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1974 DT250 wiring issues

  • Dberkner76
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1974 DT250 wiring issues was created by Dberkner76

I’m working on my 1974 DT250 and I’ve stumbled into some strange wiring that doesn’t seem to make sense.   I read a previous post about lights, but unfortunately I’m not to the point where that will help. 

First question - does the negative wire go directly to the frame or does it go out and connect with anything.  Mine is bolted to the fender(see the picture).  From what I’m looking at It almost looks like the black wires are connected to a red from somewhere.  But there are two splices that were likely done later and not correct.

The frustration for me is that the horn is the only thing that works.  None of the lights work running or not.  I’ve looked at wiring diagrams but I am just lost.  Just thought that maybe if I can start at the beginning with the battery I can sort this all out.

 
03 May 2021 18:33 #1

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Replied by Mothersbaugh on topic 1974 DT250 wiring issues

I don't have my DT250A handy (storage unit) but I can say that usually the battery ground (earth) is a bare spot on the frame closer to the battery than that fender location. Look for a Phillip's head screw nearby. And clean both the wire's connector and the frame location well.
Looks like too many red wires. Somebody been in there foolin' around...
03 May 2021 20:49 #2

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  • hydrogen sulphide
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Replied by hydrogen sulphide on topic 1974 DT250 wiring issues

I believe the horn is normally grounded to the frame via the left hand lighting switch making contact/connection to the handle bars.  So it makes sense that the only electrical functioning is the horn.  Someone has made a mess for you to sort out, start by establishing a proper ground connections
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04 May 2021 06:32 #3

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Replied by Andy L on topic 1974 DT250 wiring issues

Hello Dberkner76, I see this is your first post so 

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 to the forum.  A couple of chaps have replied to you already with excellent advice re: making sure you have good earth connections for black wires attached to the frame.  However from looking at your photo you have other significant problems too.  As Mothersbaugh mentioned there are indeed too many red coloured wires for your 'PO inherited mess' to resemble anything close to the correct wiring diagram(s) shown in the Tech'l Library which you can access on this forum.   I say this because you would NEVER connect a legitimate red wire directly to a black (ground/earth/frame) wire as shown in your photo i.e. the dual connection inside the clear rubber shrouded connector.   IF you had a battery in such a circuit it would discharge instantaneously as there would be no-load (no resistance) hence infinite current would flow and I suspect damage would be done even before any in-line fuse had time to blow.  It's very difficult to say where to start in helping you, which maybe why you've not had many responses.  My initial advice would be to disconnect the two 'splices' you mentioned.   Using a multimeter set to ohms you need to perform continuity tests and trace where the extra red wires are going (label 'em up, write on & attach masking tape tabs) and look at the wiring diagrams to compare and build a picture.  Only introduce a healthy charged battery with a new fuse once you understand where those extra red wires should be connected.  It could be the PO only had red wire available when making up the new circuits.  If all this seems gobbledeegook to you then I would advise taking the bike to an automotive electrical engineer because there is after all a safety issue when dealing with butchered wiring.  Good luck with your investigations and let us know how you get on.  Best regards Andy.  

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Last edit: 04 May 2021 14:54 by Andy L. Reason: Trying to improve the readability of the thread.
04 May 2021 12:42 #4

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Replied by Dberkner76 on topic 1974 DT250 wiring issues

Thanks for all the responses, I did start to trace wires back and found that the PO must have had extra red wire lying around so they used that and attached it to the main black wire that runs up front.  So that mystery is solved. 

I am getting a multimeter to do some tests, may be a few days before I get the time to come back with those results.  I am wondering if any of my bulbs were any good anymore, that meter will help me figure that out.  I also have a voltage regulator on order as I have read other posts that recommended adding this to prevent future light blow outs.

will give an update hopefully by the weekend.
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04 May 2021 19:44 #5

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Replied by Grussell on topic 1974 DT250 wiring issues

You will probably discover that all of the light bulbs are blown. You can check them with your ohm meter. The black wire from your battery should plug into the wiring harness were the red wire is plugged into the black 4 plug. If the bike was started without a battery it will blow the bulbs. Here is the link for the service manual you can download. You will find the wiring diagram and instructions on how to do electrical test. Keep us updated with your progress.

yamaha-enduros.com/index.php/service-man...ctory-service-manual
1978 DT400
1975 DT250
1976 YZ125C (in progress)
1976 YZ125X (in progress)
2018 YZ250FX (my race bike)
2010 Triumph America (my cruiser)
09 May 2021 08:42 #6

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Replied by Dberkner76 on topic 1974 DT250 wiring issues

I was able to confirm that all of my bulbs were dead.  I checked each light and got 6 volts with the key off at the  taillight only. With the engine running I got 6 volts at all the rest of the lights.  I thought I read on another post that the neutral light should come on with the key but I’m ok if it doesn’t.

So now I have to get the voltage regulator installed and get some new lights.  Also going to clean up some of the sloppy connections that the PO did and after everything works I’ll tape it all back up nice and tight.

The only other issue I have is (and maybe this should be a new post) is that when in neutral or clutch engaged the engine revs real nice, but it seems to get choppy when it’s moving and the engine doesn’t run real well.  My spark plugs are old and I haven’t spent much time on this yet, so I’ll have to poke around here to see if there are others with solutions.  Thanks to everyone who chipped in.  I’ll give an update once I get all the electric stuff done.
 
10 May 2021 18:57 #7

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  • Gr8uncleal
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Replied by Gr8uncleal on topic 1974 DT250 wiring issues

No neutral light fitted on that model, so you probably have a tacho from a different model. That seemed to happen a lot with the early 250s and 360s.
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11 May 2021 00:06 #8

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Replied by Dberkner76 on topic 1974 DT250 wiring issues

Thanks for your reply’s, now I’m back to where I started though. Just the horn.

I had to also reroute the wires since they’re so screwed up and the second time I did that I lost the taillight and the headlight also quit. I’m going to look at my grounds tomorrow but I’m so confused how just moving wires around the frame messed it up. I did continuity tests and have that at the taillight but not the headlight (maybe I wouldn’t with out it running).

I can tell that the brake stop switch works, but there I get no voltage readings anywhere, except the horn.

Because of this I haven’t installed my voltage regulator, want to sus out this problem before I make a new one.

I could very well be the ground, but that has been confusing for me as the black wire branches out into three different places. If one of them is not grounded would the horn still work?
15 May 2021 20:43 #9

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Replied by Dberkner76 on topic 1974 DT250 wiring issues

Update, the handle bar light on and off switch was the problem, I cleaned it up and my lights work.  But another new problem.

I hooked up the new headlight and it blew out with the engine idling and now there is no voltage at the headlight with the engine running.   I am going to install my voltage regulator and maybe something magic will happen when I do that and I’ll get my voltage back in the headlight.  Any thoughts on this issue or what to test?
 
16 May 2021 11:52 #10

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