Replacing Globes With Led

[ZAP]

I thought I would share with you a problem that I have had with my F6 since I replaced the rear tail lights with LED's, and this stopped the cruise control working.

The CC still shows up on the dash, but I cannot set any speed.

I did some research and found a aricle from Silicone Chip online :

"Many cruise control units use the brake pedal and stop lamp globes to sense when the brakes of the car have been applied, disengaging the CC unit. They use the cold resistance of the globes to take a sensing input to ground which allows the CC to be engaged. When the brakes are applied, this input goes to +12V as the lamps light, disengaging the CC.

If all the globes are replaced with LED units, the CC may not work as the input might not "see" a low resistance to chassis.

In regards to "lamp out" sensors and CCs, probably the best solution is to fit small globes in parallel with the LEDs and hide them inside the car. The rating of the lamp would be selected to allow for these sensors to work but this tends to defeat the purpose of the exercise (except for the faster turn on time of the LEDs)."

Now I am hoping that someone here with a electronics background may be able to help me get my CC working again. I do not want to add another globe, so I am hoping that a resistor or similar device may be able to fix the problem or trick the CC.

Can anyone help ?

[ORSMBA]

Zap, I had the same issue when I installed mine, went back to standard globes and installed the leds into the wifes VT. But You could fit a resistor in series with the lamp wiring. What you need is the voltage , which is 13.8v and the wattage of the standard globe. Using simple ohms law R = Vsquared/P. R = Resistance in ohms, V = volts(13. and P is Watts. I cant remember what the tail lights are but if the were 15W you would have 13.8 squared/15 = 12.696ohms.

Hope this helps

Matt

[ZAP]

The tail lights are 5w/21w.

I guess only the brake light (21w) is what I should be looking at....9 ohms ?

[ORSMBA]

id actually say its the tail light 5w . As you notice that it wont initiate crusie mode unless you have your lights off

Matt

[Benny]

I'd say that for the CC to operate correctly you need to match the total resistance of your new LED circuit to the same of the old incandecant circuit. So as what has already been said you need to measure the resistance of the old globe and make your new installation the same. This can be done by adding a resistor in series with the globe.

[ORSMBA]

Ill explain this then, with the LED's installed the cruise control works fine when you dont have your lights on. correct? It turns off when you put your foot on the brake. It fails to engage when you have your lights on. Which means it must be sensing the tail light filament, because it cant sense the main filament as it has no power going through it. Unless of course the computer is actually checking the resistance of the globe which I highly doubt.

Matt

[Adam]

I haven't put LED's in mine but my brake light fuse blew due to a trailer I was towing and the CC didn't work at all, whether the lights were on or off.

I assume it's the same in Zap's case.

[PozzE]

OK from consulting the wiring diagram, the crusie control "sense" is connected basically to +12v from the battery through the brake switch...

When the brakes are not being applied the switch is open and the line is at 12V...

When a brake lamp is not on and the filament is cold it has very little resistance, bascally zero, until the globes gets hot and starts to glow then the wire has a much higher resistance...

When the brake is applied; for a split second the line is shorted to ground before the globe heats up. This will cause a drop in the voltage on the line which the cruise controll will sense...

As an LED brake light draws 3/10ths of FA compared to a brake globe and works completely differently, it won't cause the voltage drop...

A resistance in series won't make much difference as the "resistance" (used very loosely) of the LED's is so high.

A resistance in parrallel will work... but won't increase in time and keep the line low. This is not a good thing.

As Sili Chip says a globe in line will work but defeats the purpose...

Its all the fault of the silly way the cruise controll senses the brakes, I'll have a think about it and see if there is an easy way to fix it...

[bkofoed]

Hmm, If indeed it does operate as you describe, and needs a brief spike of currnet to ground to sense brake engagement, perhaps a suitably sized Electrolytic capacitor could work? They are cheap have high current draw to charge (and negligable current flow when charged).

It would work like this:

When the brake switch is open circuit, the cap would discharge through the LED's fairly quick - perhaps a half-second brakelight soft fade out - may even look good! And it would serve to be a near short when exposed to 12v as the brake switch is closed.

My other concern with the 'resistor bank' approach is that dissipating 20 something watts into resistors (remember a 5 watt ceramic resistor is the size of a lego brick) is likely to generate a bit of heat over time, and look big and ugly - as noted above resistors don't have the non-linearity vs temp of a real globe anyway so may not fool the ecu.

I'd be interested to hear if someone can try it- I'd go with a 1000uF /25v to start with- and remeber to get the polarity right! If it's a sucess, it shall be known as BK'S LED BRAKELIGHT MOD!

I would have a go myself this weekend, but alas I need to focus on my Laser TX3 which insists on ignoring the Oxy and temp sensors and running in 'cold start' mode all the time :( (read rich as hell -enough to kill my cat in no time)

Good luck!

BK

[bkofoed]

So to clarify you would have the Capacitor in parallel with the LED globe array:

GND------LED array--\--------- ( Brake switch ) ----------- +12

GND------I|+----------/

1000uF/25v Electrolytic cap

On triggering the Brake, the cap will initially be 'seen' as a very low resistance path while the electrolytic charges, and high resistance some milliseconds later as it approaches full charge. The charge flow to a cap such as this with no series limiting resistor is very quick, in the order of milliseconds, so it would not cause a detectable delay to the brakes lighting.

On the release of the Brake, the now charged cap will keep the 12v across the brakes LED's for a brief period while it discharges. The low current requirements of LED's means you may get a half second or so soft fade out after each braking activity.

Can't wait for someone to try it!

BK