New guy here with a new to me DT250, my first classic Yamaha.

I bought the bike knowing it had an 'electrical problem' and am currently diagnosing the issues.

My first concern is the burned ground (solid black) in the harness. I noticed the negative wires going to the battery were melted, so I tracked the burn into the harness. The damage seems isolated to the back 1/2 of the bike, with the worst damage following the ground at the voltage regulator. I will replace the burned wire, but what I'd like to know is what could have caused this?


The rectifier is also burned out.

The previous owner was diagnosing the electrical issue and had made some harness cuts. I am unclear on how the ignition coil receives the spark pulse. The 77 wiring diagram in the Tech section has a CDI and I do not have a CDI.


I looked at the wiring diagram for a 75 which included the points ignition system and I think the two black/white wires that are cut should be joined for the ignition coil signal. Since the wiring diagram wasn't for a 77, I wanted to verify this is correct.

The 77 400 had the CDI... US 250 still had points as you know. Could be that diagram was from another country where the 250 had CDI... Yamaha did things like that.

Service Manual in Tech Library has correct diagram... all the way in the back. yamaha-enduros.com/index.php/workshop/se...50-dt400-shop-manual

I'm thinking someone might have connected a battery charger to the frame and to the negative battery post (instead of positive) or something like that... can't think of anything else that would melt the grounds like that... and/or maybe put the battery in backwards...

Thanks for the link! I am not sure why I didn't think to check the FSM for a proper wiring diagram.

I removed the main harness and have started repairing the damage. Soldering when it is 100 degrees in the shop isnt a whole lot of fun.

Using the 6v battery I applied voltage to the lights and horn. To my surprise everything worked. It is reassuring to know the electrical surge did not fry everything on the bike

I can't think of anything that would have done this, except what was already mentioned. When I picked my '79 up, it was missing the rectifier. It took me a little while to figure out that it was missing, but the bike ran great without it (no lights or horn, no battery charging). My '79 DOES have CDI.

Glad you are getting it all figured out, you'll love the bike.

I've managed to get the wire harness back together and I've bought a replacement rectifier (used) and ignition coil (used).

When I check for spark I get no spark or a very weak spark. I tried to check the points timing but was getting really odd continuity during the eintire revolution. I removed the flywheel for the first time to see what was going on in there and to test the points.

With the points closed I have continuity from the plug to ground. I expected this.


With the points opened manually, I still have continuity. I did not expect this.


I didn't see anything maintaining contact with the points.

Am I approaching the timing and functionality of the points incorrectly?

Yes. There is only a small change in resistance when the points open.

I'm not on my normal computer so wasn't sure I could find it but the search here works awesome!

Short video I made showing using a "buzz box" to check points opening or a VOM... I used my trusty old analog meter because all my (cheap) "autoranging" digital meters bounce all over the place and it's hard to get a clean reading at the exact time the points open.

Hi, think you said no spark then week spark but any spark is in with a chance!!. Run the black from the mag direct to the coil under the tank, at least it'll eliminate everything in-between just see if it runs. Also don't confuse the ignition black from mag with battery or loom earths. I hate digital meters but on an analogue meter there bugger all variation on the lowest scale but can definitely see the change. Lower coil--source coil-- has a very low resistance like about 2 ohms without looking it up. Pretty reliable old system & if you saw some of mine you'd think they'd never go but in fact never falter. Melted earth is pretty unusual. Not sure if battery back to front would do that, but might do a whole lot of things if it didn't instantly blow a fuse--gettin outa my depth.

Ah, thanks!

I need to find my analog VOM and try again.
The spark is inconsistent. Sometimes it is weak and another times there is no spark.

It is my understanding that the ignition wire is black/white and is switched to ground to kill the ignition source. I know I have good continuity along the black/white ignition wire from the magneto to the ignition coil I verified this when repairing the melted grounds. When I got the bike the previous owner had run a jumper from black/white wire at the magneto plug to the ignition coil. The previous owner never got the bike to run, which is why I picked it up dirt cheap

Source Coil
- 3.3Ω as tested from case ground to wire nub on the coil
- 1.65Ω +/- 10% is the spec listed in the service manual

It seems the source coil is twice as restrictive as it should be.

Do I have the correct flywheel on the bike?



I understand the 74-78 flywheel should have F0T0 1971 stamped on the face. My flywheel is stamped F1T 350.