LED brake lights
 

Anyone using them? Do they cause any problems with the cruise control unit?

Why would you think that they might cause problems with the CC unit?

Yes, I have SuperBrightLEDs. My cruise has not worked since installing them, but I did check right before installation. Rumor has it that you are correct.

Grant,

Which bulbs are you using for your brake lights? Did you do all your exterior lights aside from the headlights? How are they working? I just yesterday ordered all interior leds for my early car and wanted to do the tail and brake lights but couldn't decide for sure which onnes to get. If you are willing to share your information I would appreciate it.

Why would you think that they might cause problems with the CC unit?

Bosch CC units terminate automatic operation when they sense brake lights on. In the Mercedes world, it's well known that the control units burn out unless you use the correct factory bulbs. I'm wondering if the same is true for Porsche.

I'm using these for the tail lights:

http://www.ledwholesalers.com/store/index.php?act=viewProd&productId=597 (in red)

I've also done the "European" tail light conversion, as outlined in this thread:

http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/550305-upgraded-rear-tail-lights-using-euro-fog-slots-photos.html

And I've ordered Ebay 260662256999 for the extra sockets.

I evidently can't do the main brake lights, but I'm going to install an LED bulb in the high mounted brake light, with a strobe unit and a load resistor. This should be an improvement in safety.

That's very interesting. I figured the CC would use a separate pole on the brake light switch. But after looking at the diagrams, it appears that that there isn't even a direct connection from the CC to the brake switch. So maybe the reduced BL current resulting from the use of LEDs "might" indeed affect it. Its pretty early, but I learned something already today.

If the cruise control does not work with LED brake lights, the problem is that the conventional bulb works as a resistor when current is not applied, whereas the LED does not, so that the voltage measured on the line when the light is not active is wrongly interpreted by the cruise control. A resistor connected in parallel should do the trick, or even a small bulb. Not sure what value is required. Measuring the actual voltage on the brake light circuit will tell the entire story.

It is true that the CC will not work with LED bulbs. It senses them as a burnt out bulb. Not sure about adding resistors,etc.
I have used LED's on the interior, under Hood, and in the dash and really like them.
You cannot use them in the blinkers unless using a special flasher available at superbright LED's.

That's makes sence maybe because of the low voltage/current draw ?

Would be nice if LED headlights were more available would save some power

The issue is that the LED conducts a reasonable amount of current when it is lit, but when it is not lit, it goes to infinite resistance as the voltage across it crosses below a certain threshold, whereas the incandescent lamp draws current all the way down to zero. Thus, the "brake pedal sensor" on the cruise control will sense zero volts across an unlit incandescent lamp, but it will sense some mid voltage with an unlit LED, a voltage it will detect as "brake engaged".

If that makes sense. Just measure the voltage in the wire that goes to the brake light with a digital voltmeter, and you should see the difference.

Which type did you use? Red, obviously, but which opening angle (narrow 15/18 deg or wide 100 deg) and how many LEDs?

I have replaced all of my exterior lights with LEDs except headlights and fogs, well, maybe not the reverse bulbs -- I can't remember right now, but yes on license plate lights. I did use the flasher relay from SuperBrightLEDs. It is necessary for the front turn signals since they are dual-filament bulbs. I used the 5 watt version of these linked bulbs, which do not appear to be available any more. But I tried the 3 watts bulbs and they were fine, I just wanted to use the 5's. I could not tell much difference that I can remember - 3 years ago...

BA15 High Power 1156/1157 3 Watt LED Bulb Specifications

My question is whether your cruise would work again if you either use standard bulbs for your brakes, or add a load resistor to each side.

I have no idea. I have not tried either one. Probably the CC will work again with regular brake light bulbs.

Yes, once I noticed the CC was not working, I changed back to reg. filament bulbs and it worked again.

Each PN junction where the light is created in an LED has voltage drop of round 2.2-2.5 volts when power is applied (current is flowing). For maximum efficiency, several (3 or 4) are stacked in series, for use on 12 volt systems, along with a current limiting resistor. But when power is cut (current flow is interrupted), resistance goes infinite, like it is not even there.

LEDs that are capable of lighting your way down the road are probably still a LONG way down the road.

UPDATE: That's looking like it was a bad call. From what I see (on velocity network), they are already available on VERY high-end cars right now, albeit with 3 per side for a total of 6 per vehicle!

I have already seen a 3.3 watt single LED flashlight that put out 130 lumens. I think that's about 1/3 of what is needed per headlight for a single LED. It comes down to more a matter of getting the reflector/lens setup right to form the emission pattern of the LED into to the desired beam pattern in an efficient manor.

My thoughts.

Id have to look at the circuit more in depth but it seems that a seperate brake sensor for cruise might solve this?

If you care to read the thread, I have explained how you fix this by adding one single resistor from the brake light circuit to ground. This is all that is needed.

If you do not believe me, then by all means, you are of course free to do it differently. You will require extra wiring, and adding of an extra switch and fabricating a bracket, or finding a compatible dual pole switch. Plus you will need a resistor to ground. The choice is yours. (Of course, if you do not believe in my first solution, I am not sure why you should beileve in anything else I wrote)

From the standpoint of a contributor to this thread, I believe that what you are stating is 100% true.

But your solution leaves 1 of the 2 advantages of LEDs compromised.

The 2 (major) advantages of LED lighting are: A full order of magnitude less power usage vs. filament (incandescent) bulbs, for the same amount of light. And expected lifespan is an order of magnitude (or more) greater.

Your solution will only give you the second one, since the shunt resistor will now make up for the extra current that the filament bulb would have used.
If you are only concerned about lifespan, it IS a viable solution.

The (hot) resistance of a brake light bulb should be around 6-7 ohms (25W). But it is very possible that the circuit could be tripped by a (much?) higher resistance. The question still remains:

What is the highest value of resistance that will trip it every time?