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 on Sunday 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 yesterday 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 went out this morning 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

[Nobby the Bastard]

Your charging system is fine. The fact that the voltage goes up after usage showals that.

The battery is either fucked or theres a parasitic draw.

Charge the battery and then disconnect from the bike.

Measure voltage. Leave for two days and then measure the voltage again.

If the voltage goes down the battery is at fault. If it doesnt you have an issue somewhere outside of the charging system that's draining the battery.

Have you checked to see if you can top.up the electrolyte? Distilled water is about 50p a litre at halfords.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[WD Forte]

When doing any low resistance checks always short the meter probes together first
as the leads have resistance too y'know ( black leads matter!)
then subtract this reading from the one you get from the object under test.

Your 5000 rpm phase to phase voltages look too low to me I'd be expecting 50VAC onwards

I have YBR here I'm working on, you should get about 14V across the battery terminals when the
engine is running, anything less than 13.5V is pretty useless.


Incidentally I have an original Yamaha YBR EFI stator here from an engine
that suffered main bearing failure but had a good charging system.
It tested out perfick, I might be persuaded to part with it
____________________
bikers smell of wee

[jeffyjeff]

[Robby]

Reg/recs can fail partially. I think it's the reg/rec, but charge the battery on a charger first and then try testing.

A shunt type reg/rec (the most common sort) uses 3 pairs of diodes to rectify the AC power to DC. Over time these eventually fail. Over not much time these days it seems.
They don't all fail at once, so the reg/rec may continue to work a bit - either outputting less power, or working up to a certain rev range and then doing nothing.

Stator failure is fairly rare. Reg/recs are practically a service item. If you can, replace with a MOSFET type. Never buy a cheap (£10) item off ebay, they will either not work at all, or fail in a fortnight.

[ZebraDriver]

Just a thought. Your bike is fitted with the correct headlight bulb? Many small bikes don't have any electrical power to spare and a headlight bulb that is rated too high could be just enough to tip it over the edge.

[Cosmos_Epsilon]

I've just put a new reg/rec on it and had the same problem, but decided to keep the reg/rec as it's now one less thing to go wrong (hopefully!)

Checked the headlight bulb with what the Haynes manual recommends and it is the correct bulb.

To save forking out £135 to have the stator recoiled I bought a used wiring loom and figured if everything is unplugged, cleaned and reconnected and it still doesn't fix the issue then the only thing remaining is the most expensive thing to fix - the stator. Last night we re-loomed the bike (a 30 minute job I was told - LIES! haha), started it up and i'm now getting 14.5 V going to the battery at 5000 rpm and 12.5 V when idling so this seems to have sorted the issue. Maybe a dodgy connection or chaffed wire somewhere in the old loom.

Now I just need to figure out the wiring to reconnect the indicators and headlight

[P.addy]

[WD Forte]

Classic 'swap and hope' that
____________________
bikers smell of wee