LED Brake Lights

[deputydog95]

Originally Posted by Tremelune

Excellent comparison. Were the photos all taken at the same F-stop and shutter speed?

Does anyone know why cruise control is wired into the taillights?? That seems really weird to me.

When you tap the brakes it turns cruise off. I guess it senses a voltage change when the lights go on.

[famoroso]

Originally Posted by Tremelune

Excellent comparison. Were the photos all taken at the same F-stop and shutter speed?

According to my son, yes, all the same... F: 4.5 & 1/6 shutter.

[spoke]

Originally Posted by Tremelune

Does anyone know why cruise control is wired into the taillights?? That seems really weird to me.

As mentioned before, the CC uses the brake lights to sense when the driver touches the brakes and the CC should be disengaged.

Just to explain a little bit of what is going on with the interaction of the CC and LED brake lights take a look at a simplified schematic of the brake light circuit with CC below.

The CC uses the brake light circuit to sense when the driver presses on the brake to disengage the CC. To sense the brake pressing, the CC has a wire connected to the brake lights.

A comparator with a voltage threshold senses the voltage on the brake lights. When the brake light voltage is below the threshold, the CC can engage. When the brake light voltage is above the threshold, the CC is disengaged.

To provide fail-safe operation for the CC in case the wire to the brake lights is broken or other failure scenarios, the CC provides a pull-up resistor to 12V. If the wire to the brake lights breaks, the resistor will pull up the CC input and disengage the CC.

WHY DOES THE CC WORK WITH BULBS AND NOT LEDs?

Looking at the brake light circuit with the brake switch off (brakes not pressed), the bulbs pull the voltage at the CC input towards ground while the pull-up resistor in the CC tries to pull up towards 12V.

Putting some values to the resistor and bulbs will help determine what the off voltage at the input of the CC will be. From measurements on an '83 911 with CC, we found about 8ma of current so the pull-up resistor is V/I. For simplicity, lets assume the pull-up resistor is 1000 ohm. Across 12V, the 1000 ohm allows 12/1000 = 12ma.

A 21W bulb would have a resistance of about 12V*12V/21W ~ 7 ohm. With 2 bulbs in parallel, the bulbs look like 3.5 ohm resistance.

1k ohm pulling up with 3.5 ohm pulling down. Guess who wins. Off voltage is voltage divider between these 2 resistors: VT = 12V x (3.5/(1k + 3.5)) = 0.041V. Damn close to zero in my estimation.

To sum up the operation, with the brakes off and CC engaged, the voltage at the brake lights because of the CC pull-up resistor is 41mV. If the CC threshold voltage is a 1 or 2 volts (I don't know the exact value) or lower, the CC assumes the brakes are off.

In my next post, I'll consider what happens when LED brake lights are installed.

[spoke]

When all the brake bulbs are replaced by LED replacements either of my design or other OEM products, the characteristic impedance looking into the brake lights is significantly changed. Recall that the bulb is a pure resistance of about 7 ohms. Now the circuit looks like the one below. Here we see LEDs in series with some current limiting device.

The LED does not conduct current until about 0.7V to 1V+ is applied to it. With increasing current, the LED voltage does not change that much getting to about 2V for an amber or red LED. With 3 diodes in series as shown, the LEDs wouldn't conduct any current unit the voltage across them is probably greater than 3V.

Given the pull up resistor of 1k, it will pull up on the brake light wire until the circuit is equalized. Let's assume the LED voltage when turned on a little bit is about 1.5V and the current limiting resistor is zero.

The pull-up resistor raises the voltage on the CC input wire to 3 x 1.5V = 4.5V. The resistor current is (12-4.5)/1k = 7.5ma. With 4.5V on the brake wire and cc input, likely the CC will be disabled.

Not only will the CC be disabled, but the 7.5ma is flowing into the LEDs. An LED requires extremely small current to produce light. This is enough current to give OEM LEDs a faint glow with the brakes off. This is what has been observed by some.

The LEDs being on gently isn't necessarily a big issue as usually the brakes also serve as running lights. Where it is a big issue is with the 3rd brake light. This light has only one purpose and that is for brakes. Having the 3rd brake light on gently is very bad.

