Hi all,

 

I have fitted LED tail light clusters to my Skoda Superb.

 

As expected they have thrown up the blown-bulb warning light on the dash.

 

To cut a long story short I've given up on trying to code the CANBUS byte 18 using VCDS so I'm now going down the route of adding load resistors.

 

The cluster incorporates three lighting functions, the indicators which are normal incandescent bulbs and then the brake and tail lights which are LED.

 

The resistors need to bridge the positive and negative wires, the question is which wire is negative?

 

As far as I can tell there is no negative connection through either of the screws that are used to secure the cluster to the bodywork...

 

 

 

 

 

Any help would be much appreciated!

Edited 6 September, 201312 yr by silver1011

All the wires in the photo are positive.

 

The clusters negative connection will be through an earth to the bodywork somewhere. But you don't need to use the clusters negative.......any piece of metalwork on the car is a 'negative' connection (google 'negative earth' for an explanation), so you can simply bridge the resistors from each positive wire to any bit of bare metal on the cars body.     

All positive.?

The wires going to the incandescent bulb holder surely must be neg and positive for the indicator bulb to light?

If so then as blue is going to both then I would hazard a guess that it was the common (neg) however I would confirm with a multimeter.

 

I am of course speculating based solely on the image and happy to be corrected.

I am of course speculating based solely on the image and happy to be corrected.

 

Likewise. A multimeter confirmation would be useful.

 

But as the blue and red are the only two wires going to the brake and tail light, I assume that each must be a positive for the brake and tail.........why it then goes into the indicator light, I'm not sure. 

Edited 6 September, 201312 yr by booke23

Out of interest, is there only 3 wires coming out of that multiplug?

Yes, only three wires out of the multiplug...

I'm going to remove one of the clusters tomorrow but I'm 99% sure that there is no negative earth through any of the securing points.

 

There are only two screws that hold the cluster to the bodywork and there are no metal contacts on the cluster around any point where the screws pass through the cluster...

 

 

You can see them in this picture. Two black plastic caps hide the tops of the screws...

 

Just viewed that cluster pic in full size.....is that a brown wire going into the Brake/tail light bung? Looking closely at the multiplug, it looks like there might be a 4th brown wire there too, but It could be a shadow.

 

If that is the case, then superbdreams is right....blue must be negative. I was thinking there were only 3 wires! (in a 3 wire case they have to all be positive to operate 3 functions.....but if there's a 4th wire, that changes everything)

Slight modifation to the sketch. If one of the units has blown ,then it'll be because of excessive volts. Add load resister in series to reduce voltage .Or more elagant solution might be one of these

http://www.maplin.co.uk/constant-current-led-power-supply-511375

Good spot booke23, that does appear to be a brown wire (the photo is from the sellers site, I'm going to take some better pics tomorrow).

 

In that case that should confirm the blue wire to be the negative.

 

What is the best way to wire in the resistors? These wires are open to the elements so I don't want to use Scotchlocks.

 

Can I simply cut the wires, splice in the resistor and use heat shrink to seal the exposed wires?

Slight modifation to the sketch. If one of the units has blown ,then it'll be because of excessive volts. Add load resister in series to reduce voltage .Or more elagant solution might be one of these http://www.maplin.co.uk/constant-current-led-power-supply-511375

 

Non of the lights have blown, the CANBUS system has simply noticed the reduced resistence of the LED's (compared to the incandescent bulbs it thinks should be there) and has thrown up the blown-bulb warning on the dash.

 

 

I'm thinking of using something similar to these...

 

Looking at the photo there are three wires going to the rear / brake light: Red, Dark brown & blue. There are two wires going to the indicator light: light brown & blue. Therefore blue is the common / negative wire.

Good spot booke23, that does appear to be a brown wire (the photo is from the sellers site, I'm going to take some better pics tomorrow).

 

In that case that should confirm the blue wire to be the negative.

 

Absolutely......the blue has to be negative. 

 

 

What is the best way to wire in the resistors? These wires are open to the elements so I don't want to use Scotchlocks.

 

Can I simply cut the wires, splice in the resistor and use heat shrink to seal the exposed wires?

