Step one: Fully charge the battery from a source other than the the bike. Then let it rest 2hrs and measure the battery voltage.
It should be 12.6-12.8 Volt. If not, get the battery load tested at the battery shop.
Otherwise, connect to bike and start. Make voltage reading at idle, 2000, 3000, 4000, and 5000 RPM.
Repeat the test with the headlight fuse removed.
The voltage reading will tell you if the bike is charging or not. If it is, at some RPM and electrical load condition the battery voltage should climb rather the fall off.
You may think you already know this. But, there is information to ba gained whether the system can charge at all or if it has completely packed up and left the building. Weak vs. dead.
Most probably the rotor has become defective, and these can take the R/R with it when it goes. A new R/R can be damaged by the rotor. so start there. If you are an optimist, disconnect the wires leading to the rotor's slip rings and measure resistance at the R/R connector. A resistance of 5-8 ohms is cause for celebration, as the rotor is unlikely to be bad with such a measurement (be sure to subtract the meter's own test lead resistance from the displayed number.
If the number is below 4 or above 8 ohms. you've found a faulty circuit to break into and find cause. Low numbers can damage the R/R.
You will have to measure directly on the slip rings to the rotor. 2 ohms is a bad rotor that needs replacement.
If you find a good rotor, and slip ring connections and a good reading at the RR/connector.
Then you can verify the stator windings. Disconnected, there should be very low ohms between each pair of yellow wires. And a very high number between any yellow and the engine case. Unless there is an axe mark or cover damage,, the stator is probably alright. But, it is good to build a knowledge base about your particular bike. As the list of known good things gets larger, the list of suspected bad components gets smaller until you get to the AH HA! moment.
If the bike has experienced a battery connection error (poles reversed). The rectifier takes a permanent vacation.
The general test is to measure resistance using the diode test function on a DMM between the Red and Green wires. You should find a rather low reading with one probe polarity and a very high reading with the other probe polarity. A more thorough procedure is to make twelve tests, two for each individual diode in the rectifier. Yellow to green both probe polarities, and yellow to red both probe polarities. Again good diodes will display a low reading with one probe polarity and a high reading with the test probes reversed.
The only things left are the regulator and the bikes wire connectivity. You will have to start probing for voltage distribution losses to verify or eliminate those candidates. And the regulator can be temporarily bypassed to identify it as a problem cause. But, check the above items first and share the knowledge gained for a more directed approach to remaining possibilities.
I am assuming you already have a wire diagram for your bike to which to refer. If not, give up now and take it to a shop.
Cheers,