To sum up, with all LED brakes:
1) The CC doesn't work.
2) The LED brake lights light gently with brakes off.

Next on to a fix for all-LED brake solutions.

[spoke]

All LED brake light solution has 2 issues: the CC doesn't work and the LEDs are gently on with the brakes off. To solve these issues, the only practical add-on solution is to provide a ballast resistor as shown below.

The desire is to keep the LEDs off (voltage must be below 4.5V) and satisfy the CC unit's threshold. I don't know what the CC threshold is so just guessing at it being 0.5 or less to a couple of volts.

One issue with ballast resistors is heat generated. The lower the value the more the heat. Keeping the ballast resistors as high in value as possible gives the least heat generated. Another issue is size as the more power the resistor burns the larger it is.

Ignoring the LEDs for now, the voltage on the brake light wire is determined by the pull-up resistor and ballast resistors.

V = 12V x R_Ballast / (R_Ballast + 1k)

On my boards, I use a 300 ohm ballast resistor. This was found to be satisfactory for (I think) an '83 911. With 2 ballast resistors, the total is 150 ohm and voltage is:

V = 12V x 150/(150 + 1k) = 1.6V

This is well below the voltage needed to keep the LEDs from turning on. However, a couple of folks with '88 911's found their CC didn't work.

I tried an experiment with Frank as he detailed above by doubling up on the resistors, each ballast resistor now being 150 ohm. 2 in parallel give 75 ohm and the voltage is now:

V = 12V x 75/(75 + 1k) = 0.8V

It appears the '88 CC unit's threshold is below 0.8V. I won't double the 300 ohm ballast resistors since the power and temperature rise is more than I would like.

SOLUTIONS TO WORK WITH THE CC:

With my LED solutions:

1) If going all LED and MY < '88, CC should work

2) If going all LED and MY '88, '89, you must add a ballast resistor similar to this one. This would have to be added somewhere in the wiring. May fit in the brake cavity and crimped onto the wires of the LED board.

https://www.superbrightleds.com/moreinfo/headlight-load-resistors/led-light-load-resistor-kit-led-turn-signal-hyper-flash-warning-fix/190/831/

3) If MY > '84-'85'-'86-'87 (only have data on an '83-CC works; and '88-CC doesn't work) Leave the 3rd brake light as a bulb to satisfy the CC.

Again for the CC, it seems the CC unit was changed between MY '83 and MY '88. The '83 having a higher threshold than the '88. If anyone knows the differences, please enlighten me.

[famoroso]

I will test this...

3) If MY > '84-'85'-'86-'87 (only have data on an '83-CC works; and '88-CC doesn't work) Leave the 3rd brake light as a bulb to satisfy the CC.

...on my 1988 next weekend.

If it works, I'll swap an OE / incandescent CHMSL light bulb in for long road trips. Besides I-5, I don't use CC anyway. That being said, kudos to Spoke for having the intellectual curiosity to continue pursuit of a fully baked solution.

[spoke]

For those of you who are interested in front turnsignals for '74-'89 cars, I've been reminded that some (non US?) cars may not have front turnsignals acting as running lights.

One of the first adopters of the front turnsignal has run into this issue. Here's his front turnsignal bulb and a pigtail from my front turnsignal LED board. It is no issue to run the LED board with this socket. I just need to know if you need the BAY15S (single) or BAY15D (dual) bulb base when you order.

If anyone can shed light on what variation this is, please enlighten me. BTW, this vehicle is located in Australia.

[APachon71RN]

Spoke, I will look to see what I have in my front right now.

~Andy

[universeman]

I have the dual contact bulbs in my front turn signal housings. Texas, USA.

[APachon71RN]

dual prong on an 83 911SC

[Arne2]

RoW cars should all use the single filament bulb in the front signal, as the running light is the city light inside the headlight. Only US/Canada spec cars will have dual filament bulbs in the signals.

[famoroso]

Originally Posted by famoroso

I will test this...

3) If MY > '84-'85'-'86-'87 (only have data on an '83-CC works; and '88-CC doesn't work) Leave the 3rd brake light as a bulb to satisfy the CC.

...on my 1988 next weekend.

