Is there any point retro-fitting LED's

[190]

I mean any point retrofitting LED's from an energy saving perspective.

From what I can gather if I replace a filament bulb with a standard LED the bulb failure warning system won't recognize the LED as it draws insufficient current.

There are LED's with additional parallel resistors to fool the system but if they have to draw the same current that the stock filament lamp would then the whole exercise seems pointless.

I guess my question is are these load resistors really designed to simulate a filament lamp by pulling the full stock current or can the bulb failure system be fooled at some lower current which still makes LED's viable.

I should say I'm not remotely interested in fitting LED's for how they look only for reliability and to save energy.

 

[Hawkwind]

There might be a reliability advantage

 

[carat 3.6]

There might be a reliability advantage

Hit the nail on the head. They last much longer, and they are brighter too.

 

[Chrishazle]

I've put LED's in the front side/parking lights and most of the rear of my S204. The parking lights are superb - parked on a country lane near the pub last Saturday they really showed up - and there was a very large tractor and trailer running to and fro all night! They're also brighter/whiter than filaments, so my reversing light is better, the rear lights are brighter.

However, there are good and bad LED's, especially if you need P21/5W rather than P21W. Also P21W amber LED's are much more expensive than white, and on my 968 the white ones caused the relay to think a bulb was gone, so I would also need to change the relay!

For your 204, I'd say changing the rear P21W's for these would be good :
4x No Error Canbus CREE White #C LED Turn Light Bulb BA15S 1156 P21W | eBay
For the W16W reversing light, these are good :
2 x 7.5W White Error Free T15 W16W LED Projector Reverse Light Bulbs Canbus | eBay
And for the front side/marker lights, these are good :
T10 501 W5W CAR SIDE LIGHT BULBS ERROR FREE CANBUS 6 & 10SMD LED XENON HID WHITE | eBay
but they are too long for the rear lights on my car, don't know if they're OK on a saloon.

 

[David J]

To save energy ?? Im confused ? yes they may draw less current but what will you gain in the energy saving ?

 

[Diesel Benz]

There are LED's with additional parallel resistors to fool the system but if they have to draw the same current that the stock filament lamp would then the whole exercise seems pointless.

I guess my question is are these load resistors really designed to simulate a filament lamp by pulling the full stock current or can the bulb failure system be fooled at some lower current which still makes LED's viable.

The "intelligence" in "CAN bus" LEDs is that those are designed to draw the same current as original filament bulbs (mainly with parallel resistors). No energy saving there.

 

[190]

Thanks Guys. It's as I thought then, the so called error free LED's consume the same current as stock filament lamps.

Why do I want to save energy if LED's are fitted ? because it just goes against my engineering principles to install efficient light sources then dump most of the consumed current as heat. I accept though that correctly selected LED's can provide better illumination.

Don't get me wrong I am a fan of LED's as on my 1979 BMW motorcycle LED's are commonly fitted to reduce the load on a marginal charging system. But of course there is none of this bulb monitoring nonsense so it can be done efficiently.

I presume later cars where LED's are standard will have been more efficiently designed.

 

[190]

I forgot to mention that one reason I had thought about for fitting LED's if they had pulled less current was in the rear light clusters. I've heard the earthing arrangement on W204's is inadequate and can lead to overheating.

 

[GadgetBoy]

It's nothing to do with current. It's the total resistance of the circuit that's measured by the lighting fault system.

LED's (even with resisistors) will draw considerably less current that the incandescent lamp they replace.

 

[190]

It's nothing to do with current. It's the total resistance of the circuit that's measured by the lighting fault system.

LED's (even with resisistors) will draw considerably less current that the incandescent lamp they replace.

Now I'm confused although I do understand there are series resistors used to limit the current through an LED and the parallel resistors that are used to make them "error free"

If we take a specific example of a 6w incandescent bulb, that will pull 0.5 A at a nominal 12 volts. So the running resistance must be 2 ohms.

If the error free LED bulb/parallel resistor combination has the same resistance of 2 ohms how does it pull less current.

