For safety during both the day and after dark, motorcycle lights should always function correctly. In most states, it isn’t illegal to have a headlight out, but a traffic stop for any other reason could get you a ‘fix-it’ ticket.
Sometimes replacing a bulb seems like the quick fix, but a new bulb hasn’t solved the issue and you’ll have to dig a bit deeper into the electrical circuitry that runs your headlight.
Electrical work sometimes seems above our skill level, but to quote an electrician friend of mine-“When you turn off the power, electrical work is just work.”
If you have a motorcycle headlight that just won’t work, we’ll help you troubleshoot the issue and provide a solution to fix it!
Table of Contents
How Do Motorcycle Lights Work?
The original automobile headlight was literally a flame behind a glass enclosure, powered by acetylene gas.
We’ve come a long way from this primitive technology and now halogen or LED motorcycle headlights are powered by an electrical circuit that runs off the battery.
Most motorcycle headlights should turn on when you turn the key in the ignition. The headlight switching only changes your standard beam to your hi-beams or brights.
A motorcycle headlight circuit begins at the battery and is fed through your wiring harness.
Often, a relay transforms the incoming voltage to the proper amount of power for your bulb, but not all wiring harnesses have a relay.
Here are some reasons your motorcycle headlight doesn’t work:
1. Bad Grounding
Every automobile and motorcycle produced after 1970 has a grounding system known as a floating ground.
A ground wire, or series of ground wires, creates an extra pathway for any voltage surges that might damage existing circuits.
This extra pathway then returns the excess charge or voltage back to the negative side of the battery.
Anytime there is damage to the grounding system, there can be electrical dips and rises. These dips in voltage can cause a shortage of the electricity needed to power your lights.
This can cause your lights to appear dim, or won’t carry enough current to power your lights at all.
Because grounding can cause a variety of different electrical issues on your bike, it is important to test your grounding with a voltmeter.
To test the grounding of your motorcycle’s lights, find all circuits associated with the headlight, turn signals, and tail/braking lights. Attach the red, positive probe to the battery and use the black, negative probe to test several points along your circuit.
If you are testing your motorcycle lights for a faulty ground connection, be sure to test any connectors to and from the harness or relay.
If any of these grounding locations show signs of low voltage (correct battery voltage should be 12.6-13.5 volts), you may have found a grounding short in your lighting circuit.
Simply replace that portion of the bad wiring or the bad wiring connector. This should solve the issue if you have a bad ground.
2. Wire Harness Shorts and Bad Connections
The electrical circuit that controls your motorcycle’s lights consists of battery-powered wiring that connects to your headlight module to keep the lights shining.
Often, the wiring is held in the harness and uses several connectors to relay the electricity throughout the circuit.
If any of these circuits have experienced an overload of voltage, it can ruin the connectors and damage the wiring.
Damaged wiring is easy to spot because the surge of higher than normal voltage will burn the wires.
You will most likely be able to see this where the wire connections are made, but sometimes a nicked wire can cause a problem somewhere in the line.
You will see discoloration or burn marks anywhere in the wire in this case.
Visually inspect the light circuit wires in the harness and the connectors. If you suspect a short in the wiring or bad connections, but can’t see any external damage, use a voltmeter to test the connections.
It is rare that an entire circuit will be overloaded and destroyed. Just find the section of the circuit that has been damaged and replace it, keeping you from having to do more extensive electrical repairs than you might have to.
3. Dimmed Lights Due to Accessory Overload
Every power source, like your motorcycle’s battery, is rated to give enough electricity to the bike. This has been done for your bike by the electrical engineers who designed your wiring.
Every bike makes sure to use the correct amount of voltage to power the right amount of electrical systems.
When you add accessories such as radios, hand-warming grips, or phone chargers, they can put a strain on the available amount of electricity.
Once this electricity becomes too taxed by too many load points, it can reduce the amount of electricity available for every piece of equipment.
This can be more of a problem for older, halogen bulb systems that draw significantly more electricity than modern LED lighting packages.
Unfortunately, we can’t upgrade battery sizes to accommodate more accessories because it causes problems in other electrical systems that are only rated to handle the amount of electricity that is put out by the battery specific to your bike.
If you’ve added electrical accessories and suddenly your lights don’t work, consider taking the bike to a certified professional to diagnose and solve this problem. The installation of a relay might solve this problem.
4. Bad Fuse
Motorcycles use a system of fuses to disrupt power surges, just like the ones in a car or truck. A fuse is simply a fail-safe device that keeps any electrical system from being destroyed due to overload.
If a power surge or electrical abnormality occurs, the fuse blows out before the high current can get to the mechanism that it is in place to protect.
Find the fuse box on your bike, and use your owner’s manual to locate the fuse that is connected to the malfunctioning lighting system. Remove the fuse and inspect it.
Inside the plastic casing is a small piece of metal that should be intact. If the fuse is blown, the small piece of metal will become disconnected.
Fortunately, fuses are inexpensive and easy to replace but don’t be too sure that this will solve a longer-term issue.
If a lighting circuit fuse continues to blow out, it is indicative of a potentially more serious problem with your electrical wiring and should be diagnosed by a certified professional.
5. Bad Relay
A relay is essentially a small transformer that uses a series of contacts to convert the power coming in from the battery to the necessary amount of voltage needed to power your lighting system.
If a relay has gone bad, the incoming current won’t signal the contacts to switch over to create the needed voltage for that lighting circuit.
If you suspect a bad relay is a reason your motorcycle lights are not working, test the relay with a voltmeter.
Make sure that the voltage readings are in step with the lighting circuit specifications that are unique to your make and model.
If the readings are low or read as 0 volts, you’ll need to replace your relay.
6. Bulb Is Burned Out
One of the most easily diagnosed issues if your motorcycle lights don’t work is burned-out bulbs.
Motorcycle lights function the same as any other light source from a halogen bulb. The electricity excites molecules that travel through a filament to create light.
If the filament is busted, or there is any other damage to the bulb (like cracked glass), the bulb will need to be replaced.
An easy way to know you have a burned-out bulb is to visually inspect the other lights on the bike.
If a single bulb is out in the turn signals, tail/braking lights, or headlight, it’s probably just that one bulb that needs to be replaced.
A handy tip to remember when changing out halogen bulbs-use a pair of gloves or a cloth to remove the bulb from the package and replace it in the bike.
The oils from your fingers can leave a hot spot on the bulb that, when heated over time by turning on the light, can cause the bulb to explode.
7. Bulb Is the Wrong Wattage
The lighting circuitry on your motorcycle is designed to carry a certain amount of power to a device that uses that specific amount of electricity.
If you mis-ordered a part and the wattage is incorrect, the light bulb won’t work with that circuit and will need to be replaced with a bulb rated for the correct amount of watts.