Headlight Burn Out?

So I'm riding in Bellingham, WA late last night. I turned onto a country road going about 45mph and switched on my High Beam. Not 10 seconds later it's suddenly totally dark. My headlight went out. Thankfully I was on a straightaway. I pull the clutch and shift down, pull to the side of the road and kill the engine. I find that my Low Beam is still working and start up again, turn around and start heading back to the city, a moment later the Low Beam goes out too. The LB going out right after the HB makes me think it must be a wiring issue rather than the bulb being at the end of its use.

This is a recently buttoned up Honda 350 Cafe, running one of THESE 55/60w Hi/lo H4 headlights that I modified to fit a 35/35w bulb by cutting a small provision where the bulb is seated. I did the entire wiring harness from scratch and so far have not had a blown fuse or any issues. I only ran a single fuse coming off of the + terminal of the battery. The charging system has been modified between the stator and R/R by combining the yellow and white wires from the stator to connect with the yellow from the R/R so the battery is also being charged when the headlight is off. This is a fairly new bulb, I've only been using it for a couple weeks and have riden with the light on for 50 miles or so with no noticeable issues.

So I'm trying to figure out why it blew. Did it act as a fuse and melt when drawing too much current? Could this be resolved by using a higher wattage bulb or a resistor?

What size wire is run to it?
Have you checked your alternator output?

Sounds like you might have a couple of different issues.

If you're losing bulbs in a short period of time, the culprit is usually a bad regulator. Too much voltage means too much current and too much current makes things pop.

That said, it should be pretty hard to blow a 55W or 65W bulb since your charging system usually isn't capable of powering one, let alone creating more power than one can handle.

The first place to start would be getting a new headlight bulb onto the bike (I don't recommend anything over 35W, switch to HID or LED if you want a brighter headlight) and then hooking up a voltmeter. Make sure you're not going over 15V, even when the revs get up there. If you're using an AGM or LI-ION battery, 14.5V is really as high as you want to see.

SONIC. said:

What size wire is run to it?
Have you checked your alternator output?

Click to expand...

The wiring run goes from the battery to the keyed ignition, to a 14ga bus into a 16ga wire at the RH control cluster, which leads to the LO/HI switch on the LH control cluster, and then to the headlight. All is 16ga except for the 14ga + bus.

I haven't checked the output of the alternator yet. How would I go about doing that?

Sonreir said:

Sounds like you might have a couple of different issues.

If you're losing bulbs in a short period of time, the culprit is usually a bad regulator. Too much voltage means too much current and too much current makes things pop.

That said, it should be pretty hard to blow a 55W or 65W bulb since your charging system usually isn't capable of powering one, let alone creating more power than one can handle.

The first place to start would be getting a new headlight bulb onto the bike (I don't recommend anything over 35W, switch to HID or LED if you want a brighter headlight) and then hooking up a voltmeter. Make sure you're not going over 15V, even when the revs get up there. If you're using an AGM or LI-ION battery, 14.5V is really as high as you want to see.

Click to expand...

The R/R is a modern combo unit (from Sparck actually!) I'll get a new 35w bulb and see how it runs/measure the voltage. Could I verify the R/R is functioning properly by measuring voltage between the green and black wires? (I'm unsure because of the conversion between AC and D.C. that takes place in the unit.)

Without an adjustable power supply or other testing tools, diagnosing the regulator can be difficult. The best method for the home mechanic would be to measure voltage across the battery terminals with the bike running. Give it some revs and you should see the voltage increase, but then cap out at a max of 14.5V.

Our unit is bench calibrated to 13.9V, which usually ends up being about 14.2V in the real world, but due to stacked tolerances this number can be higher or lower. What you're looking for is that the voltage never exceeds 14.5V, regardless of how high you rev it.

Sonreir said:

Without an adjustable power supply or other testing tools, diagnosing the regulator can be difficult. The best method for the home mechanic would be to measure voltage across the battery terminals with the bike running. Give it some revs and you should see the voltage increase, but then cap out at a max of 14.5V.

Our unit is bench calibrated to 13.9V, which usually ends up being about 14.2V in the real world, but due to stacked tolerances this number can be higher or lower. What you're looking for is that the voltage never exceeds 14.5V, regardless of how high you rev it.

Click to expand...

The R/R appears to be functioning normally. The battery measured around 13.3V and climbed no higher than 14.2V revving. I suppose the next step is to test the voltage across the 35W headlight.

Unless there are some added electronics to the system that are not stock, it shouldn't be possible for the system voltage to read higher anywhere than at the battery.

Vibration issues would be another possibility if the voltages all check out.