 

[Ted]

It doesn't.
Blame Mr Ohm and his law.
"Ye canna change the laws of physics captain"

It's nothing to do with current. It's the total resistance of the circuit that's measured by the lighting fault system.

This statement contradicts the above and is incorrect.


Series resistor to limit current through led
Parallel resistor to shunt current to ground and 'fool' the bulb monitoring.

 

[GadgetBoy]

You don't need to limit the current through an LED, it's self limiting, an LED will only draw whatever current it needs, the voltage however, is important.

But that's not the point.

If, as you say, the current is used to determine if the lamp is good/bad then the circuit would have to draw a consistent current all the time even when the lamp is question is off! Remember, the monitoring system will tell you a lamp circuit is faulty even when it's off.

I reiterate, it's the resistance (and by definition - voltage) of the circuit that's important. The current is irrelevant for the monitoring system.

 

[Ted]

You don't need to limit the current through an LED, it's self limiting, an LED will only draw whatever current it needs, the voltage however, is important.

Yes you do. As a semiconductor device, as long as it is forward biased it will pass as much current as it takes to destroy it. This is limited by a series resistor.
Many commercial LEDs, particularly those manufactured for the Automobile market have a series resistor such that they will pass the correct current at 12(or 24)v built in.
It doesn't change the fact that an LED requires a current limiting resistor.

If, as you say, the current is used to determine if the lamp is good/bad then the circuit would have to draw a consistent current all the time even when the lamp is question is off! Remember, the monitoring system will tell you a lamp circuit is faulty even when it's off.

The monitoring systems that I have seen will often pulse a circuit to check if current is flowing. The normal method of checking if a bulb is working is to measure the voltage across a low value resistor. If the required voltage is measured across the resistor then current is flowing and the circuit (read this as bulb) is working correctly.
There may of course be other methods.

I reiterate, it's the resistance (and by definition - voltage) of the circuit that's important. The current is irrelevant for the monitoring system

If you have the resistance and voltage of a circuit then you know the current. It's a simple equation.

That is all I will say on the matter. I now bow to your greater experience.

 

[markmifsud]

Just to add to this, according to MB tech docs, the monitoring system is carried out by N10/1 and the check is based on the the current consumption.

 

[MikeAMG]

You don't need to limit the current through an LED, it's self limiting, an LED will only draw whatever current it needs, the voltage however, is important. But that's not the point. If, as you say, the current is used to determine if the lamp is good/bad then the circuit would have to draw a consistent current all the time even when the lamp is question is off! Remember, the monitoring system will tell you a lamp circuit is faulty even when it's off. I reiterate, it's the resistance (and by definition - voltage) of the circuit that's important. The current is irrelevant for the monitoring system.

I suggest you put an ammeter in the circuit with the led in then again with the bulb in to test it. They will both be pulling the same current BECAUSE both circuits will have the same overall resistance (to keep the bulb check happy) and the same voltage feed.

 

[Diesel Benz]

I forgot to mention that one reason I had thought about for fitting LED's if they had pulled less current was in the rear light clusters. I've heard the earthing arrangement on W204's is inadequate and can lead to overheating.

The W204 rear lights issue is more on the rear lights connector design.

But since you mentioned rear lights, one of the reasons MB started to use LEDs on rear brake lights (centre mounted and main brake lights, I think starting from pre-facelift W211 Avantgarde models, or about the time) is the fact that they emit light "much faster" than an bulb.

Cannot remember how big the difference is but the driver behind gets alerted a fraction of a second earlier if the car in front has LED brake lights.

And you are right, those cars that have LEDs from the factory, do not have any useless parallel resistors and do consume less current (energy), perhaps not a big deal but an advantage anyway.

 

[Uky]

An LED brake light has an "attack" time compared to an ordinary bulb which represents 5 meters of driving at a speed of 100 km/h. Therefore, replacing the ordinary bulbs with LED's even if you have to add extra load resistors to keep the light warning away is a good deed.