When, for example, the smoke escapes from an electrical component (i.e., say, a Lucas voltage regulator), it will be observed that the component stops working. The function of the wire harness is to carry the smoke from one device to another; when the wire harness "springs a leak", and lets all the smoke out of the system, nothing works afterwards.I'm on nights at the moment. I rode home on Saturday morning and parked the bike up outside the house. When the time came to go back to work on Saturday evening, I togged up, put my bag in the topbox and switched the ignition on. Nothing. No dash lights, no starter, no go. The ignition switch is a bit dicky, so I fiddled with the key a few times, but then gave up and took the car. (Yes, I checked the kill switch!) This afternoon, after my sleep, I thought I would see what was wrong.
The Yamahaha, being a trail bike, has a very simple electrical system . There is a wire from the battery to the ignition switch, with a fuse close to the battery end. That fuse, just the one, protects every circuit on the bike. A quick check with a test lamp showed that there was juice in the battery, and as far as the near end of the fuse carrier. Then I checked the far end of the fuse carrier and there was still power. So I now suspected a break in the wire under the tank, or a malfunction with the switch itself. Investigating that was going to involve some dismantling, so I decided to do a few easy things today before work, and leave the other stuff to my next day off.
So I took the fuse out to check it. Remember that there was power both before and after the fuse, indicating that the fuse was good, and the fuse itself was unblown. I took the fuse out - all OK - and put it back. I checked the ignition switch, more in hope than expectation, as there was no reason to think anything would be different, and lo and behold there was light. The bike started first time, all the lights worked, and everything was back to normal. So I took out a perfectly good fuse, put it back, and solved the problem.
I know why. The fuse lives in a little box next to the battery, a space which is also shared by the indicator relay. Last week I replaced the relay, as the old one was flashing like a Club 18-30 disco at about 300 bpm. The fuse was simply sulking, as it hadn't had any attention.
It's fine now.
 Simple is good, but the disadvantage is that, if the fuse ever blows for any reason, it takes out the engine and lights, everything. I had the headlights fail on a car once, and managed to find a safe place to stop using the indicators. On the Yam, even that wouldn't work. It's a poor design for a bike that is mainly used on the road, and if I ever get round to any serious restoration work on the bike, I'm going to try to design a new scheme that doesn't have all the electrical eggs in one basket.