Yamaha YBR125 losing charge

[Cosmos_Epsilon]

Hi all,

I bought a 2012 Yamaha YBR125 a couple of weeks ago second hand under the description that it was a good runner... fast forward a couple of days later and I see that the engine management light had come on and flashed to let me know it was code 46 - an issue with the charging system.

I checked the battery with a multi-meter and found that the battery was sitting at 10.6 V and no matter how many times I bump started it I couldn't get the battery to charge.

After watching some videos and reading forums I figured it was either the battery, the reg/rec or the stator that was causing issues. When I unplugged the connection to the reg/rec I noticed that one of the pins was badly corroded and so after cleaning up the connection and running it, the battery would sit in the low 12 V region even up at 6000 rpm. I ordered a new battery and installed it and all seemed to be well, could it have been as simple as a dodgy connection? We did check the wiring of the alternator with the multi-meter along with a visual check of the stator coils. We also checked the diodes within the reg/rec with the multi-meter and we were getting expected results.

Anyway, took it out a run yesterday and covered about 90 miles over 3/4 hours and there were no issues. We stopped every now and again and checked the battery with the multi-meter with the following results:

Battery out of the box: 12.4 V

Battery after 20 minutes of riding: 12.7 V

Battery after another 20 minutes riding: 13.06 V

Battery later in the day (with engine off) after ~80 miles: 12.8 V

These numbers would suggest that the battery was being charged and holding the charge throughout the day.

I became quite happy that the issue was solved quite cheap and managed to get a day out of it. However, when it got dark on our way home I rode with the full beam on for most of the journey home (approx the last 10 miles). I did notice towards mile 10 that the full beam light started to flicker and then once again the engine management light came on (not flashing for code 46, just a solid light which i presume shows low voltage). I managed to get home and check the battery and it was showing 11.9 V.

I went out this morning and checked again and the battery has risen slightly to 12.2 V. I thought maybe the full beam had drained the battery and so I started it up to see if the battery would charge if I took it out later to charge on a ride.

When I rev the engine (from idle to 6000 rpm) the maximum the battery voltage rises to is approx 12.6 V before dropping to ~11.9 V and steadily falling when idling - this would lead me back to a charging system failure.

I'll charge the battery up properly and see where it sits when fully charged but it seems a bit strange that I got so many miles covered and it was only when I used the full beam for a few miles I drained the battery.

Any advice would be helpful.

Stephen

[Mississippi Bullfrog]

You really want to be seeing 13.5v and above at 6000 rpm.

It sounds like the charging system is just about coping but not when you put full beam on. If the voltage drops under load then the battery will gradually discharge.

[WD-40]

It could be that the corroded terminal on the reg/rec causing high resistance so you're getting low voltage and current out of it. You need to check the AC voltage going into the reg/rec and the DC voltage coming out of it. If the AC voltage going in is low the stator or the wiring between the stator and the reg/rec is the problem. If the AC voltage going into the reg/rec is good but the DC voltage coming out of is low it's a fault in the reg/rec or the wiring between the rec/rec to the battery. There is a procedure you can follow. The numbers and wire colours will be different but the procedure is the same

https://www.r1-forum.com/threads/how-to-charging-system-fault-finding.294687/

[Cosmos_Epsilon]

I have carried out the following steps on the charging system and think I have narrowed it to a faulty stator coil.

First I checked the regulator/rectifier with the diode setting on the multimeter. With the black probe on one of the (i'm assuming) regulator outputs and the red probe on each of the three phase inputs I got a reading of 500 for all three phases and then 1 for all three phases when the black probe was placed on the other regulator outputs. When this process was reversed and the red probe was put on the regulator outputs I got all 1's and then all 500's when reversed. This would lead me to believe that the reg/rec was ok. I had ordered a new reg/rec which arrived and gave me the same readings when probed but just to be safe I installed the new reg/rec just incase. As the new reg/rec didn't increase my charging voltage (12.6 V max at 5000 rpm) I moved onto the stator.

On the stator I performed the following checks:

- Measurement of wire resistance between input A and B, A and C, B and C. All came out at 1.9 ohms (Haynes manual said the max value should be roughly 0.67 ohms so roughly three times higher).

- Checked if the stator was grounding out but got no reading so i think this is good.

Now the interesting part, I unplugged the stator and measured the AC voltage against all three phases while the bike idled and also at 5000 rpm with the following results:

At idle

probing A and B phase: 13.2 V AC

probing A and C phase: 13.5 V AC

probing B and C phase: 17.0 V AC

At 5000 rpm

probing A and B phase: ~15 V AC

probing A and C phase: ~33 V AC

probing B and C phase: ~26 V AC

This variation and low voltage values would lead me to thinking that the stator has had it. I did have the coils out a few days ago to inspect and all looked ok but I know a small break could go unnoticed. My main question is are there any further checks I can do to confirm the stator is at fault before I fork out £100+ on having the coils rewound? I don't really want to pay for a cheap chinese copy, especially since the delivery times are 2+ weeks and i'm itching to ride.

Cheers

[WD-40]

I think going by the AC voltage measurement you can say the stator needs to be replaced. The voltage across A and B didn't really increase when you increased the revs to 5000RPM. It should increase and all three phases should be the same

For your resistance measurement did you take into account the resistance of the meter leads? If you didn't, you should touch the two probes together and whatever reading you get take that away for your measurement.