xpab wrote:. . . but I'm going to go ahead and check those coils now that it's light out. As far as the battery terminals and cable ends, they all seem to be in pretty good shape.
I'm supposing that you won't find 12V on the positive side of your coils. And I'm supposing further that the battery negative is well connected to the frame since your battery cables are in good shape and tight.
At this point I would start following the +12 volts from the battery to find out where it gets lost. Starting from the battery the +12V goes through the battery cable, and connects to the starter solenoid under the engine top cover. From there a smaller red wire goes to the starter relay socket (which is on the left hand side of your frame tubes under the tank). At the socket that red wire is connected through the relay itself to another socket pin which connects to another red wire. That red wire goes into the headlight bucket where it attaches to the ignition switch.
I need to add a caveat here. I'm familiar with a '76 R75 as well as a '77 R100/7 and '77 S bike. My concern here is that a '75 /6 just might be different here from the '76 and later bikes. So what I'm about to say applies to those, and hopefully yours.
That starter relay socket is notorious for giving symptoms just like yours. It happened to me once on my R100/7 and the fear of the bike suddenly stopping farther than you can walk home is daunting. What goes on with the socket is that it is in a dirty environment what with the under tank master cylinder leaking and the brake fluid getting into the socket. Thus the starter relay pins don't complete the circuit between the two red wires I talked about earlier, and your bike goes dead.
The fix is usually to clean the socket contacts as well as the relay pins. The socket is difficult to clean since the contacts are recessed and hard to get at to clean them decently. Nevertheless they have to be reasonably clean. I go at the contacts with contact cleaner and a toothbrush. There's probably a better way to go about it and somebody else may have a better idea.
Now then, I'm going to go off on a bit of a tangent to the trouble shooting. On my /7 I was very worried that my starter relay socket cleaning wasn't going to be good enough to get rid of any oxide on the contacts. So I devised a hot wire so that I could start the bike around the starter relay socket connections or for a lot of other possible problem areas.
What I did was to run a wire directly from the battery positive post to a terminal on a single pole single throw toggle switch. And from the other terminal on the toggle switch I ran a wire to the positive side of the two coils, attaching to the same green wire with some color tracer that I talked about earlier.
This hot wires your bike when the toggle switch is switched to "on". In effect you are bypassing the ignition switch and other wiring and getting the 12V directly to the place where it needs to be, on the positive side of the coils. With the toggle switch "on" several other things will work properly, including the starter. Some other things won't work, but I don't recall which. But as I recall things like the brake lights will also work. Of course the ignition switch will do nothing and you will have to turn the toggle switch off to kill the engine. But you will get home to fight the wiring another day.
That's about as far as I'm going to go for now. My next step, when I'm sure that the 12V is getting through the starter relay socket into the headlight bucket is to start looking around in there.
And again, some others might chime in with some ideas that you may want to follow.
Ken
