DRL bulb replacement....

[Madmonkee]

Hid bulbs in the fogs?? Thought they were just halogens??

[PetrolDave]

Hid bulbs in the fogs?? Thought they were just halogens??

Those are not HID bulbs, it's (yet another) misleading description on eBay. I've even seen LEDs described as HID bulbs by some sellers.

It seems some will describe a bulb as HID if it has an HID-like colour, eBay needs to be more rigorous in it's enforcement of accurate descriptions (I have to admit I've given up reporting inaccurate descriptions as eBay have done nothing to get them corrected).

[Stoo.]

Anyway, back to error-free LEDs...

I'm sure I read that the Audi bulb failure detection system works by generating very short (a few ms) pulses of lower than normal voltage (approx 5V) across the bulbs - short enough and low enough voltage to not generate any visible effect, even at night - so surely it should be possible to design a circuit that simulates the off resistance of the bulb when a short duration and/or low voltage is applied and removes that low resistance when a normal voltage is applied for a normal duration? That would give the aesthetic, low power and fast start-up benefits of an LED bulb, but without the power loss of a parallel resistor.

Audi used to use 'cold bulb monitoring' failure detection systems on the filament bulb circuits.

Nowadays most are LED so use hot monitoring.

Cold monitoring is where the central electrics control unit (J519) which controls the exterior lighting, checks for complete circuits when the ignition is turned on.

It sends 5 pulses of voltage down the circuit, one every half a second. As its doing so, it monitors the current on that circuit. If there is no current, it assumes open circuit or bulb failure and turns the supply to that circuit off and puts an error message on the DIS. If it measures current too high, assumes short to earth. Again, error message and turns the circuit off. If the circuit is fault free, no action.

The voltage pulse is typically 12v, for around 10ms. Vehicle dependant.

If you have LEDs in place if the filament bulbs, the current would be too high and the bulbs would flash when the system checks the circuit.

[MikeFish]

By that logic none of the LEDS I get a warning for would work at all because it should turn the circuit off if it thinks there is no bulb. I get warnings but the LEDs still work.

[PetrolDave]

By that logic none of the LEDS I get a warning for would work at all because it should turn the circuit off if it thinks there is no bulb. I get warnings but the LEDs still work.

A "properly designed" (which the majority are not) LED bulb (which is not just an LED, it has additional circuitry) detects the bulb monitoring test and conducts enough current during the monitoring pulses to fool it into thinking there's still a filament bulb there.

Some LED bulbs just have a resistor in parallel to draw excess current all the time (and generate unnecessary heat) - these wortk but you have the problem of finding a home for the (hot) resistor if it's not built-in.

Some LED bulbs are just LEDs and don't draw enough current to keep the bulb monitoring circuit happy - but are fine in cars without bulb monitoring (I use theses in my Citroen C1).

[MikeFish]

I appreciate that but the post said that if the car detects no bulb you get a warning then it turns the circuit off. My car detects no bulbs but it doesn't turn the circuit off as they still turn on.

[PetrolDave]

My car detects no bulbs but it doesn't turn the circuit off as they still turn on.

Weird???

[Stoo.]

Tbh, it depends how old the car is.

Older ones Didnt.
Tried it on an old b6 a4 the other day and it didn't turn it off.