 

Yes, cutting the wires and splicing in the resistors is probably how I'd do it......but they could do with soldering for reliability. Having said that it's not the end of the world if a connection should fail in future......you'll know as you'll get a bulb out warning. Definitely heat shrink the connections, and be aware of how hot the resistors get....I'm not sure if it's enough to melt plastic, but it's probably worth keeping them away from any plastic.       

Sounds like a plan, thanks!

 

Next question...

 

Do I need 25W or 50W resistors?

 

If I play it safe and go 50W will it cause any issues (can I reduce the current too much so that the LED's dim too much etc.)?

 

EDIT: Ah the power of Google!

 

"Resistors are also rated in terms of how much heat energy they can dissipate without overheating and sustaining damage. Naturally, this power rating is specified in the physical unit of watts."

 

So the watts rating shouldn't have any effect on the resistance, a higher wattage simply allows for greater heat dissipation, its the Ohms I need to look for.

 

Will chosing a higher watts rating (50W vs. 25W) mean that the resistor won't get as hot?

 

Looking at resistors online all the automotive LED resistors don't mention the ohms rating, surely this is important as I have both brake and tail light functions...

Edited 6 September, 201312 yr by silver1011

In a 12v system, a 6 ohm resistor will draw 2 amps. Watts = amps x volts, so this equates to 24W.

 

So in theory a 25W resistor would be fine. However I think for a 100% duty cycle you need to double the wattage of the resistor for safety...........for this reason they always recommend 50W resistors in these situations. So that's what I'd use. But you'd probably get away with a 25W resistor on the indicator circuit as it's not on continuously.....and when it is it's only a 50% duty cycle as it flashes on and off.     

Good stuff, I'll go for 4x 50W 6 Ohm resistors then, two for each cluster (one each for the tail and brake wires).

 

I'm guessing the point at which I splice into the negative wires doesn't matter. I'll be splicing the blue wire twice, once for each resistor.

 

I just need to be careful where the resistors sit as I don't want them touching the plastic cluster if they get as hot as a bulb!

BUT the resistor NEEDS to be in SERIES , not paralel ..Series resistors reduce the current ,and hence the voltage .

through the circuit. Heat in the comanant is from a factor of resistance x resistance x curent. Bigger watage resistors will simply reduce the heat in the circuit. You need to know the ideal working voltae of the LED ( that across tge LED, from makers data at the specifiewd current ) ,and at what current.Then using Ohms law workout the value needed to drop the voltage. Onceat that stge , you need to calculate the power ( IxIXR ) NEEDED FOR THE DEVICE. AND then decide on the wattage of the resistor. It's done by maths clculation ,NOT guestimation .

Sorry to be blunt , but it's not that easy . And high power resistors often need metal to dissapate heat. That's why I sugested voltage droppers/stabilisers.

If you work out the figures at nomnal 12v ,you get one figure. Run at max volts in a car, you need a differing drop and curent/ resistance etc .And of course heat dissapation in the droppers.

BUT the resistor NEEDS to be in SERIES , not paralel ..Series resistors reduce the current ,and hence the voltage . through the circuit. Heat in the comanant is from a factor of resistance x resistance x curent. Bigger watage resistors will simply reduce the heat in the circuit. You need to know the ideal working voltae of the LED ( that across tge LED, from makers data at the specifiewd current ) ,and at what current.Then using Ohms law workout the value needed to drop the voltage. Onceat that stge , you need to calculate the power ( IxIXR ) NEEDED FOR THE DEVICE. AND then decide on the wattage of the resistor. It's done by maths clculation ,NOT guestimation . Sorry to be blunt , but it's not that easy . And high power resistors often need metal to dissapate heat. That's why I sugested voltage droppers/stabilisers. If you work out the figures at nomnal 12v ,you get one figure. Run at max volts in a car, you need a differing drop and curent/ resistance etc .And of course heat dissapation in the droppers.

The resistors are NOT there to reduce the voltage, but to increase the resistance. 

Fitting an LED produces a Bulb Out warning as the Bulb Failure System senses the reduced resistance in the circuit as if it is a blown bulb. Fitting the resistor in parallel puts that resistance back into the circuit. 

