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CBF 125 Alternator Noise - No Lights

Blog Date - 17 January 2015

Queer. Really queer.

I ride the bike to work in the morning, all systems normal, all things as they ought to be. I leave it locked up for 9 hours then return to it. I start it and the first thing I notice is a peculiar noise. This is hard to describe but I'll try my best. Imagine the regular noise of a CBF 125 engine but quietly there is some doing an impression of a monotone ghost. Remember as a kid ghosts made "oooooooooooo" noises, you'd vary and alter the pitch in a spooky manner. Make the same noise but just one note, fairly quietly while the engine is running.

Hmmmmm, thinks I. Camchain? stuck starter gear? some balancer shaft gear stuck? A gear loose on it's spline? I restart the motor to see if it's the starter. Same problem, there's a monotone ghost in the engine. Well, it doesn't sound healthy but I'll risk running it home. No I won't, there's no lights either. Poop.

The CBF 125 is unusual in that the lights run straight from the alternator and not off the battery...as such.There's 4 wires to the regulator rectifier. 1 inbound from the alternator. 1 outbound to the general electrics like the battery, computer, fuel pump and so on. 1 Outbound that feeds the headlight, side light, rear light and clock light. 1 earth. This means while the fuel, computer and brake light can run off the battery the running lights need the engine to be running to be lit. I suspect this is a super simple way of ensuring buffoons like me can't leave the lights on by mistake. The downside is 2 fold, at tickover the lights dim somewhat and if you stall then you're left in the dark.

My first suspicion is that the flywheel that also holds the magnets for the alternator has come loose, aka woodruff key failure. As the crank spins the flywheel is not turning, this might cause the spooky noises and the lights to fail. The rest of the bike is still working but just on the charge in the battery. I have to make the decision that the RAC is taking me home tonight. Damn.

Back home feeling downbeat, tired and grumpy I remove the alternator cover, expecting to find a mashed up woodruff key, the flywheel and magnets hanging off and a trashed stator, the bit with all the coils on. Nope. Not only does everything look fine but I note that the crank position sensor is in there and on the flywheel are blobs of metal. These blobs as they pass the crank position sensor tell the computer where the crank is, for example top-dead-centre, going up, going down etc etc. If there was anything wrong with the flywheel and how it is mounted the computer would get all the wrong signals and the bike would not start.

It's definitely not the woodruff key. 

After some sleep I return to the problem. Everything checks out as per my Haynes manual. The ohms across the 2 wires from the alternator (earth and feed to the regulator rectifier) are within spec. The charge seems to be a little lacklustre at 13.5V giving it some revs but it's enough. The battery without load is 12.4V, acceptable. 

None of these electrical problems explain the noise either. I disconnect the alternator so there's no load on it in case there's a short on the lighting side. The noise is there. Is it possible 2 problems have arisen at exactly the same time? Possible, but unlikely.

Perhaps...perhaps the starer motor is jammed on in some peculiar way? This is causing the motor to act as a dynamo but with reverse polarity causing some weird load or cancelling effect. As you can tell I'm clutching at straws now. I disconnect the starter motor wire. No difference. I even remove the starter motor gears to eliminate the possibility of random electrical magic, no difference.

I inspect the stator again. No gouges, no scoring, nothing untoward other than a little crusty baked on oil near the terminals. Perhaps, but that wouldn't make noise, would it. 

In a final process of elimination I remove the stator and manage to keep the crank position sensor in place. There's a hole now in the crankcase where the wires pass through but I can live with that for a short run of the engine. I start the motor. Apart from the little crankcase puffing through the hole the motor sounds back to normal. Yippee!!! Sort of.

the stator hanging outside of the engine (cbf 125)
Well, hanging the stator outside the motor makes it quiet. Won't charge like this though.

Do alternators make noise? I mean car ones have bearings that rattle as they get old. Alternator belts squeal. I know when I put the lights on in my car I can hear the engine revs drop just a hint as the load on the alternator drags the engine down. But how can they make noises? Perhaps when there's a big load some eddy currents cause vibrations? But there's no load, not if the alternator is not connected to anything at all. Perhaps...perhaps...clutching at straws but perhaps there's a short in the coils and the current is going to earth? Maybe the short is somehow making the coils generate MAXIMUM power? With the bike being computer controlled fuel injection the computer is compensating for the drag of the coils by upping the air valve.


Reader's Comments

Max said :-
My son has exactly the same problem with his CBF125. We have changed the rectifier/regulator without effect. He is currently charging a (new) battery every night as it seems to be discharging whilst running. The noise is intermittent. It is going into the workshop tomorrow so we hope the mechanic can trace the problem. Most baffling.
1/1/2000 UTC
Ren - The Ed said :-
I changed to alternator stator - the bit with the windings and all was well. Not a cheap part though.
1/1/2000 UTC

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