Motorcycle electrical systems have come a long way since the days of the original gas-lamp design.
Today we have everything from multi-faceted LED-comprised headlights to simple-but-bright, tried-and-true halogen setups, both of which are powered by the motorcycle battery.
Most motorcycle headlights illuminate automatically at the turn of the key. If it doesn’t, electrical troubleshooting can be a tough nut to crack, leaving some skillful home mechanics asking themselves, why won’t my motorcycle headlight turn on?
Here’s Why Motorcycle Headlights Won’t Turn On:
If your bulb and battery are both in good condition, your charging system components could be to blame. It could also be fray or short in your wiring harness or a blown fuse. An electrical inspection is part of routine service; a blown fuse or shorted-out wire needs to be replaced asap.
Blown Headlight Fuse
Your motorcycle’s electrical system includes a fuse box.
Contained in its fuse box is a fuse for each individual circuit that forms your bike’s wiring harness or circuitry.
The fuse’s sole function is to serve as a safety net.
When there’s an increase in amperage within the circuit, the corresponding fuse absorbs the excess charge before it damages your motorcycle’s ECU or other electrical components.
Once absorbed, the fuse breaks or blows, shedding the excess power.
Your motorcycle’s headlight may share a circuit with some other electronics, instrument displays, etc., or it may run on its own circuit. Regardless, your headlight’s course has a fuse.
If a there’s surge in the headlight’s circuitry, the headlight fuse will absorb it and blow to save your motorcycle’s electronics from damage. Your motorcycle’s headlight won’t turn on until the blown fuse is replaced.
Determining if your headlight’s fuse is the reason it won’t illuminate is as simple as inspecting your fuse.
The impact will have split the metal strip in half if the fuse is blown. If it’s in good shape, the metal strip in the fuse will be one solid piece without breaks.
If the fuse in your headlight’s circuit was blown, replace it with a fuse of the same rating, as not all fuses are the same.
Consult the wiring harness diagram for your specific make and year model in its owner’s manual for a comprehensive list of the fuses for each system, including your motorcycle’s headlight and the rating for each fuse.
Note: Installing a fuse with an incorrect rating can further damage your electrical system. Electrical work is a specialized skill; if you’re uncertain about uninstalling your fuse box, we suggest taking your motorcycle to a pro for a safe and speedy fuse examination and upgrade.
Aftermarket Accessory Leeching/Overloading
The engineers behind the electrical harness for your motorcycle designed it to integrate with a specific size battery that delivers the precise amount of electricity for your bike’s circuitry.
Therefore, each motorcycle uses a particular amount of voltage to power the amount required for their stock design.
Adding aftermarket upgrades like Bluetooth sound systems, amps, and speakers; air ride lifts; heated grips; and an electric locking trunk taxes your motorcycle battery with excess charging.
If your motorcycle’s battery and wiring system aren’t modified to coordinate with your aftermarket accessories, it could be stealing power from your headlight.
- If your motorcycle has halogen-bulbed headlights, the typical headlight bulb style, your headlight actually pulls quite a bit of power from your battery.
- On the other hand, LED-based headlights pull less power than halogen bulbs, but their wiring harness is designed with that in mind.
- Therefore, it’s not as simple as swapping out your battery for a more powerful one, regardless of which type of headlight your motorcycle uses.
If your motorcycle headlight doesn’t work or is dimmer than usual, and you have extra accessories wired into your battery, take your motorcycle to a pro for electrical diagnostics to see if the aftermarket upgrades are why.
Breaks in Your Grounding System
The electrical systems of modern motorcycles incorporate what’s called a grounding system or floating ground.
Your motorcycle’s grounding wires function as extra pathways for potentially damaging voltage surges.
The grounding wires then route the excess voltage back into the motorcycle battery via the negative terminal.
Suppose your ground wires fray or short out, whether through everyday wear-and-tear or through malfunctions elsewhere in your bike’s electrical system. In that case, the whole system will experience dips and surges in electrical flow, which can make your motorcycle headlight fail to turn on.
In some cases, your lights will get dim and then bright, back and forth. In other cases, your headlights might not light at all.
If this sounds like your problem, the next step is to troubleshoot your grounding wires with a voltmeter.
- You can test the grounding of your motorcycle lights by locating all the headlights, turn signals, taillights, and breaking light circuits.
- Connect the positive red probe to the battery.
- Connect the negative black voltmeter probe to various points along the ground wires.
- If any of these ground wires test with a voltage below 12,6 and 13.5 volts, your headlight circuit could have a bad ground somewhere.
