R75/6 Electrical problem

[xpab]

New to the forum here, so just wanted to say hello to everybody.

I have a '74 R75/6 that up until a few days ago had been running perfectly. I took it out for a few hundred miles maybe three weeks ago and parked it, only to uncover it this past weekend with nothing running. With the key turned on, there's no headlight, no horn, no starter, no blinkers, nothing. Won't kick start either. I have digital ignition so I figured that the battery might be dead (odyssey battery) but it was holding strong at 12v when I did a load test. Thought that it might be the fuses in the headlamp, but both were intact. I changed them anyway, to no avail.

Any ideas what could be up?

[Ken in Oklahoma]

New to the forum here, so just wanted to say hello to everybody.

I have a '74 R75/6 that up until a few days ago had been running perfectly. I took it out for a few hundred miles maybe three weeks ago and parked it, only to uncover it this past weekend with nothing running. With the key turned on, there's no headlight, no horn, no starter, no blinkers, nothing. Won't kick start either. I have digital ignition so I figured that the battery might be dead (odyssey battery) but it was holding strong at 12v when I did a load test. Thought that it might be the fuses in the headlamp, but both were intact. I changed them anyway, to no avail.

Any ideas what could be up?

That's a hard question to answer, for two reasons. First there are a lot of different ways to approach the problem and secondly, we don't have a clue about your electrical knowledge. It will help if you tell us a bit about your troubleshooting capabilities. Do you have a DVM? Can you read a wiring diagram? Do you have a wiring diagram? I think you get the idea. If we know how handy you are with electrical stuff we can shortcut the process and give you better, more do-able ideas.

Whatever you do, you will need to develop a methodical approach. To get started here are a couple things I would do.

The first thing I would do is to make sure that the problem is electrical. I would turn the key on and then measure the voltage on the positive side of the two coils under your tank. The coils are wired in series and you will probably looking for a green wire with some color tracer which goes to one of the terminals of your two coils. (The negative side of the two coils will be a black wire which goes to the points.)

You should have about 12 volts between that green wire and chassis ground. You probably don't. If that's the case you will have to find out where the 12 volts is getting lost.

You probably don't have 12 volts. Your task now is to find out where it's getting lost. That can be both on the positive side or that the negative side of the battery if it isn't well grounded. A prime suspect is always battery cables. The cable connections must be good and the cables need to be in good shape. By good shape I mean the connections bright and no corrosion on the cable ends. Often the corrosion is actually hidden under the cable jacket so the ends need to be carefully inspected. There are four places to check the cables, on the positive lug of the battery, on the negative lug of the battery, and at the connection of the negative cable where the speedometer cable goes into the transmission. The fourth connection is under the top engine cover which will have to be removed to check. The good news is that particular connection seems go give less trouble than the other three.

I'm going to quit here since at this point the possible problem areas multiply. With those 4 connections known to be good you can now start some serious trouble shooting.

Like I say, there are a lot of way to skin this cat and different people will have different ideas.

And welcome to the forum.

Ken

[Airbear]

Welcome xpab. Sorry for your woes.
I'm glad that our Ken in Oklahoma has initiated a useful approach to the problem in his typically thoughtful way. I'll add that I can supply a couple of 'maps' of /6 wiring in PDF format that may be helpful when you start tracing wires. Just shoot me an email or a PM with an email address and I'll attach them in reply. (Hit the little envelope icon over to the right of this post)
Good luck.

ps: You'll find one of these very useful when you go looking for those pesky electrons - about $6 at auto parts stores:

[Zombie Master]

Does you bike have a kill switch?

[Boertjie]

Does you bike have a kill switch?

damn switch

happens to me

[Major Softie]

Does you bike have a kill switch?

damn switch

happens to me

I have NEVER done that.

For years...

[richard t]

the 75 R75/6 doesn't have a kill switch

[xpab]

No kill switch on the bike, but I was hoping it would be that easy.

Airbear sent me some great wiring diagrams, can't thank you enough for that. Going to get them printed out this morning.

And Ken, thanks for the detailed jumpstart. As far as my capabilities, I have a dvm and am able to read a wiring diagram. I've spent a lot of time working on my 71 Triumph--rewired it and installed a boyer, but that's about as far as my electrical experience goes. Those seem to be a lot simpler than this /6. I live in NYC so I have no garage, 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.

Thanks guys, hopefully I can figure this one out soon

[Ken in Oklahoma]

. . . 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

[wirewrkr]

the 75 R75/6 doesn't have a kill switch

?????
Since when?
his bike is a 74.
A 75 does have a kill switch, it's the first year.