Headlight dims with increased engine rpm - Honda Rebel Forum
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post #1 of 8 Old 05-16-2019, 12:23 AM Thread Starter
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Headlight dims with increased engine rpm

I started having the following electrical problems a couple of days ago:
The first thing was that the starter stopped working. I can still run and pop the clutch to start the engine.
Tonight I noticed that the headlight dimmed (a lot) as soon as I increased the engine rpm. It is much brighter with the engine idling.
Also, the headlight dims when the brake light is on.
Also, the indicator lights for the high beam headlight and neutral light do not light up when the engine is shut off and the ignition key is on (as usual).

I have not had a chance to check the starter solenoid and its 20 amp fuse.
I'm thinking maybe the battery is bad??

Thanks for any input that you can give me!
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post #2 of 8 Old 05-16-2019, 06:05 AM
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it could be all of the above plus a bad ground somewhere

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post #3 of 8 Old 05-16-2019, 09:14 AM
 
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it does sound like not enough V+ in the electrics. bad battery or bad connection from battery negative to the frame.


what's the battery voltage just sitting with everything off?
what's the battery voltage drop to when you hit the starter button (yes I know it doesn't start but that could be from not enough juice)
when you push start it, and let it sit in N in the driveway, and disconnect the Negative power cable from the battery, what's the voltage on the battery cables. not the battery but the cables.
and does revving the motor with the battery disconnected cure the headlight getting dim?

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post #4 of 8 Old 05-16-2019, 09:50 AM
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If you can start it by jumping from a car battery, the bike battery is weak. Do not do this with the car engine running, it can damage the bike's electronics. Yuasa makes top quality batteries for the Rebel.
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post #5 of 8 Old 05-16-2019, 12:10 PM Thread Starter
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Thanks for the ideas. The results of voltage measurements:
Battery with everything off: Little over 10
Battery after turning on ignition key: less than 1
Battery with starter button pressed: around 1
After starting, in N and negative cable disconnected, measuring across cables:
Idle: Little over 10
Increase rpm to around 1300 (guessing by ear): Little over 13
Increase rpm over around 1300: Drops to 6
Revving with battery disconnected does not solve headlight issue, it still reduces brightness

I went ahead and replaced the battery since it is a few years old. I also increased the idle rpm to around 1300 or so, at the point where I measured the max volts on the cables. This solved the starting and headlight problem for now.

However, It seems to me that I have an alternator problem or maybe a ground problem.
Your thoughts?

Thanks for your help!
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post #6 of 8 Old 05-16-2019, 12:21 PM
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You are getting those readings with a new battery?

Recommended idle is 1,400 give or take a few hundreds. Needs to be set when the engine is fully warmed up.

Keepin' all the left over parts. Gonna use 'em to build another bike!

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post #7 of 8 Old 05-20-2019, 08:32 AM
 
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10v with everything off, drops to 1v when you hit the starter button.
definitely a bad battery.


moderate rpms 13v is ok, buthigher rpms and the disconnected line voltage drops to 6v.....not good ... might be why your battery went bad.
sounds like probably the voltage regulator is bad (not cheap).

or less likely one of the coils in the stator (generator, also not cheap) is bad.

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"The bravest thing for me to do is admit when I am wrong" - unknown
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post #8 of 8 Old Yesterday, 01:05 PM
 
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Sounds like the problem I had that we decided was the rectifier(Voltage regulator). Before I got it replaced, it fried a battery that was only about 6 mos. old. Found a reasonably priced wreck-ti-fier online. Bon chance.

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