Good stuff, I'll go for 4x 50W 6 Ohm resistors then, two for each cluster (one each for the tail and brake wires).

 

I'm guessing the point at which I splice into the negative wires doesn't matter. I'll be splicing the blue wire twice, once for each resistor.

 

I just need to be careful where the resistors sit as I don't want them touching the plastic cluster if they get as hot as a bulb!

 

Yes, 2 for each cluster....assuming the indicator isn't monitored by canbus?....then you'd need a resistor for that too.....3 per cluster. And you're right. You can splice them into the negative wires anywhere you want. I'd like to hope they won't get as hot as a bulb........but who knows.

 

 

BUT the resistor NEEDS to be in SERIES , not paralel ..Series resistors reduce the current ,and hence the voltage . through the circuit. Heat in the comanant is from a factor of resistance x resistance x curent. Bigger watage resistors will simply reduce the heat in the circuit. You need to know the ideal working voltae of the LED ( that across tge LED, from makers data at the specifiewd current ) ,and at what current.Then using Ohms law workout the value needed to drop the voltage. Onceat that stge , you need to calculate the power ( IxIXR ) NEEDED FOR THE DEVICE. AND then decide on the wattage of the resistor. It's done by maths clculation ,NOT guestimation . Sorry to be blunt , but it's not that easy . And high power resistors often need metal to dissapate heat. That's why I sugested voltage droppers/stabilisers. If you work out the figures at nomnal 12v ,you get one figure. Run at max volts in a car, you need a differing drop and curent/ resistance etc .And of course heat dissapation in the droppers.

 

No!.........you're misunderstanding the issue here. These LED clusters are OEM units that simply plug and play. As Llanigraham says, the problem is that this cars CANBUS system cannot (at the moment) be coded so It knows the rear lights are LED's. So it thinks (due to the much higher resistance of the LED circuit) that all the rear bulbs have blown, and displays a constant warning on the Maxidot.

 

The aim is to reduce the resistance of the circuit to fool the CANBUS into thinking all the rear lights are ok......so the resistors have to be in parallel.  

I might be wrong, but I understood that the 'bulb out' system worked by measuring the resistance of the lighting circuits, if a bulb fails I would thought the resistance would be less? Adding resistors to the circuits will increase the resistance.

I might be wrong, but I understood that the 'bulb out' system worked by measuring the resistance of the lighting circuits, if a bulb fails I would thought the resistance would be less? Adding resistors to the circuits will increase the resistance.

 

You're correct......but if you add resistors in parallel (as in this case) it reduces resistance. 

Right, just been out into the garage and taken plenty of pictures.

 

Yes, the brown wire does exist so there are positive feeds to the tail (either red or brown), brake (either red or brown) and indicators (yellow).

 

Blue is therefore the negative earth for all three functions.

 

Looking on the back of the clusters the wattage for each function is stamped onto the cluster.

 

The indicator is a normal non-LED bulb and is a standard PY21W.

 

The two LED feeds are listed as "LED 12V 4.5W / 0.6W".

 

I therefore assume that the brake light LED's are pulling 4.5W and the dimmer tail light function is pulling 0.6W?

 

The brown wire is now clearly visible...

 

 

The non-LED incandescent indicator bulb is a PY21W...

 

 

...and the LED feed is 4.5W and 0.6W...

 

Edited 7 September, 201312 yr by silver1011

correct, they need to be in parallel however you will find that they will be 6.8 ohm as this is a normal value doubt you will find any 6 ohm ones. You only need them on the LED bulbs but bare in mind that 6.8 ohms is for an original bulb wattage of 21W so if this is for a side light then you need to get a different value based on the original bulb wattage.

 

John

Excellent. I didn't realise the indicator was a standard bulb.....so only 2 resistors needed per side needed. 

 

I'd agree with your assumption on the brake/tail light being 4.5W/0.6W. 

As for the resistance of the resistors......looking on google most off the shelf 50w load resistors seem to be 6 ohm. 

 

However there is probably a range of resistance values that would work.......I imagine anything between 6 - 8 ohms would work fine. The CANBUS system will probably have a certain tolerance built in to allow for bulbs ageing etc.