Yamaha WR450 03 Headlight dims when reving

Hello all,

I have a standarf WR450F 2003 Model. I noticed today that when I rev it the lights go slightly dim. The harder I rev it the dimmer they seem to go. battery has over 14volts when bike is running. any ideas? :p

Regulator / rectifier?

That's generally a sign that the battery isn't maintaining a full charge. How old is the battery? With 14v (DC) going to the battery whilst the bike is running, the charging system is probably fine. It wouldn't hurt to check/reseat the connectors and terminals to make sure there's no corrosion on a terminal, connector, or ground.

Try charging it overnight and see if the headlight still dims. If everything else looks good, replace the battery. Remember that you should charge the new battery for a couple of hours (off the bike) as the bike's charging system really isn't designed to handle charging a dead battery, running lights, and powering the CDI at high RPM all at the same time.

I took the battery off the bike with it running. Wwhen I held a multimeter to the connectors it was only showing 7.1volts going to the battery. I then tested the reg/rec via what the manual says and it only had 7odd volts coming out of it and also when i reved the bike the voltage dropped.....

Any ideas?

Check the Stator and regulator voltages. Check for AC on the regulator output. The battery acts as a capacitor so expect to see "pulsating DC" on the regulator output. You should not, however, have any AC on the regulator output.

I'm curious how you had 14v + going to the batter before, but only 7.1v now. In any case, put the battery on a trickle charger overnight. It should read 12.6-13.2v in the morning (slowly dropping to 12.6v). If it does not, it's toast as well.

Voltage drops when you rev the bike because the CDI creates more load since it's firing more times per second. Ignore that part for now and stick with addressing charging and battery voltages at a fast idle.

I didn't have 14v I think that was my mistake. The battery had 14v with the bike running as it was being charged. It was still only charging it at 7.1 volts though. I was going to check the coil resistdence and see if it's that. If not it must be the ref/rec

Took rectifier off. Started the **** up. Reved it a little lights are going bright as they should.

I have been told that the lights are only bright at low RPM on this bike and as soon as you rev it up the regulator kicks in to stop the lights blowing hence they dim slightly..

A few things.

First, go over ever single connection, ensure every terminal is clean and good. Pay extra attention to all ground connections.

Next, You need to test the electrics, starting at the stator exactly according to the service manual. Do not jump about or second guess it. For example, you cannot do a charge output test with the battery disconnected or low on charge or a bad battery. The battery must have a good charge on it (over 12.8 volts, engine off, over 12.5 engine off, bike switched on). At idle, power to the battery should be at least 13.5 volts, at 4,000 or high rpm, 14.5 volts.

Depending on how your bike is wired (US bikes have a seperate circuit for the headlight and it is AC) You must have the stock sized bulbs installed. Being there is no 'AC Battery' there is nothing to absorb power fluctuations and the system is 'balanced' to work properly. So certain wattage bulbs are required. Again, do voltage tests at idle then at speed.

I am wondering if its because I have now fitted a 2008 headlight. I have only put in a 25.5watt bulb...

But this shouldn't effect it. As the rear light still dims slightly without the front headlight connected. Although when I first start the bike and rev it the light brightens as noramal it seems that when the battery gets back up to full charge it dims slightly when I rev it

I was going to put in the brighter 35.5 halogen bulb.

William do you know if the 2007-onwards put more power out?

Here is your first error

1>Jumping around instead of methodically testing and checking

And the second:

2>Not using standard components withouth confirming compatibility

I cannot be of much help unless you test it correctly. The headlight is AC and being there is no battery on it, it is critical the system is properly balanced with load to function properly.

I assume the wiring is stock too. If not, you will have a very hard time resolving it without a greater knowledge of wiring and lods.

You need to check the wires and all connections. You have to perform the tests in the exact order they are listed in the manual.


I have checked all connections :thumbsup:

I have tested everything on the charging system,stator,coil,regulator,cdi box,battery...The charging system is working probaly the battery is charging at the correct voltage!

The only thing I have changed is the headlight...So I am guessing it could be this...and the fact i had ran the bike without the headlight could like you say "unsettle the way the volts is passed around the bike" I will connect the stock headlight back up and see if the problem goes away...Cheers mate

remember when you rev up the bike is making more charge so maybe its just the rectifier trying to keep an already 14 volt battery from roasting due to overcharge, 14 volts still seems a little high unless the battery is low and its trying to charge, does the battery die? had any problems?

Man I wish guys that start these threads would come back and let us know what the end results are!!! :banghead:

I have the same issue......

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