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1974 DT250 Wiring Question

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1974 DT250 Wiring Question was created by Yamalol

I'm having some trouble sorting out the wiring on my DT250. When I got it, everything except what's necessary for it to start was disconnected. I've put the headlight, taillight, and horn back on, and replaced the bulbs in the tach and speedo. So far, I've only been able to get the taillight to work, and only in night mode. Neither the front, not the rear switch seem to be working... if I even have them connected properly.

I have the color wiring diagram from the tech library, and the wiring diagram from the service manual. It seems like I've got extra wires, or wires of the incorrect color, or something like that. I might just be misinterpreting the diagrams, but I'm not sure.

For example, by the rear brake, I've got two black wires coming out of the switch. One connects to a green and yellow wire, and the other connects to nothing. The only disconnected wire is brown (could be faded red), but I don't see any indication that a brown (or red) and black wire are supposed to connect in that area.

Similarly for the headlight, I've got the green and yellow wires coming from the high beam switch to the light. The black wire I connected was the only one I saw, seems to be coming from the meter lights, even though the wiring diagram makes it seem like that black wire should be coming from the ignition coil. Also, there's another yellow wire at the front that's disconnected and I don't see where it should connect. This is sort of visible in the photo.

Any thoughts or ideas are appreciated! Thanks.
'74 DT250A, '74 DT250A (another one), ‘72 Honda XL250, ‘69 Honda SS125A, ‘88 KLR 650, '57 Harley Davidson Sportster, '46 Harley Davidson WL, '79 Harley Davidson Shovelhead.
25 Sep 2019 08:10 #1

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Replied by Gr8uncleal on topic 1974 DT250 Wiring Question

Here we go!

1) Headlight only works with the engine running.
2) Rear lights (both), horn and indicators run off the battery and so will work without the engine running.
3) If your rear brake light switch has 2 black wires, it may be aftermarket. Anyway, one wire should have a male bullet connector and the other a female. The female connects with the male bullet connector on the green/yellow wire from the main harness and the male with the female one on the brown wire from the harness.
4) Amongst various other wires, coming off the main harness into the headlight area should be a black wire with a triple female end and this takes the black wires from the headlight and the idiot lights from the speedo and tacho.
5) My headlight bulb holder has 2 single female bullet connectors and these connect with 2 males, from the headlight switch and main harness (I think!).

Hopefully this will help!
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Last edit: 25 Sep 2019 11:00 by Gr8uncleal. Reason: spelling
25 Sep 2019 10:59 #2

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Replied by Yamalol on topic 1974 DT250 Wiring Question

Thanks! The aftermarket taillight switch makes sense. Based on what you said about the front, it looks like I may need to do some modifications as my black wire only has two bullet connectors. I think my green and yellow headlight wires are going to the right place, but that doesn't explain the additional yellow wire at the front. currently not connected to anything. From the looks of the diagram, the only yellow wire with an available end should be the one that goes into the headlight.
'74 DT250A, '74 DT250A (another one), ‘72 Honda XL250, ‘69 Honda SS125A, ‘88 KLR 650, '57 Harley Davidson Sportster, '46 Harley Davidson WL, '79 Harley Davidson Shovelhead.
25 Sep 2019 11:53 #3

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Replied by Gr8uncleal on topic 1974 DT250 Wiring Question

In your first photo, it looks like you have 2 single yellow wires - 1 male currently outside of the headlight and 1 going into the back of it. Does the wire going into the back of the headlight have a male connector as well? Maybe post a photo of the wires inside the headlight shell/bucket.

In your second photo, the female connector on the end of the black wire can receive 3 male connectors - 2 at the front and 1 at the back.
Last edit: 25 Sep 2019 12:31 by Gr8uncleal. Reason: more text
25 Sep 2019 12:24 #4

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Replied by MarkT on topic 1974 DT250 Wiring Question

Not sure what diagrams you're looking at but the ones I see clearly show brown on one side of the brake light switch... even many different years and models were wired that way. Not bashing you... just mentioning in case you somehow got some oddball diagram for a Yamaguchi Nokumura 250 somehow.

All the harnesses should be fed into the headlight bucket and connected up inside... not hanging behind headlight. Get all the harness connectors into the bucket where they belong and it might make more sense.

Yellow from engine connects to male yellow harness that runs up to headlight switch on handle bar. Female yellow out of the handlebar headlight switch harness connects to small plug harness that goes on back of headlight.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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25 Sep 2019 16:20 #5

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Replied by Gr8uncleal on topic 1974 DT250 Wiring Question

Yes, the individual wires with bullet connectors should sit inside the headlamp shell. However the block pin connector with the large covering sleeve in your last photo actually sits just behind and to the left of the headlamp shell.

