Why Do My Headlight Bulbs Keep Blowing?!

Why Do My Headlight Bulbs Keep Blowing?!

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Most of us with vehicle ownership experience will know that changing light bulbs on a vehicle should be as easy as just removing the cover from the back of the light, and slotting a new one straight in; easy peasy, right? Unfortunately not, some vehicles can become your nemesis when it comes to bulb changes. Who would have thought that a very few specially selected vehicle designers have a unique ambition in life to make headlight bulb changes one of the most frustrating, time-consuming, expensive, hand-torturing maintenance processes there possibly is to do on a car?

The point is proven when last week you spent most of Sunday morning just removing the front bumper and 100,000 of those dreaded, rusty bottom engine cover bolts just to swap out some pesky little light bulbs!


Reason #1 – Faulty Voltage / Alternator Regulator (A.K.A Voltage Stabiliser)

The voltage regulator is a critical piece of electrical equipment in a vehicle, the part can usually be found attached to the side of the alternator or mounted separately. In easy-to-understand terms, it limits the maximum amount of voltage that runs through the electrical system to a safe and usable amount for all components to utilise.

If the voltage regulator is intermittently having problems then it can be the source of your bulb problems. This is due to spikes of high voltage jumping through the system which in turn blow the bulb filaments and/or headlight fuses.

We must keep in mind this article is about bulbs blowing constantly, but regulators can also fail and cause other problems, such as the battery not charging. This usually occurs when the diodes built into the regulator fail which allows electrical current to be reversed; this can in the end drain the battery or cause the running voltage (engine on) to even be lower than when the engine is off (battery resting voltage).

You can perform a simple test to check if the voltage regulator is faulty, however you’ll need another person to help you rev the engine whilst you take a multimeter reading.

  • Acquire a multimeter if you don’t have one, you can usually purchase one for under £10
  • Open the bonnet and remove the positive red terminal safety cover from the battery (if there is one)
  • Set your multimeter to 20 DC volts as shown in the photo. The DC Volts symbol is a V, and next to it there should be three dots with a straight line on top
  • Don’t use the symbol with a V and a squiggly line next to it, as that’s AC volts. Useful tip – think of the squiggly line as alternating current (AC) to remember this!
  • Before you begin testing, make sure the ignition and all of your vehicle’s lights are off.
  • Connect the red + (positive) multimeter test lead to your battery’s positive terminal.
  • Then, connect the black – (negative) multimeter test lead to your battery’s negative terminal.
  • Have your assistant turn on the engine (check it’s in neutral first)
  • The multimeter reading should rise to around 13.8 volts whilst the car is idling, this means the alternator is charging the battery correctly
  • Ask the person helping to slowly rev the engine until it reaches 1500 – 2000 RPM
  • Check the multimeter reading and jot it down, the voltage regulator should cap the amount to 14.5v. If the reading is over 14.5v then your voltage regulator is most definitely faulty.

Another known problem in the same category is that some poorly manufactured alternator belts can cause power surges due to static build-up. This problem can be found by performing the check above and hopefully fixed by replacing the alternator belt with a premium brand, or by installing resistors to the low and high beam circuits. This is a common ‘online forum’ notable fix for some vehicles. Try replacing the regulator first to see if it fixes the problem.



Reason #2 – Loose wiring connections to the bulb holder/bulb

This can cause the current or ‘flow’ of electricity to contact intermittently on and off, this can in turn, cause an increase in heat. Temperatures that go beyond what the bulb is designed to cater for will easily cause the filament inside to blow.

Vibrations in the headlight can also increase this problem, so check to make sure all headlight bolts are tight, that no headlight mounts are cracked and that the electrical connectors to the bulbs are secure. We recommend fitting new connectors to the bulb to completely eradicate them as the source of problems, especially if they look burnt, melted or show signs of corrosion inside.

H4 Plug Connector

Reason #3 – Poor Quality Bulbs

This one might sound like a bit of a cliche, however, the truth is that very cheap bulbs are nowhere near the quality standards of high-end brands and this is why we will never sell them. The wire filaments are usually made from a far thinner gauge of tungsten which can see them fail in a matter of hours. Vibrations are the most common cause of failure on cheap bulbs, and this combined with thin gauge wire results in easy failures, especially if voltage fluctuations are going on with a dodgy regulator. It’s an obvious sign of household technology put into a car bulb, so failures are bound to happen easily, have you ever seen the wire wobble around inside a household bulb? Stick to a high-quality German brand such as OSRAM, they’re available in our shop.

Reason #4  – Touching the glass of the bulb on installation

Halogen bulbs get very hot during operation and the chances are that by the time you get to the stage of actually fitting the bulb, your hands look like this. Bulbs need to heat evenly throughout the whole of the bulb’s surface for an optimum lifetime, leaving traces of dirt and grime or even the oil from your skin can sometimes cause uneven heating when the bulb is in use.

Dirty Hands
Touching bulbs with hands this oily is a recipe for premature failure

In some extreme cases, dirt on the glass of the bulb can cause a complete structural failure and they end up exploding! Use clean latex gloves and only try to touch the bulb’s metal base when installing, this can be very tricky when sometimes they’re hard to position correctly with limited amounts of space.

Reason #5  – Excessive vibration

Vibration is not a good thing for halogen filaments, which is also another reason why Xenon HID bulbs excel on lifetime as they don’t have any! Check the bulb holder’s retaining spring is clipped correctly in place and as said before, check the headlight is secured nice and tight. In extreme scenarios, it may be worth checking the condition of wheel bearings, suspension springs and wheel balance, literally, anything that can cause major vibrations at the front usually contributes to bulbs blowing.

Reason #6 – Excessive condensation in the headlight

Too much condensation in headlights can cause electrical shorts and diminish the lifetime of bulbs rapidly, so it’s important to check if they’re getting too damp inside.

Headlights are usually ventilated through the top and bottom of the unit through small holes or tubes usually found with a 90-degree bend to them. They’re designed to only allow a certain amount of air to flow in and out for heat dissipation but also to help reduce moisture in the lens. If the bulb covers on the back of the headlight aren’t mounted correctly or any seals have broken around the lens, then you can expect a higher amount of condensation in the headlight than normal.

Headlight Condensation

Aftermarket Headlights & Condensation

Some after-market ‘copy’ headlights are made so poorly that a large amount of condensation can be seen after just a couple of weeks. It’s worth noting that minor condensation is perfectly normal, however, when ‘pools’ of water form at the bottom corners it should be checked out ASAP. The staff here have their own experiences with aftermarket headlights and can safely say they’re a waste of time. For this reason among others, we only sell original (OEM) branded headlights due to wanting the very best for our customers.

Have you had a really bad experience changing headlight bulbs? Let us know in the comments section below.