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1978 cb750k rained on now all the lights only come on halfway bright

One has to be able to use a continuity checker and/or a voltmeter first, and be able to read a wiring schematic.

Just to illustrate a small bit of what I was talking about and the lack of detail in posting by OP...............there are TWO disconnected two pin wires in the pic, the one he is talking about and the OTHER END of the same which goes to a different place. It's the lack of detail there that will absolutely melt every wire on the bike if missed. We've already confused continuity in and of itself with continuity to ground in the first couple posts and two totally different things. As well, several things on a bike are intentionally wired continuity to ground or they will not work and others aren't and the schematics do not point that out unless the person looking at it knows what to look for. OP says he knows how electricity works but DC or AC?, two different things again and big trouble there. If he truly knew DC the battery or tender would never have been connected backwards and it as well would have been precharged.

I suggest going to a library and checking out a basic book on DC power, maybe on cars, which work pretty much the same as bikes and doing some reading.

The below alone is wonky and misleading.............the testing method and tool are determinants of the outcome and not stated.

'If you don't get continuity between two ends of the wire, then check both ends of that wire to ground.'

You will get continuity if checking both ends of a grounded-in-the-middle wire, it's just that both will also show continuity to ground as well and COULD be a short but if intended it isn't then. Not getting continuity across a length of wire is the opposite of a short or an OPEN. And AGAIN the results depend on checking using vehicle power or using internal power like a voltmeter.

OP is pretty much right, I never asked anyone, everything I got was from the books and able to do this work at 12 years old. That is not my fault. Some recognize when the job is too big and the skills don't cover it, others don't (usually out of desperation) and then drag things on forever and ever and still to no good resolution. I'm hoping of course that doesn't happen here but things are what they are. The machines do not care about owner money situations or skills at all and do what they will. I see it like forever on all the computer controlled car sites I post on. There are poster to poster issues that the internet makes worse instead of better. Lack of communication from OPs is the most common cause.

Again, trying hard not to insult someone. Reality will not be denied though.

I would NOT be looking for continuity at all myself, it being obvious something is shorted by melting wire, which is caused by TOO MUCH continuity and in the wrong place. I instead would break the system up into much smaller pieces to get that shorted piece out of the loop and then slowly add piece of harness to piece of harness until the problem shows back up and then the indicator of where it is. That means yanking all fuses and wiring connectors on the bike and then adding them all back in a piece at a time until the offender is found. Key off, add a part, key on and the result. If good do it again. Over and over.

And if that week plus old rain dripped inside connectors to make a short by itself then you may STILL have shorting going on for a long time. It can take a month for a water drop to evap out of a closed space. It doesn't matter what the rain forecast is, you ALWAYS cover up an open machine to the weather and the carb openings as well, I've seen mice take trash into open carb inlets to lock up engines and the openings were only like that a day. If the carb holes got water in them then the engine should be locking up right about now and that could easily make the wiring issue a moot point.

Wanting to learn how is fine but learn to do it in the best way for you, a basic book or the internet read in minutes will tell you more than a month here online and the bike didn't burn up doing it.
 
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Just to let you know, I did what the other posters said and checked continuity...There was a load wire that had rusted loose and was making contact with a ground. I found it by testing voltages on the various grounds. SO get rekt asshole.
 
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