Got a "tiny" charging problem...

[Kermit]

Hello there community,

Hope you're all doing fine, enjoying your bike(s).

I come here with a charging issue on my 1986 R65RT. I'm usually not a big fan of electrical issues, but stubbornness have me think this time I'd like to fix this one myself. Which leads me here asking for knowledgeable people for some guidance.
I'm pretty sure I bought this bike with the issue already there, but never really had a problem that forced me into having to fix it... until last month when I was left stranded on the hard shoulder of the motorway with no possibility to really feel safe.

Anyway, I'm currently running total loss as the battery is not charging at all. So I opened the front lid and started conducting tests for the rotor and stator. There might be other issues on other components, but for peace of mind, I would like to make sure each work individually. So logically, I started with what produces power. Tests are showing rotor and stator are ok (no shorts, correct resistance values where there should be), but I would like to make sure they both work well together.
Am I correct in saying that I should be able to measure AC voltage with a multi-meter from the 3-wire output on the bottom right of the stator casing, with
- engine running
- the said 3 wires unplugged from the stator casing
- all other wires untouched
- probes on only 2 separate spades

Cause I'm currently reading 0 whatever the couple of spades I'm choosing.

Thanks a lots for your help !

[Kermit]

This is how the alternator part of the charging system looks right now

Screenshot from 2025-10-19 14-50-12.png (786.99 KiB) Viewed 278 times

I have just unplugged the green, red-ish and white wires and hopped to measure some AC current from there. But nope.

[Kermit]

Also, checked the brushes are touching and with enough length, and cleaned the rotor slidings.
Everything looks fine.

[barryh]

Am I correct in saying that I should be able to measure AC voltage with a multi-meter from the 3-wire output on the bottom right of the stator casing, with
- engine running
- the said 3 wires unplugged from the stator casing
- all other wires untouched
- probes on only 2 separate spades

Cause I'm currently reading 0 whatever the couple of spades I'm choosing.

Thanks a lots for your help !

Yes you should be able to read an AC voltage across any of 2 of the 3 phases although I wouldn't disconnect them to take the reading. If you read nothing and the stator checks out otherwise then it's likely a problem with the voltage regulator. You can test if it's faulty by briefly linking D+ to DF which will should then give an unregulated output so leave the link in place only long enough to do the test.

ChargingSystem.jpg (69.47 KiB) Viewed 268 times

[Kermit]

Thanks Barry for your time and input on this. Really appreciated.

I've spent my afternoon on trying to understand what's going on. I kept on and performed some more tests, because the charging light on the dash does not light, never. It should when turning the key, right?

Since no light on the dash, I made the test where you link the 2 rotor slidings. No light on the dash.
Then the one where you earth the DF brush terminal. No light.
Then the one where you earth the blue wire from the voltage regulator connection plug. No light (same with brown).
So I turn to the dash to check the bulb, but it tested ok.

Does the alternator only produces AC if the full circuit is closed (which doesn't seem to be the case given the dash bulb does not light)? Or does it produces AC even if unplugged from everything?

I'm a bit lost I must admit.

Thanks again for any help

[gspd]

The charging light, the oil pressure light, and the neutral light all get power from the same circuit when the ignition is switched on.
The neutral and oil lights illuminate when grounded by their respective switches.
The charging light illuminates when grounded through the blue wire at the voltage regulator.

The first thing you have to do is get that charging light working.
These antiquated systems need the resistance of the charging bulb to function correctly.

[Kermit]

That's interesting as well. Thank you.
So it's not worth conducting tests on the circuit if the bulb doesn't even light on.
I might have to start the other way round, by first having it lit with a power pribe on the dash, then follow the harness to the next junction.
Any other ideas on places where to test this bulb between the voltage regulator and the dash is welcome.

Eh! Thanks a lot for tackling this with me.

The charging light, the oil pressure light, and the neutral light all get power from the same circuit when the ignition is switched on.
The neutral and oil lights illuminate when grounded by their respective switches.
The charging light illuminates when grounded through the blue wire at the voltage regulator.

The first thing you have to do is get that charging light working.
These antiquated systems need the resistance of the charging bulb to function correctly.

[gspd]

If you unplug the voltage regulator and jump the blue wire to ground the charge light should go on.

[Kermit]

If you unplug the voltage regulator and jump the blue wire to ground the charge light should go on.

I tried this one. And it doesn't unfortunately.
This is what made me look up in the dash for a faulty bulb or connection, but only found a working bulb.

It gets tricky really.

[barryh]

The stator won't produce any output until you get some current flowing through the rotor. It sounds like that isn't happening initially because of a break in the wiring somewhere. It might help to break things down into sections. For example, using the diagram above take a temporary feed from the battery +ve to the green blue side of the charge lamp. If it lights up by being earthed via the regulator and rotor you will know that side of the circuit is OK and you can then trace back in the other direction to find the break.