- Test the connector between your harness and relay as well.
If your voltmeter test finds breaks in your ground circuit, replace the damaged, shorted, or frayed wires, and your headlight should shine on back to its stock illumination.
Frayed or Shorted Out Wiring Harness
If the headlight circuitry in your motorcycle’s wiring harness has a short, loose connection, or frayed or damaged wires, your headlight won’t turn on.
In modern motorcycles, the lighting systems are getting more and more advanced.
While headlights used to be a single flood unit on a solo circuit with its own breakers, the integrated light, display, and infotainment systems on today’s moto market may be on shared circuits.
These more intricate modern circuits use multiple connectors to course electrical current through the bike’s circuitry and to the headlight.
If the wiring in your harness is frayed in any of the circuits shared with your headlight, your headlight won’t get the full voltage that’s being channeled to it. In some cases, the underpowered headlight’s illumination dims, while the motorcycle headlight stops working in others.
You’ll know if you have shorts or frays in your wiring harness first by visual inspection.
The wire will be damaged, nicked, scorched, stripped, or frayed.
Also, keep an eye out for discoloration, burn marks, or melting.
To be clear, you’re inspecting the circuits that interact directly with the headlight first and foremost. Still, wiring issues elsewhere in the circuitry can leech power from your battery, affecting your headlight by proxy…
…which takes us to our next section.:
Faulty Headlight Relay
Your headlight relay is a small transformer that uses a succession of connectors to transform power from the battery to the voltage required to power your headlight.
Like a fuse, most electrical systems in your motorcycle have some form of relay that transfers power to it from the battery. The ground wires revert the extra power back to the battery it came from, as we discussed earlier.
Suppose a relay fails to transform the power. In that case, the battery current won’t activate the connectors needed to generate the voltage that illuminates your motorcycle’s headlamp, and your headlamp won’t turn on.
If a faulty headlight system relay is your prime suspect, you’ll have to test it with a voltmeter to be sure.
We suggest consulting a copy of the service manual for your specific make and model moto, as you’ll need to cross-reference the relay test results with the spec readings for your bike.
Not all light circuits are of the same voltage rating, sufficient to say that if the relay reads zero, it needs to be replaced.
Still, you’ll have to know what rating to replace it with.
If you don’t have the right tools, electrical experience, and service manual, we suggest taking your motorcycle to a pro technician; electrical mishaps can be hazardous to you and your bike.
If your motorcycle’s battery is expired and needs to be replaced or, for some reason, won’t hold a charge, your headlight won’t get the power it needs to turn on.
In some cases, a battery might just be depleted due to parasitic drain or faulty charging system components. IF that’s the case, you’ll be able to take your battery into a motor parts store and get it charged after they verify that the battery is indeed in working order.
If the battery is expired, it won’t hold a charge at all. In that case, replacing your battery is how to get your motorcycle’s headlight to turn back on.
Once you replace the bad battery, the new battery should be enough to power the headlight back up.
That said, contaminated, lost, or rusted battery terminals can be enough to make your motorcycle’s headlight fail to light up. Inspect your battery, terminal, and wiring conditions per your owner’s manual guidelines to ensure your motorcycle lights stay lit.
Dead or Wrong Wattage Headlight Bulb
We saved the most basic for last, but it’s important enough to be included, to be sure.
Headlight bulbs can burn out for various reasons. You’d be surprised how many motorcyclists assume the bulb they just put in a week ago is problem-free and can’t be the problem, only to find it was a dud after 8 hours of troubleshooting.
Just like any other halogen light bulb, a halogen motorcycle light bulb burns out eventually after enough use. That said, a crack in the glass or damaged filament can cause even a brand new motorcycle headlight bulb to burn out.
The fastest way to troubleshoot a bad headlight bulb is by examining the other bulbs on the motorcycle.
So, if all the lights but the headlight are in working order, and you’re sure they all share a circuit, chances are your headlight bulb just needs to be replaced.
One of the reasons a new bulb can burn out early is that leftover finger oil residue gets hot enough to break the glass.
Therefore, we suggest using a rag or a bandanna to remove the bulb from the box and screw it into your motorcycle.
Even some well-seasoned riders don’t realize that all halogenic motorcycle bulbs aren’t the same wattage; installing a headlight bulb of the wrong wattage will render it useless, and your motorcycle headlights won’t light up.
The headlight’s circuit is engineered to transfer a particular wattage to a bulb of a specific rating—altering that circuit by using the wrong bulb impacts the process until your motorcycle headlights won’t work.