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The regulator, looking at this diagram doesn't rely on the frame. The stator input is AC so no ground required there. The output is DC using the engine casing to join the regulator to the battery.
I am assuming the wires to the stator will be the ones that head across to the primary drive side running under the engine tir rods...and the power and ground is the pair that runs down the front downtube and under the frame.
I'll check the ground, as one connector should have a good ground and the other should connect to the main fuse. That's currently pulled while I am playing with the thing cleaning up battery terminals etc.
So the other two wires should check at around 0.3Ohms resistance and not earth, plus I should be able to measure my AC voltage there. I don;t think there is a stator issue, as every time I have had that happen, the result has been low voltages, battery discharging, or problems with additional load like headlights etc. I owned MotoGuzzis for many years, and electrics are something you get familiar with......still, no reason NOT to test since I can do it from the regulator plug that's coming out.
Lest I forget ...thanks for all the input here. I am learning a lot from this, and while some of it I knew ( and it always helps to have what you think you know verified) a lot of it is also giving me new things to check and new info.
This old bike has kind of stuck with me, and when I got it some year sago, it had only done 6000 miles. Now it;s nearly 20k, and I am planning to keep it a while yet. So the more I know about her, the better!
The regulator, looking at this diagram doesn't rely on the frame. The stator input is AC so no ground required there. The output is DC using the engine casing to join the regulator to the battery.
My 1991 regulator grounds on the frame, as there are only the two stator wires coming in, and the wire to the circuit breaker going out. No fourth ground wire. Good catch Andy!
See it endless times on here. Loose battery terminals will cause the regulator to raise voltage.
A voltage regulator knows how much to charge by constantly monitoring the system's voltage and adjusting the alternator's magnetic field strength by rapidly switching current to the field coils, aiming for a steady voltage (around 13.5-14.7V).
That keeps the battery topped off and power accessories without overcharging.
It acts like a smart switch, turning the alternator's output up or down many times per second.
Last edited by Jackie Paper; Jan 14, 2026 at 11:40 AM.
Got the new unit fitted, and it's all working fine. Volts are back where they should be and the speedo light is back. Needed it today riding home from work in the torrential rain. Now, the pic below is the back of my old unit.
I ...er...think I can see the issue. It's HORRID!
The corrosion bottom left goes right in and is lifting out the resin they have filled the component with. Must be tracking moisture inside.
Not surprised it was dead.
Also checked the stator before fitting. Checked out good on resistance and continuity so all good.
Well, the new regulator is in and working fine. The stator was checked for resistance and continuity while everything was unplugged, and that was all good. When I had a good look at the old unit and turned it over....er....I think I know why it failed now!
The corrosion was right under the rubberry material they fill the back with and I could just pull a chunk away at the bottom left. Corrosion was all the weay through and there was a nice chunk of powdery oxide in there. The circuit board was easily visible and showing corrosion too. Actually, surprised it had managed to last as long as it did with the recent rain we had in the UK.
Running as well as ever now!
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