Why LED auxiliary lights flicker, fog up or fail early

Nine times out of ten the LED chip is fine. Here's how to find what's actually wrong, and which of the three problems you're looking at.

LEDs are solid state. There's no filament to break and no gas to leak, so when one misbehaves the cause is usually outside the chip. That's good news, because the things that go wrong are things you can check in a driveway with a multimeter.

Flicker

Work through these in order, because they're listed roughly by how often they turn out to be the answer.

• A bad earth. The most common cause by a distance. The light finds a path back through something else, which makes it dim, flicker, or light up when you brake. Sand the mounting point back to bare metal and refit the ring terminal. If the housing is powder coated, the bracket itself may not be making contact, and a separate earth wire is the fix.
• A connector that's worked loose. Vibration does it in a few thousand kilometres. Pull each one apart, look for green or white corrosion, and refit with dielectric grease.
• Low voltage at idle. If the flicker clears above 2,500 rpm, that's the alternator not keeping up at low revs, and it's normal on a lot of bikes. It's not damaging anything.
• A CAN bus or PWM circuit. Some bikes pulse the tail light circuit, and a light tapped into it will flicker in time with the pulses. Move the trigger to a clean switched source.
• A switch that's carrying too much. If the switch is in the main power path instead of the relay trigger, it gets hot and starts to make intermittent contact. That's also a fire risk, and it's why the relay belongs in the circuit.

Moisture inside the lens

There are two versions of this and they mean different things.

First, light condensation that clears within about ten minutes of switching on. That's normal. Air trapped in the housing holds moisture, and a cold lens on a damp morning will show it. The breather vent equalises the pressure and the heat clears it. Nothing to do.

Second, water that stays, or droplets running around inside the lens. That means there's a way in, and there are three usual candidates. The breather vent is clogged with mud or insect debris. A seal has shrunk or split, usually after a pressure wash. Or moisture is tracking backwards along the cable into an unsealed connector.

To dry one out, take it off the bike, put it somewhere warm and dry with the connector end down, and leave it for a day. Rice is a myth. Then find the entry point before refitting, or it'll be wet again inside a week.

Early failure

When a light dies in the first year, look at heat and voltage.

• Heat. LEDs don't like heat at the chip, and the housing is the heatsink. A light buried behind a fairing, or mounted where it gets no airflow, will cook. That's also why fanless aluminium housings last longer than a fan cooled one on a bike, since the fan is the part that fails.
• Over-voltage. A regulator-rectifier on its way out can push 15V or more into the system. Lights rated for a narrow voltage window suffer first. A light rated DC 8-80V rides through it, which is a real reason to prefer a wide input range over a narrow one.
• Vibration at the board level. On a single cylinder or a hard-used adventure bike, solder joints crack. It shows up as intermittent operation that gets worse over bumps.
• Water at the connector. Corrosion in the plug raises resistance, which makes heat, which finishes the job.

Symptom to cause, at a glance

When to replace rather than repair

If the housing is cracked, the bracket is stripped, or the driver board has corrosion on it, replace it. Soldering a board inside a housing that's been wet usually buys a few weeks.

If you're replacing, it's worth thinking about what failed and buying against that. A light that died of heat wants an aluminium housing with more mass. One that died of vibration wants a rubber mounted bracket. One that died of voltage wants a wide input range. The 7000LM fog light pair covers the voltage side with DC 8-80V and the heat side by being aluminium and fanless, and the 4-inch pods come with an adjustable bracket that takes some of the vibration out.

Rest of the range is in the motorcycle driving lights collection.

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