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I am having a weird problem with my headlamp on my 1983 Honda FT500. When I go out to start it in the morning, insert my key and turn it (engine still not started tho), the headlamp comes on as one would expect, and is quite bright. When I start the motor, however, the headlamp immediately dies. I think I have noticed that if I let the bike run for several minutes that the headlamp will come on again.

Update: Unfortunately I do not think it is a problem with the battery. Today I started the bike and let it idle for about 20 minutes. The light still didn't come on. Then I drove it around (locally only) for about 20 minutes. The light did not come on. When I got home I took the battery out and used a multimeter to test the voltage and it showed 12.85, which I understand to be quite strong. The bike starts quickly, too. So the fact that the bike starts quickly, and the battery maintains a strong charge even after a 40 minute idle + a 20 minute ride makes me think it is staying charged as well.

What would be causing this?

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  • If the light goes from bright before starting to completely off when started, I'd look for electrical connectivity problems, like a loose cable or switch, or maybe a weak relay.
    – barbecue
    Commented Oct 15, 2015 at 1:25

2 Answers 2

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Your battery might have died or is in its last legs, when you start the motorcycle the start motor consumes a lot of energy and thus it causes shut down on the headlights and other electronic stuff, in my case it happened to be my instrument cluster.

After running a while the alternator will charge the battery again and so the lamps come on.

To test the battery use the multimeter/voltmeter which will have two prongs sticking out red(+ve) and black(-VE) turn the middle knob to 12 or 20 volt scale, connect the prongs to the corresponding terminals on the battery the reading should be around 12.24 to 12.66.

    12.66v . . .  100%
    12.45v . . .  75%
    12.24v . . .  50%
    12.06v . . .  25%
    11.89v . . .  0%
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  • This would be distressing to me since the battery is brand new as of 2-3 months ago. However, I do have a trickle charger, perhaps I should put it on the slow charger for a couple days to bring it back up to strong charge? Your answer does make sense tho, thanks!
    – zeeple
    Commented Sep 25, 2015 at 16:47
  • If your battery is new then I would suggest checking the alternator or the rectifier. It might not be charging the battery.
    – Shobin P
    Commented Sep 25, 2015 at 16:59
  • Unfortunately I do not think it is a problem with the battery. I just started the bike and let it run for about 20 minutes. The light still didn't come on. Then I drove it around (locally only) for about 20 minutes. The light did not come on. When I got home I took the battery out and used a multimeter to test the voltage and it showed 12.85, which I understand to be quite strong. The bike starts quickly, too. So the fact that the bike starts quickly, and the battery maintains a strong charge even after a 40 minute idle + a 20 minute ride makes me think it is staying charged as well.
    – zeeple
    Commented Oct 14, 2015 at 20:47
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I have exactly the opposite problem. My battery has good charge and shows good charge when at idle. When I turn the ignition key to on and the bike still not started the headlight comes on gradually, and only dimly at that. When I kick start the bike the headlight is bright, the neutral light is bright, the turn signals are bright. This is a 1986 Yamaha SRX600 and has only kick start, no electric start. I do notice that when I engage the turn signals that the headlight and neutral light flicker when the turn signal engages. This is leading me to believe I need to replace the turn signal canceler (yes, this old crappy bike has THAT! LOL). Another thing leading me to this conclusion is the fact that when I start the bike the turn signal is on - even though I KNOW I did not have it on when I shut down the bike! Also, I revved the bike to about 5000rpm and the turn signal turned OFF! This was at a stand still and should not have cancelled the signal.

As to your problem, have you checked your fuses for current draw? There is a YouTube video about that. Albeit he's testing a car, this method works to tell you where to check on your bike when you get a reading from a fuse that should basically be at zero when the bike is off and key out of ignition. This is known as parasitic amperage draw.

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  • As far as a bad regulator/rectifier goes. That's a pretty simple battey test. Start the bike, put the black test lead to the black or ground of the battery and the red lead to the positive or red lead of the battery. If the readout is less than 14 volts it is likely you have a bad regulator. Common problem in any bike. I upgraded mine to a MOSFET unit from China - bought on Ebay for a fraction of the cost, and it works splendidly on my 1986 Yamaha SRX600. MOSFET regulator/rectifiers operate at lower heat levels and are way less prone to shorting out due to overheating issues.
    – Stan
    Commented Oct 27, 2015 at 14:20
  • Also, inspect your connectors for the regulator/rectifier. Any burnt indication is a sign of heat overload and not only do you need to replace the rectifier, you need to splice on a new connector. Many companies realize this and like the one I bought it came with the entire connector (both ends - including the spade connectors). You simply cut the connector off your bike, crimp the fresh wires to the spades, and the spades lock into the connector. Then connect to the rectifier and re-test your battery at idle. Should be between 14-15 volts and no more than the 15 volts.
    – Stan
    Commented Oct 27, 2015 at 14:25

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