If it works, I'll swap an OE / incandescent CHMSL light bulb in for long road trips. Besides I-5, I don't use CC anyway. That being said, kudos to Spoke for having the intellectual curiosity to continue pursuit of a fully baked solution.

Tested.
Confirmed.

1988 Carrera...
With Spoke double resistor CHMSL LED and double resistor tail lamp / brake lamp LEDs = inoperable CC.
With incandescent OE CHMSL bulb and Spoke double resistor tail lamp / brake lamp LEDs = functioning CC.

[spoke]

Originally Posted by famoroso

Tested.
Confirmed.

1988 Carrera...
With Spoke double resistor CHMSL LED and double resistor tail lamp / brake lamp LEDs = inoperable CC.
With incandescent OE CHMSL bulb and Spoke double resistor tail lamp / brake lamp LEDs = functioning CC.

Frank,

Thanks for the info. I tried to satisfy the CC with 300 ohm, then 150 ohm (on your boards) on each board, but it's obvious the voltage threshold on the '80s CC is very low.

The CC input pulling up on the brake light wire is too much for the 150 in parallel with 150 (2 brake boards) = 75 ohms provided by the brake LED boards. Recall that incandescent bulbs of 21W variety have like 7 ohm resistance.

I think the next test would be to add one of those 50W 6 ohm ballast resistors sold at places like superbrightleds.com. This could be added behind a brake board or right at the CC brain in the interior. The brake light wire is available at the CC brain and may be more room to mount.

Does anyone have the wire diagram for '80s cruise control units?

Spoke

[DRACO A5OG]

If you like, I can take a pic of the HELLA LED CHMSL that I updated on my car.

[Uwon]

Spoke,
I'm emailing you the CC wiring diagrams for 84-89. Hope this helps.
Johan

[wwest]

The CC circuit has a failsafe feature, with the brakes off there is a resistor in the circuit that provides a small current to the brake lights to detect an open brake light circuit or blown bulbs. Not enough to show any brightness for incandescent bulbs but a ~1.2V drop across an LED.

This small voltage drop, often barely visible, is enough to disable the CC.

A cold 21W incandescent's filament resistance is a lot lower than a powered bulb.

[imoto]

Originally Posted by Arne2

RoW cars should all use the single filament bulb in the front signal, as the running light is the city light inside the headlight. Only US/Canada spec cars will have dual filament bulbs in the signals.

This is correct the front turn signal bulb is single filament on ROW cars like mine purely a single purpose lamp. The what we call front "parking light" is a small bulb at the base of the headlight.

[JohnJL]

Hi Spoke,

Have you installed your whole set of led replacements in a bare harness yet?

I am finishing up another car, I am going for LEDs wherever i can. I originally planned to clear epoxy a set of smd boards to the backs of a set of lenses to see how that worked, but I'd be willing to try yours first.

I see there are some complications where cruise control and other components are involved.

I build my own wiring harnesses out of 22759 wire and solid-state relays. The harness is bare-bones...just enough circuits of just enough capacity to keep everything as light as possible. The target car weight is around 900 kgs...

Here is a page from my wiring diagram. I know all the symbols are not engineering-correct but they work for me.

Do you see any reason why your lights wouldnt work here, or more precisely if there are any known issues with installing on a bare harness without any incandescents/connections to cruise control/fans/etc?



Do you have everything now for front and rear running lights, turns, brake and reverse?

[spoke]

Originally Posted by JohnJL

Hi Spoke,

Have you installed your whole set of led replacements in a bare harness yet?

Do you see any reason why your lights wouldnt work here, or more precisely if there are any known issues with installing on a bare harness without any incandescents/connections to cruise control/fans/etc?

Do you have everything now for front and rear running lights, turns, brake and reverse?

Right now I have brake, 3rd brake, and front turnsignal LED boards. Plan to add rear turnsignal boards in the near future. Won't have anything for reverse or most of the side lights. Those are available on multiple sites as universal plug-ins.

All the LED boards I have should work with your system as long as the flasher is LED-compatible. My 930 has LEDs on brake, 3rd brake, front and rear turnsignals with no issues.

[RocketWrench]

PM sent.