The short version

Is anybody able to check the voltage across the supply to the front side light bulbs, say with an oscilloscope, and tell me whether there is any noise (e.g. ripple or spikes) on the line? It's a bit of an odd request, I know, but I don't have access to a driveway, and I have some most peculiar bulb-failure symptoms.

The long version

I recently replaced the standard front sidelight bulbs with these LED "bulbs". The devices have a plastic moulding which imitates the standard "wedge" shape, then four 3mm white LEDs are mounted on a forward-looking circuit board, along with two identical resistors. I do not (yet) know how they are connected, but I would expect series-parallel (two series circuits, in parallel).

At first both sides worked fine, but after a few days (with lots of use - I drove the best part of 1000 miles) they both started flickering (first one side, then the other). At first I wondered whether the sidelights were driven with a modulated (PWM) supply, but then I noticed that only two of the LEDs were flickering (out of the cluster of four). As I watched, the flickering became "worse" with more "off time" and less "on time", making the light appear to get dimmer. The frequency of the flicker also varied; I would estimate between 5 and 20 Hz.

Now the right side has two LEDs flickering and two off completely, and the left side has two LEDs flickering at one rate and two at another rate. I wondered whether the bulb itself has some PWM circuitry in it, but I can't see anything other than a pair of resistors.

Curiously, the two identical LED bulbs I put in the rear numberplate lamps are working perfectly.

Frankly, I'm stumped. Four years studying electronics tells me this should be impossible! :confused:

The only thing I can think of is that there are some electronics somewhere in the bulbs, and the sidelights' supply is either modulated or noisy, creating a big mess of electrical signals within the bulb. Phew! Hence my request for somebody to put a scope across the side light supply!

I would imagine it's something to do with the car's bulb failure circuit.

I fitted a Cruise Control on a Fabia a few days ago fitted with these type of bulbs, the cruise wouldn't work as it was seeing a signal from the brake switch. Put the normal bulbs back in and all is well. The LEDs were lightly glowing all of the time when the ignition was on.

just put the normal bulbs back in , problem solved surely

I would imagine it's something to do with the car's bulb failure circuit.

OH, I hadn't realised I had a bulb failure module. I thought that was introduced with the mk II. That goes some way to explaining it, although it's still a weird failure.

just put the normal bulbs back in , problem solved surely

I know, I know; I just don't like being beaten, particularly by something I don't understand!

I've not really taken much interest in the subject of LED bulbs as they don't currently interest me. However I have heard talk of fitting a resistor somewhere in the circuit which may help - More research perhaps, could be a nice little challange for a man of your experiance

I have heard talk of fitting a resistor somewhere in the circuit which may help

A shunt resistor, yes. That's a good idea. Here's the background:

A bulb failure module does its job by monitoring the current going along the wire to the bulbs. It "knows" that, for example, a pair of 5W bulbs will draw just under an amp ((2*5)/12), so if it sees significantly less than that going past it assumes a bulb has broken.

LEDs are more efficient than bulbs (they're better focused and don't get as hot) so they don't draw as much current. This can fool bulb failure modules into thinking a bulb has broken.

A shunt resistor is just a big resistor connected in parallel with the LED unit, to draw some extra current - enough to make the bulb failure module happy again. In effect, the resistor generates the heat that the bulb's inefficiencies would have done.

My understanding was that when a bulb monitor detected a blown bulb it would just show a warning on the dashboard, but maybe some modern ones get a bit over-excited and do other things. I've heard that, for example, the Octavia II will cut off the supply to any bulb it deems faulty (how annoying).

could be a nice little challange for a man of your experiance

Ha! I like the complement but I'm not sure it was justified! I don't really have the time for projects, these days. My engineering days stopped the week after I graduated when I decided to become a teacher instead. Much more job satisfaction but no time to scratch my *rse!

The sites that sell these bulbs usually warn that if they are used for a fitment where the sidelight is mounted close to the bulb for the main beam that it will overheat and fail starting to flicker.

Hope this helps

Simon

ill check at work on monday if you want but try the old resistor trick

The sites that sell these bulbs usually warn that if they are used for a fitment where the sidelight is mounted close to the bulb for the main beam that it will overheat and fail starting to flicker.

ahhHHhh. The site that I bought mine from didn't (and still doesn't - I checked). That sounds like the cause. How annoying.

Tuck, thanks for the offer but I've taken Rick's advice and just put the old bulbs back in. I didn't really like the colour of the light from the LEDs, anyway - it was a little too blue-ish for my tastes (despite the "white" label) and looked silly beside the non-HID bulbs.