If you're finding things too fiddly so start with, you could remove the headlamp - this would give you plenty of space to do a dry run and shouldn't stop anything working that should be. Once you have sussed things out, disconnect the wires, re-fit the headlamp shell, feed the wires through the hole at the back and re-connect them.

I am attaching a couple of images of a typical headlamp bulb socket with 2 female bullet connectors - 1 is 2 individual connectors and the other is 1 double.

A statement of the obvious (sorry!), but do make sure that all of the bulbs are 6v and working properly.

If a previous owner hasn't done it already, and if it wasn't fitted as standard, you might want to consider retro fitting a 6v voltage regulator. I am attaching a link to Yamaha's own bulletin on this (from the forum's tech library) and it will need to be fixed direct to the frame to earth properly.

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Last edit: 26 Sep 2019 01:24 by Gr8uncleal. Reason: spelling
26 Sep 2019 01:18 #6

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Replied by Yamalol on topic 1974 DT250 Wiring Question

Ok, here's what I've got. I didn't bother taking a picture, but I connected the brown wire to the second black wire coming out of the rear taillight switch and it worked.

I realized I don't have the socket you pictured, so I tried to make my own. I tried a variety of connections at the front, including splicing the green wires together, even though I can't see how that would make sense. The headlight never turned on, nor did the horn make sound, but connecting wires that weren't previously connected did prevent the taillight from turning on.

Again, I know next to nothing about the history of this bike, but I do see some things that make me thing the wiring, at least for the tach and speedo might be off a different bike. See the picture with the blue light coming out. I see nothing on my wiring diagram that indicates a blue wire should be entering that area. I also have a green wire with an attachment for three additional splices. not sure what that could be either. Took photos of everything I could. Really appreciate the help.
'74 DT250A, '74 DT250A (another one), ‘72 Honda XL250, ‘69 Honda SS125A, ‘88 KLR 650, '57 Harley Davidson Sportster, '46 Harley Davidson WL, '79 Harley Davidson Shovelhead.
26 Sep 2019 17:23 #7

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Replied by MarkT on topic 1974 DT250 Wiring Question

Connecting the yellows together like that is not correct. I already explained it.

Engine must be running to have headlight.

Blue wire to instruments is absolutely normal and correct.... runs the instrument lights but you'll never get power to the blue correctly unless you hook up the yellow through the headlight switch like it needs to be.

Yamaha sometimes screws things up in their diagrams... especially seems to be common around 1974. Maybe look at an older diagram or two or a newer one. Wiring did not change much and Yamaha stayed with the same basic color scheme for many years.

Good luck.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
26 Sep 2019 17:42 #8

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Replied by Yamalol on topic 1974 DT250 Wiring Question

I know it doesn't make sense based on the diagram or explanation, but the photo of the socket with female connections for multiple wires confused me. First time tackling a project like this, I may end up taking the gas tank off and tracing and labeling every single wire. I guess I've been taking the diagrams too literally.
'74 DT250A, '74 DT250A (another one), ‘72 Honda XL250, ‘69 Honda SS125A, ‘88 KLR 650, '57 Harley Davidson Sportster, '46 Harley Davidson WL, '79 Harley Davidson Shovelhead.
Last edit: 26 Sep 2019 18:19 by Yamalol.
26 Sep 2019 18:19 #9

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Replied by Gr8uncleal on topic 1974 DT250 Wiring Question

Brake light working - great, that's progress and one less thing to worry about.

Forget about trying to get the headlight to work until the engine's running. However, assuming that the second yellow wire that we haven't seen the end of has a male bullet connector, then once you get the headlight socket you should have all you need.

If you just want to test the headlight direct from the battery then, looking at it from the riding position (ie from behind), left hand terminal is earth (black), right is drive/low beam (yellow) and top is pass/high beam (green). And yes, your headlight has been replaced at some stage, hence the non standard colours on the wires. Typically this would happen due to the voltage not being regulated properly (or maybe a short circuit somewhere) and, as the original was almost certainly a sealed beam type (ie you can't just replace the bulb), the whole unit had to be replaced. Crazy set up in my opinion!

Turning now to the horn, first up just test it direct from the battery to make sure that it's ok. From memory, for this purpose you can attach either battery wire to either horn wire and it should work. If it doesn't, report back and we'll try and fix it!

If it does work, when you wire in the brown, it has to go to the lighter of the brown wire multiple female connectors - clean all of them off if you're unsure which is which. The light brown is a positive source from the battery and the darker brown is for the left hand indicators/flashers.

It is a good idea to remove the tank to check the general condition of the wiring.

Keep us updated and, remember, one photo is worth a dozen words!

All the best.

Alan
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26 Sep 2019 23:52 #10

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