voltage dropping from alternator when revving up bike

[777]

hi,

i did a check on the alternator using the battery terminals, without the battery connected. when i rev the engine up the voltage drops, is this normal? its starts at 13.4v on tickover and drops down to about 8v at high revs. i bought a new battery because the other one wouldnt turn the starter motor. now the new battery is doing the same, its at about 12.4v. any ideas?

its a yamaha ybr 125 btw

[Pete.]

Sounds like it's not charging and the extra load of revving the engine is pulling the battery voltage down. Un-plug the rectifier from the engine loom and check the AC output of the alternator with the engine at bout 5k rpm. Should be up around 50 or 60 volts AC at a guess.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[777]

hi,

yes, im gettting about 55v at the regulator.

[Pete.]

Then check over the plug connections for the regulator rectifier and make sure none appear burnt or corroded. If they are ok check the DC output of the reg/rec at the plug. If there is none the reg/rec is dead, if you get a fair output from the reg/rec you probably have a fault on the neutral side.

I trust you have checked the battery connections are clean and tight?
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[777]

yes, ive done this and they are all ok. not burned or corroded and the battery terminals are fine

[777]

yeah, im only getting about 7v at the rectifier

[Pete.]

Reg/rec is toast then. Easy fix, just swap it out.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[777]

cool, thank you

[777]

ok. i replaced the voltage regulator, but did a re check on the old one. it seems to be putting out the correct voltage, now. i must have checked the wrong pins. how do i fix the earthing problem?

[777]

[Nobby the Bastard]

Make sure the earths to the battery and the engine are secure to the frame.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[777]

[doggone]

[777]

the small wire just goes into the loom, so no idea where it ends up, anyone have any idea where this goes?

there is continuity from the engine to the frame and the battery, also

[Pete.]

[777]

[777]

[777]

fixed it.it was just the relay contacts needed cleaning. no need for a new regulator