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DT175MX- 1979 Magneto


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Hi all,

returning member, been renovating g so long my membership ran out......so finally renovated my DT and she looks lovely (lockdown project, pics to follow) bought in field bike condition and am now at the final wiring stage on lights, ignition, indicators etc. She starts and runs fine. Looking at the wiring loom and the Haynes manual it appears to have 4 wires coming from the magneto and I have three! The one I am missing seems to be the one to recharge the battery. Are there different versions or is it likely over time this wire has snapped off? Trying to understand if some models were like this or if I have to take magneto off or even replace it?

all help appreciated 

cheers

scott

 

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Posted (edited)

Hi welcome back Scott!

You say it was a field bike in which case it may not have all its correct OEM parts! which might explain the discrepancy in the number of wires!

I have a DT175MX 1978 with the round tube swinging arm and I may be able to help you. You see the RED wire is only found on the earlier model with two source coils. Its actually the WHITE wire which is part of the charging circuit. (NB: The outgoing Green/white wire changes colour to WHITE within the plastic connector)

If you have the same two source coils stacked on top of each other as you can see in my photo, then you will have 4 wires going to the CDI as in the haynes manual page 167.  If you look in the corner of this photo (yellow circle) you can see all the wires within the generator before they disappear from view within the loom wrapping. Here the wiring is least likely to be under tension and therefore more likely to still be intact even if one wire has snapped off within the loom. You can see and count them and be able to trace them back to the coils where you can see the red wire going to the upper source coil in the photo. There should be 6 wires visible here: x4 (RED, Brown,Black and White/Red) from the two stacked source coils and x2 wires from the dark red lighting/charging coil (Green/white and Yellow). Note the red wire in this layout goes to the CDI. Not directly to the battery. The red wire you see on the battery comes from the rectifier not from the CDI circuit.

However, if you have the later generator with only one source coil then it will only have three wires as in the diagram page 172.  Note there is no red wire to the different CDI. however there is still a red wire from the rectifier to the battery. Therefore there will be only 5 wires visible: x3 from the single source coil  (Brown, Black and White/red) and 2 wires from the dark red lighting/charging coil. (Green/white and Yellow)

 gen

To explain the charging circuit on the WHITE wire take a look at one of my earlier posts on converting my DT from 6v to 12v. It's a long post but I do explain in detail how the charging circuit works along with detailed pictures.

 

Hope this helps

Edited by NE0
Clarity and brevity once I re-read it the next day!
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Posted (edited)

Thinking about it....

You said in your first post ..that she starts and runs fine....if you DID have a snapped off RED wire,: i.e you have the Two Source coils version, and your red wire was missing THEN it probably wouldn't run!

The two source coils are responsible for maintaining the performance at low and high speed with the correct corresponding CDI. It was soon replaced with the single source coil and a different CDI more common on the later models.  The CDIs are not compatible with each other.

I'm not sure why two coils stacked on top of each other was used. On the one hand it seems over engineered to deal with it, and on the other hand with two coils surely there was two chances of failure?. Either way it didn't last long and obviously not a successful method, otherwise they would have kept it and continued to develop it.

The simple answer is, probably someone came up with a cheaper simplified solution of the single coil which matched the performance of the two coil version.

So in view of the above, I'd venture to say, the fact you have it running succesfully, the reason why the RED wire is missing is you have the later single source coil and you're looking at the wrong circuit diagram.

Hope this helps, look forward to seeing the photos you mention in your first post. Well done bud.

 

I must confess i enjoyed answering this one, got me thinking.

Edited by NE0
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