A typical scooter charging system such as on the Modenas Elite 150 / 125 (Gen One) consist of a 2 phase charging system.
The Primary phase produces AC (Alternating current) Voltage of about 25 Volts, before the regulator rectifier converts that current to DC Voltage. This is for charging the battery and supplying power to the spark plug via the CDI.
The 2nd phase will produce approximately 6 Volts AC which does not get converted to DC but reduced to a 1VAC and provides for regular menthol filament bulbs. Yes the filament bulbs will still light up though a lower AC current because the amperes will go up. This is what makes adding a relay for additional accessories such as LED running lights a little tricky. Most 4 or 5 pin auto relays require a 12VDC signal to trigger the relay. When converting to LED headlamp bulbs or Brake bulbs, it becomes essential to take the pre switch power supply from the 12 VDC output. Naturally, this would also reduce slightly the charging power delivered to the battery and spark coil.
To identify a faulty rectifier / stator coil / wiring for these types of 2 phase (4 pin), firstly disconnect the socket. Using continuity mode (key ignition switched off), identify and establish the Ground Wire. Leaving the black probe in that pin, set the meter to DCV 20V and use the Red probe to identify the pin which returns to the battery / CDI for charging and spark plug supply. You should get a reading of 12. 6 to 12.8 VDC +- on a healthy battery. Once this has been established, switch the meter to 200 VAC. Leaving the ground / black probe connected to the ground pin on the socket, start the engine and quickly move the red probe to the other 2 socket pins. One pin should read 25 VAC+- while the other pin should yield 6VAC +-.
NOTE: This is okay to do for short while only as the battery will drain quickly without charge being provided.
If it does, the fault is most likely in the rectifier unless the wiring has been damaged and unable to carry the intended load. If does not read, the fault could be in the stator coil having shorted out / failed or the wiring from stator coil to regulator rectifier has been damaged. Since these rectifiers are Below RM 20.00. I would just buy a piece to see if it resolves the issue. If it’s not the issue, I can keep it or carry it as a spare.
Note: The meter cost around RM 40.00 on-line and serves to check many other house hold items too.
Motorcycle Maintenance editorial contribution by: Shamalan Jeyasingam