XS650: Charging Systems

Throughout production Yamaha basically installed the same charging technology. Rotor spun inside a battery induced magnetic field creating an AC current converted to DC by a slicone rectifier (6 diodes).

Pre 80 models had a mechanical regulator controlling voltage before the rotor.

80 on used a solid-state rectifier/regulator combi which controls voltage after the rotor.

These systems have provided reliable service for a long time now. Most problems stem from bad battery, worn rotor brushes, dirty or broken wire connections, dirty or worn ignition cylinder. If you have charging problems best fix it. Better sooner than later.

A bad battery kills a good charging system. A bad charging system kills a good battery.

Although easy to troubleshoot and repair good parts are becoming harder to find. Rewound rotors expensive and not necessarily reliable.

Earlier adaptations of later model RD350 alternators provided relief – but were still old. Advantages were instant current production and the ability to run battery-free. Today, off-the-shelf bolt-in kits are available.

In the garage are 2 bikes with PMA conversions (1 an RD350 conversion, the other from Ants – both points), 1 with total loss, and 2 with original systems (1points, 1 Boyer). One of the original chargers, 79, with mechanical regulator has decided to rebel. Battery flat after a couple of hours riding, headlight on.

I have a spare RD350 conversion boxed, in the corner. Do I put it in?

If your battery is suspect, the ignition-block, fusebox or wiring damaged or dirty – nothing you do will solve your problem. When you are absolutely positive these are ok, check them again. This is actually most of your work done.


What do you know about your bike? Curly rulz!!


Curly 2.2


Battery voltage: Static? (around 12.5VDC) … Active? (max. 14.5VDC @ 2000 rpm)

A quick Slap-test gives an indication how good your Regulator and Rotor are. I hang a 17mm ring spanner off a screwdriver, poition it close to the alternator cover and turn the key on. It should pull into the cover and release when the key is turned off.  The more positive it reacts the better. Mine was good.

Check voltage at your brushes. Green wire. Key on. Motor not running. It should be max. 0.3VDC less than your battery reading. Weak  voltages create weak magnetic fields. Check your wires. Battery, earth, fusebox, ignition block. No real point in going any further until you solve this. First time I had 5.4VDC. Cleaning the 6 wire connector gave me 11.5VDC. Battery earth strap both ends sanded and washed, 11.9VDC. Battery +ve, heavy end to starter solenoid. red to fusebox and ignition-block. Washing the ignition-block out with brake cleaner and cleaning the brush connection gave me the rest.

You may, now, only need new brushes. Check them. Min 7mm. The outside brush will be shorter than the inner. Its’ path each rotation is longer.

I usually remove the stator.

Gives direct access to the rotor. Meter on Ω, Ohms. Ring/Ring should be 5.0 – 5.5 Ω. Ring/Earth, infinity. Inspect for damage, discolouration. The dirtier and darker the windings the more likely the rotor is defect.

Check the stator. Damaged wiring? Discolouration? Smell? Mine was quite oily. Where did it come from? Crankseal? Cable-pass hole? Gave a quick sqiurt with brake cleaner. Unplug the 6-wire connector. White/White should be around 0.5Ω. All 3 combinations. If not disconnct the single yellow wire and repeat (tests the safety relay). White/Earth – infinite. Plug together. Motor running. Meter to VAC. White/White 10.5-11.0 VAC. Low readings indicate a grounded Stator. High readings, a bad Rectifier.

TBC: pics, rectifier and regulator tests

So, if you’re having problems with your original charging system don’t simply throw it away and get a PMA. Check it. When you find a problem keep checking – there may well be more than one issue.

For me here, once I sorted the voltage at the brushes I was fine. A quick readjustment of the regulator and all was good – didn’t even need to replace he brushes.

If you do decide to replace it, be aware that if you have electronic ignition you will need to replace that too.

Ants produces a direct swap kit for points models that i am personally very happy with. Powerdynamo also have a combi PMA/Ignition.

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