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Headlight dimming at 3000 rpm's...
- Thread starter katmol
- Start date
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- Tags
- electrical
Have you measured the voltage as you rev to 3000 rpm?
I have not measured the voltage. I'm guessing it's dropping and that what's causing the light to dim. Just wondering what would cause it to drop at 3k rpm's.
You should not assume the voltage is dropping. Maybe it is and maybe its not, that's why you should measure it. You could have a defective headlight bulb/sealed beam. You also could have a loose or corroded headlight socket connection. Vibration is higher at 3000 rpm, which could be shaking some of the connections.
Does the voltage rise at 2000 rpm and at 2500 rpm, then suddenly drop at 3000 rpm? That would indicate a charging system problem.
So I am clear; the light is bright, gets brighter as you rev then drops at 3000 RPM? Check your voltage at several places, battery terminals, at the headlight connections, maybe at brushes and ignition coils. Also move the ground probe; battery terminal, engine case, frame, headlight ground connection, etc. as always clean and inspect every electrical connection you can find.
CDNTX650
XS650 Guru
Edit....Nvm. ...read your post wrong.
ScooterMagoo
XS650 Addict
This is a bone stock '75? You disconnected the battery and it kept running? Isn't that impossible?
When my 75 was stock it would quit as soon as I unhooked the power. Once battery power is removed the current flow through the rotor stops. This stops alternator output. No alternator output and the bike quits.
I would check the grounds and other connections.
leo
I would check the grounds and other connections.
leo
Hey Leo, OK then what has been done to my bike so that it runs with the battery disconected?
You are both correct in what you are seeing.
With these bikes, the voltage from the alternator is proportional to the engine rpm. As an example, if the rpm is a little low at say 1100 rpm, the alternator output drops off to maybe 11 or 12 volts, depending on what lights, etc. are on.
If the rpm is higher at say 1300, then alt output may be 13 or 13.5 volts.
If Leo's bike was running at 1100 rpm or less with lights on, the voltage may have decreased enough that the ignition HV became too low and the engine stopped.
If katmol's bike was running at say 1300 rpm, then the alt output was high enough to continue to give good ignition HV, and keep the engine running.
That's the reason, that the battery and alternator are connected in parallel. There is that area around 1200 rpm, where its a balancing act. When the alternator voltage falls below 1200, the battery takes over and supplies the current to the ignition and other loads. As rpm goes above 1200, the alternator takes control, supplying current to the ignition and other loads, and in addition starts feeding current back into the battery to charge it.
ScooterMagoo
XS650 Addict
Still doesn't make sense how the engine runs with the battery disconnected. No battery, no current to excite the field, no spark. Am I wrong? If he disconnects the battery and it continues to run, it has a PMA installed........or a second hidden battery somwhere, LOL.
Disconnecting the battery while the engine is running is a normal part of the regulator adjustment/test. The alternator supplies enough to fire ignition and be self-powered (barely enuff at idle). Of course, this only applies to the stock 70-79 models with points ignition...
Attachments
You're missing the fact that the battery and alternator are in parallel, and either one can supply current to the ignition coil to keep the engine running. Disconnect the battery and the current from the alternator is still present, as long as the rpm is high enough.
Even the 80 to 84 years should still keep running, with the battery disconnected, as long as the rpm is high enough.
Having said that, IMO its not a good thing to disconnect the battery with the engine running. The battery acts as a filter to even out the pulsating DC coming from the rectifier.
I was trying do the testing as the book called for, I unplugged the fuse and the engine quits.
I didn't try it at higher rpms. Maybe some rotors have a bit of residual magnetism which allows them to create some power when running that is enough to keep it running.
Leo
I didn't try it at higher rpms. Maybe some rotors have a bit of residual magnetism which allows them to create some power when running that is enough to keep it running.
Leo
Without a battery, the alternator output supplies the current that goes to the alternator rotor and to the ignition coil, keeping the engine running. As I said before, this only works if there is high enough rpm.
There is some residual magnetism, but that is very tiny and has no effect.
During normal operation, with the battery and alternator both connected, the battery never supplies current once the rpm goes over about 1350/1400 rpm. Its only when you stop at an intersection, and the rpm drops back to 1200, that current is drawn from the battery.
ScooterMagoo
XS650 Addict
OK, that makes sense. So, if the RPMs are high enough, it's self sustaining. It's producing enough of its own current to keep the magnetic field alive. Drop below a certain threshold of RPMs, and the engine dies. I think I got it. Boy, I sure do love the idea of the PMA. I got a headache.
I've been driving motorcycles for 23 years, and they all used electro-magnet rotors in the alternator. In that time I've never had any charging problems. The stock XS650 alternator and charging system works extremely well for me.
Yes you are correct, if anyone is foolish enough to disconnect the battery, and run engine down below 1200 rpm , the engine will almost certainly die. Which proves what???
