Head Lamp Bulbs Blowing

[kulta]

Why do my headlight bulbs blow after a week or so?
I use the correct bulbs as per manual.
No other bulbs are affected.
2001 Ford Ka 1.3

[soapnumpty]

Could your headlight be cracked and letting in water? I had a bulb on my car that kept going every few weeks and that was the reason.

[bhg481]

One of the favourite causes of bulbs blowing is alternator output too high - it should be less than 14.5 volts with the engine running. Wouldn't explain why these are the only bulbs affected.

[TWR]

Check the connections.

[chas2008]

the 2 main reasons we come across -
1- the bulb has been handled (you should not touch the glass on a halogen bulb)
2- cheap bulbs( ie Ring or nasty unbranded imports)

Same problem you get with cheap household bulbs, they just dont last..

best to use a well know brand , may cost more but as they say you get what you pay for..

[ToraToraTora]

this is almost certainly caused by rapid variations in temperature on the bulb which can be made worse by poor quality bulbs. Check that the headlight itself is not cracked and letting in air/water.

[davewr]

I had a problem with main beam filaments blowing in both headlight lamps at once. It turned out to be the switch "arcing" due to dirty contacts.

[bhg481]

That's an interesting comment Davewr. I think most headlamps nowadays are operated via a relay. Since the OP implies BOTH headlights are affected, a problem in the control circuitry seems likely and changing the relay should be a fairly cheap and simple job. The car is 13 years old, so it feels a good bet.

[davewr]

A work collegue had a similar problem a few years ago and I remembered a story about a loose battery connection being the cause. This turned out to be the culprit, although I don't know the science behind it.

[bhg481]

Resistance increases with temperature. At the moment a bulb is switched on the filament is cold and takes more current than it does in normal running when the temperature has increased to white heat. This is the reason that most bulbs fail at the moment they are switched on.
An intermittent connection is like switching the bulb on and off rapidly, hence the increased risk of failure.

[chas2008]

so indicator bulbs fail more ? dont think so..

[bhg481]

I'm not certain, but I think that the indicator relay is not just a simple mechanical relay and the switching is controlled electronically, rather than contacts making and breaking, hence not such a shock ob the bulbs.

[jomifl]

A friends car blew it's headlights if he left them on for long with the car stationary. Apparently on some makes they can overheat if there isn't a cooling stream of air...might be complete dog's ******** though