there, dirty connectors, ancient repairs, tangles, and all oily dirty, the type of grime where badness resides. So I set about cleaning all the wiring, venturing into places never ventured ever before since day zero. It looked a bit iffy here n there. Other owners had clearly left their home made repair hacks over 50 years. But gradually I found this component and that component and got my head into it.

The big shock was under the seat where the main trunk of wires runs, it threads under the frame, ie right onto the top of the exhaust!, and under this frame cross member 8 of the 15 or so wires had melted into a solid block and many had corroded through........a real mess. Amazing anything had ever worked. I'd never noticed because It was just too dark and dirty under that bit. So I had to chop that section out and rewire that 4 inch section.

By this time my parts had arrived. I replaced the rectifier which was just a little one inch square bit of plastic which had 'blown', blistered open. I fitted a new battery. I used many new connectors. The whole thing started looking orderly and tidy, none of that grimy badness, just now happy goodness.

Eventually it was time to turn on the ignition and see what didn't work, but it did work.....indicators, stop lamp, horn, and neutral light. Fantastic. At last some reward and a weight off me.

Now there is just the head and tail light and however the battery gets charged. I realized this will need me to have the engine running, which means outside in the winter cold. I plan to try it, but if it needs some time on problems then I may have to wait till warmer weather. But for now I will take it outside and try the lights.

One other thing i did discover: I knew the  two-stroke oil tank was showing low in its little window, so I topped it up; then for some reason i decided to just have a quick look at the oil pump and so took the cover off. As soon as i did all the oil spilled out all over the floor in one big flood...a mug full. I cleaned the oil pump and casing dry and went to top up again so i could see the leak....BUT nothing, no leak, tank still full. Bone dry. Not a drip! I left it for two days and still the same. So I think it must only leak when the engine is running. Anyway, I have bought new oil pump oil seals and gaskets and this is next weeks job.  Saved a fortune in beer haha

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23 minutes ago, lotty said:

Another update. I began to get my wiring head on by reading the manual and just looking at the loom and the components. What a mess. looming tape falling off, odd wires here n there, dirty connectors, ancient repairs, tangles, and all oily dirty, the type of grime where badness resides. So I set about cleaning all the wiring, venturing into places never ventured ever before since day zero. It looked a bit iffy here n there. Other owners had clearly left their home made repair hacks over 50 years. But gradually I found this component and that component and got my head into it.

The big shock was under the seat where the main trunk of wires runs, it threads under the frame, ie right onto the top of the exhaust!, and under this frame cross member 8 of the 15 or so wires had melted into a solid block and many had corroded through........a real mess. Amazing anything had ever worked. I'd never noticed because It was just too dark and dirty under that bit. So I had to chop that section out and rewire that 4 inch section.

By this time my parts had arrived. I replaced the rectifier which was just a little one inch square bit of plastic which had 'blown', blistered open. I fitted a new battery. I used many new connectors. The whole thing started looking orderly and tidy, none of that grimy badness, just now happy goodness.

Eventually it was time to turn on the ignition and see what didn't work, but it did work.....indicators, stop lamp, horn, and neutral light. Fantastic. At last some reward and a weight off me.

Now there is just the head and tail light and however the battery gets charged. I realized this will need me to have the engine running, which means outside in the winter cold. I plan to try it, but if it needs some time on problems then I may have to wait till warmer weather. But for now I will take it outside and try the lights.

One other thing i did discover: I knew the  two-stroke oil tank was showing low in its little window, so I topped it up; then for some reason i decided to just have a quick look at the oil pump and so took the cover off. As soon as i did all the oil spilled out all over the floor in one big flood...a mug full. I cleaned the oil pump and casing dry and went to top up again so i could see the leak....BUT nothing, no leak, tank still full. Bone dry. Not a drip! I left it for two days and still the same. So I think it must only leak when the engine is running. Anyway, I have bought new oil pump oil seals and gaskets and this is next weeks job.  Saved a fortune in beer haha. Ps always had plenty of exhaust oil smoke, so haven't been running it 'dry'.

 

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Good work, especially under the seat. I had similar problems with my wiring loom and ended up replacing the whole thing with a replacement loom from the bay!

It's no fun working on a bike in the cold weather, been there done that! mind you, when they do break down, it's normally in the winter, after the sun has set and miles from home!! You can ride for miles in the summer without so much as a hint of trouble, but come with winter...different story!

Keep up the good work and enjoy.

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