Battery Not Charging

The battery died on my WR250. but I'm fortunate to have access to another identical WR250 so it was easy to do some cross-checking.

On the good bike, the voltage across the battery was 14.6v with the engine running - as expected.
On the bad bike, the voltage was 12.1v - ie the battery was not being charged, so that seemed to be the problem and I just needed to figure out why.

I followed the troubleshooting guide in the manual:
- unplugged the stator/regulator connector and measured the stator resistance at the connector = 0.5 ohms.  Within spec, so stator looks ok.
- checked the stator wires in the connector to ground and no resistance, ie no short to ground.
- reconnected the stator/regulator connector
- unplugged the other regulator connector and measured the voltage output with engine running = 14.6v.  Within spec, so regulator seems ok.
- using the charging circuit diagram in the manual, I checked the continuity of the wiring harness.  All ok.

Now starting to get confused!

- Next, started the engine and measured the voltage across the battery.  Still only 12.1/12.2v but when I disconnected the battery while the engine was running the voltage across the battery wires (not the batter itself) went up to 14.5/14.6v.

So, it seemed as if the battery was 'dragging down' the voltage from the regulator.  Regulator or battery fault?

Having access to a second WR250 I was able to swap the regulators, which is where things got even more confusing.
I put the regulator from the good bike on the bad bike but it made no difference.  The voltage across the battery was only 12.1/12.2v with the engine running, ie not charging.
I then put the regular from the bad bike on the good bike and charging was ok, ie about 14.5v across the battery.
So this would seem to prove that both regulators are ok.

So then I swapped batteries.  Unfortunately, one is a lead acid battery and the other is Lithium so they are not identical, but they were all I had.
On the good bike, the voltage across EITHER battery with the engine running was about 14.6 volts, ie charging was ok
On the bad bike, the voltage across EITHER battery with the engine running was about 12.1v for the lead acid and about 13v for the lithium (which is the voltage when not connected to anything).

So, the bad bike doesn't charge the battery because it doesn't put a charging voltage of about 14.6v across the battery, even though
- I've swapped the regulator (both work ok on the good bike)
- I've swapped the battery (both work ok on the good bike)
- the bike generates 14.6v when not connected to a battery
- the stator resistance is within the manual spec

The batteries themselves seem ok because they can be charged with a separate 'smart' charger (NOCO) and will start the bike ok many times, until they get run down through not being charged by the bad bike.

I'm pretty much out of ideas now and am thoroughly confused!

Any suggestions before I take the bike to a dealer to investigate?

KingArthur wrote:

- unplugged the stator/regulator connector and measured the stator resistance at the connector = 0.5 ohms.  Within spec, so stator looks ok.

The spec is 0.168-0.252 Ohms, so why did you conclude that the stator was good?

johnkol wrote:

KingArthur wrote:

- unplugged the stator/regulator connector and measured the stator resistance at the connector = 0.5 ohms.  Within spec, so stator looks ok.

The spec is 0.168-0.252 Ohms, so why did you conclude that the stator was good?

Where did you get that spec from?

The spec I used was from the Yamaha WR250F 2021 Owners Service Manual, stated as 0.368-0.552 ohms.

My meter reading was within that spec, hence my conclusion that the stator is ok.

This is the relevant page from the manual:

The question over the stator resistance made me realise that one test I didn't do was to test the stator on the good bike.

Just to recap, the 'bad' bike is a 2021 WR250 and the 'good' bike is a 2020 YZ250.  Basically the same but not totally identical.

Anyway, I've just checked the 'good' bike stator resistance and it's 0.8-0.9 ohms.  This is much higher than the 0.368-0.552 figure for the 'bad' bike according to the service manual, yet it works correctly (ie, puts 14.6v across the battery when the engine is running).  I don't have the service manual for the 'good' bike so I don't know the actual spec for the stator resistance.  All I know is that it works.

Curiouser and curiouser.

Quick update:

I found the YZ250F 2020 Owners Service Manual.

Checking the stator is the same procedure as for the WR250F, ie disconnect the stator coupler and measure the resistance between the two white wires.

The coil resistance spec is stated as 0.512-0.768 ohms.

So, higher than the WR250F spec and my measurement of 0.8-0.9 ohms suggests it is (slightly) out of spec, yet it all works and correctly charges the battery.

KingArthur wrote:

The spec I used was from the Yamaha WR250F 2021 Owners Service Manual, stated as 0.368-0.552 ohms.

This entire forum is about the WR250R, a street-legal dual sport bike that shares no components with the WR250F.

Thumpertalk has a WR250F sub-forum with lots of knowledgeable people.

My apologies.

Perhaps a moderator could delete this entire topic?

Thanks

who has a meter that can accurately read a half an om and less? I don't, and I'm a professional mechanic (over 40 years in aircraft)
anyway, a good method for testing stator windings is by using a lightbulb. hook a 12v incandescent (not LED) between pairs of stator wires. check the light at various RPMs but be careful, the voltage is unregulated and can easily burn out the bulb. you can also measure AC voltage between windings at different RPMs, like idle, 1/4 throttle, 1/2 throttle. . they must match pretty close.