Losing electrical power

[Bluethumb]

The bike is a ‘76 R90S. I started a new tread as I believe I have the problem better isolated, it’s a baffling problem, and has nothing to do with most of the details I put in the other post.

The issue is loss of electrical power. Details…

I am getting a consistent drop in voltage when the ignition is turned on.
The battery is good. I removed it and had it tested at NAPA. All grounds have been checked as have all connections.

At the ignition switch I get 12.8V at the red wire - terminal number 30. But when I turn the ignition on, I get 8V at both green wires and terminals, numbers 56/58 and 15. If I remove those green wires from the terminals I get 12.8V.

At the ignition switch with the green wires connected and both fuses removed, I still get 8V at terminal 15 or 56/58.

There are no telltale signs of a short such as a blown fuse, wires getting hot or the insulation softening, components getting warm, or smoke - nothing.

Something is causing the bike to “brown out”. With the lights off I can turn the ignition off, I can measure 8V at the ignition switch. If I add any additional load such as turning the lights on, hitting the brake switch or turn signal and the volts drop to zero.

I’ve repeated the above tests with the headlight relay removed, or the starter relay removed, or the starter disconnected. All with the same results.

The bike is consistent in that I can replicate these results.

Any suggestions?

[srankin]

I have only seen a post by you regarding the clutch switch. I figured you and Kurt were figuring that out and did not reply.

So, I have read this post a couple of times and I am confused as to what your problem is? Is it still to do with the clutch switch or something else?

Do you have problems with the headlight or other lights not working as they should? You say you are having a brown out? Does this mean as you turn on lights while the bike is running they dim? The headlight dims? Quit working?

Are you seeing a drop in voltage at the factory dash voltmeter and wonder if it is excessive?

I am sorry it has been a long time since I took classes in ohm's law and the theory of electrical trouble shooting. So I can't account for the voltage drops you are seeing on your meter.

I do recall dimly something about grounding and changes in voltage in a 12 volt system. A couple of years ago I found a very good video online explaining it. Sadly I didn't book mark the link so I can't supply it.

If your lights work as they should, the bike runs as it should I would say don't worry about it and ride it.
I am keen to hear more from you and who knows maybe I can help. St.

[Darryl.Richman]

There is a relatively common problem with these bikes, which might be the cause of your problem.

The battery positive lead goes directly to the starter motor solenoid before travelling to the diode board under the motor front cover. From there, it goes to the starter motor itself, then into the headlight bucket to the circuit board, and finally it goes to the ignition switch.

In particular, the wire running from the starter solenoid to the diode board connects to the diode board with a quick disconnect fitting. For whatever reason, this fitting is often a magnet for corrosion. The corrosion creates resistance, and this lowers the voltage available throughout the rest of the bike. You might notice other things like the turn signals not flashing or only flashing a couple times, for example.

If you're going to follow up this tip, be sure to disconnect the battery before removing the motor front cover. Because this power is not fused, it's easy to accidentally ground the diode board with the front cover and this will destroy the board.

[srankin]

Not sure if this helps as it is car oriented. Looks like it wouldn't take much to adapt to a MC St.
https://www.youtube.com/watch?v=APLic9F7ZsA

[Bluethumb]

Darryl nailed it, the d+ wire at the diode board was corroded. When I checked for continuity there was considerable resistance. A good soaking in vinegar, followed by baking soda solution and cleaning got a measurement of infinity and the voltage drop disappeared. You’re a genius!

[srankin]

Good news! But I must admit in my years of owning my bikes and hanging around the shop, I haven't seen this happen.

I guess I am real lucky considering I ride in the winters here with a ton of road salt on the roads from time to time. Anyway it is a good thing more than one person answers the questions posted on the forums. St.

[Darryl.Richman]

Darryl nailed it, the d+ wire at the diode board was corroded. When I checked for continuity there was considerable resistance. A good soaking in vinegar, followed by baking soda solution and cleaning got a measurement of infinity and the voltage drop disappeared. You’re a genius!

Glad to have been able to help! Somebody gave me this advice long ago, and I was able to use it on my own R90S. Now I'm happy to have been able to pass it on.

[Darryl.Richman]

Good news! But I must admit in my years of owning my bikes and hanging around the shop, I haven't seen this happen.

I guess I am real lucky considering I ride in the winters here with a ton of road salt on the roads from time to time. Anyway it is a good thing more than one person answers the questions posted on the forums. St.

My R90S is a west coast bike and has been in my hands in California for more than 25 years, yet it had the corrosion bad enough that, when I went to disconnect the wire, the quick disconnect came off in my hand with a burst of green powder. I think it has more to do with condensation behind the motor cover.

[srankin]

I guess I don't have condensation, maybe I ride too much and the engine never cools down? LOL. St.

[Darryl.Richman]

I guess I don't have condensation, maybe I ride too much and the engine never cools down? LOL. St.

I never really understood why I had the problem, because at the time, I had put about 40k miles on my bike, and riding it pretty regularly. Seems like that would generally keep things hot and dry here in California.