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Thread: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

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    Senior Member Jlaa's Avatar
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    LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Has anyone successfully fitted rear red LED bulbs in the brakelights and successfully fooled the bulb out warning circuit?

    By fooling I mean - no inelegant load resistors, and no cheating by pulling out the warning lamp bulb.

    I ws under the impression that the circuit works by measuring the differential in voltage drop between the L and R. Meaning if both sides fail, the bulb out circuit will not catch the failure.

    So i fit both sides with an LED bulb .... no nice. Then I fit all three positions with LEDs....(chmsl). Also no dice.

    many thanks for any thoughts...
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Jlaa, there is a mod for the headlights when relays are used, of course the drawback is you lose the headlight bulb-out warning.

    I assume the same could be done for the tails with enough detective work, and experimentation on the module. But AFAIK, nobody has done this, at least on this forum.

    Do the LED's offer enough advantage (higher lumens, etc) to make it worth the hassle?


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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    I run aftermarket tail lights with LED's built into them. They also have resistor packs to fool the bulb out indicator.

    The light output of LED's is much higher than conventional bulbs.
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    Senior Member Jlaa's Avatar
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Quote Originally Posted by 2phast View Post
    I run aftermarket tail lights with LED's built into them. They also have resistor packs to fool the bulb out indicator.

    The light output of LED's is much higher than conventional bulbs.
    Thanks 2phast, I surmised as much reading through your website.

    Quote Originally Posted by gsxr View Post
    Jlaa, there is a mod for the headlights when relays are used, of course the drawback is you lose the headlight bulb-out warning.

    I assume the same could be done for the tails with enough detective work, and experimentation on the module. But AFAIK, nobody has done this, at least on this forum.

    Do the LED's offer enough advantage (higher lumens, etc) to make it worth the hassle?

    I'd like to keep my OE tail light lenses ---- I don't really want to modify the look of the car just LEDs. My main driver is that I have two pairs of 1156 compatible LED bulbs that but put out a SHEET-TON of light. The use Cree drivers I think --- anyways they bright! HOLY SMOKES bright. I've used these to good effect in my other hobby car where the brake lights weren't that bright to begin with ---- gotta prevent the distracted person behind ya from rear ending you donchaknow! I'd like to see if I can use these and not have them go to waste.

    I did a little testing ----

    + FOGLIGHTS --- both bulbs removed --- does not trip the bulb error light
    + FOGLIGHTS --- one bulb removed --- DOES trip the bulb error light

    + REVERSE LIGHTS --- both bulbs removed --- does not trip the bulb error light
    + REVERSE LIGHTS --- one bulb removed --- DOES trip the bulb error light

    + BRAKE LIGHTS --- all three (including CHMSL) bulbs removed - DOES trip the bulb error light (??)
    + BRAKE LIGHTS --- L/R bulbs removed (CHMSL populated by an incandescent bulb) - DOES trip the bulb error light (??)

    I was hoping for some kind of combination where I could equally populate L/R brake lights with LEDs and not trip the bulb error light .. but that seems not to be an option.

    As well, I pulled up some w124 circuit diagrams --- these treat N7 - EXTERIOR LAMP FAILURE MONITORING UNIT as a black box, so I cannot really tell what's going on in there -----
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    E500 n00b nocfn's Avatar
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    FWIW I used red LED that superbrightLED dot com said would work in older MB to not trigger the indicator - to no avail. So they got sent back and I use the LED for the dome lights, trunk, surprisingly the license plate and the puddles.
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Quote Originally Posted by nocfn View Post
    FWIW I used red LED that superbrightLED dot com said would work in older MB to not trigger the indicator - to no avail. So they got sent back and I use the LED for the dome lights, trunk, surprisingly the license plate and the puddles.
    Thanks for relating. I did the interior lighting as well --- front/rear dome lights and the trunk/puddles lights --- with warm white LEDs. I find that the light output is much increased (argghhh - this was the year I started bifocals) and yet the color temperature is still period-correct (not too blue)

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Quote Originally Posted by Jlaa View Post
    I did the interior lighting as well --- front/rear dome lights and the trunk/puddles lights --- with warm white LEDs.
    Would it be too much to ask which bulbs you used for which applications? For example, which bulb did you use for the front dome light? Also, did you replace the bulbs in the front hvac vents? Oh, and how about the rear reading lights?
    Greg
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Well my car is not here - getting trans rebuilt. Cool white torpedo shaped iirc
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Pics attached of the modification to the module for headlight relays.

    I assume a similar mod would be required for the tails, cutting different circuit board traces.

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Quote Originally Posted by gsxr View Post
    Pics attached of the modification to the module for headlight relays.

    I assume a similar mod would be required for the tails, cutting different circuit board traces.

    Thanks for digging those up GSXR. Ugh, changing the function of the N7 - EXTERIOR LAMP FAILURE MONITORING UNIT is not what I had in point. I was hoping to workaround it in an elegant way. Might you have the original poster's writeup on how they reverse-engineered the circuit schematics for N7?

    thanks.
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    OK for posterity's sake, here is the ULN24545A IC used in the N7 - EXTERIOR LAMP FAILURE MONITORING UNIT.
    Since this IC is just a bunch of OR-gates (If any condition is true, then the bulb out indicator goes on) then the nipping of leads in the
    N7 - EXTERIOR LAMP FAILURE MONITORING UNIT is just a matter of snipping inputs to the correct OR-gates.

    One of these days I'll harvest a
    N7 - EXTERIOR LAMP FAILURE MONITORING UNIT from the junkyard and experiment. For now, I'm putting this IC data-sheet here so that I don't forget.
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    I am surprised there is no mention here of a simple elegant way to double the amount of W124 brake light illumination...
    Wire the brake light circuit, inside the tailights at the rear bulb holders, to include both rear fog light locations.

    For LHD the fog bulb holder on the left needs to be tied into the left brake light wire; the right side (where no bulb holder resides) needs an additional easily added MBz bulb holder tied into the right brake light wire.
    With an added pair of standard 32cp/21w bulbs, no bulb warning light triggered, illumination doubled and looks better balanced from the rear when braking.

    And, with a pair of diodes strategically placed on either side of the left sided fog light wiring, you can retain normal left rear fog light function from the headlight switch as usual.

    I have done this with many European cars (MBz, BMW, Audi, Volvo) that have a pair of rear fog light lenses.
    Last edited by northNH; 3 Weeks Ago at 09:18 PM.
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Quote Originally Posted by northNH View Post
    I am surprised there is no mention here of a simple elegant way to double the amount of W124 brake light illumination...
    Wire the brake light circuit, inside the tailights at the rear bulb holders, to include both rear fog light locations.

    For LHD the fog bulb holder on the left needs to be tied into the left brake light wire; the right side (where no bulb holder resides) needs an additional easily added MBz bulb holder tied into the right brake light wire.
    With an added pair of standard 32cp/21w bulbs, no bulb warning light triggered, illumination doubled and looks better balanced from the rear when braking.

    And, with a pair of diodes strategically placed on either side of the left sided fog light wiring, you can retain normal left rear fog light function from the headlight switch as usual.

    I have done this with many European cars (MBz, BMW, Audi, Volvo) that have a pair of rear fog light lenses.
    This sounds like an excellent topic for a HOW-TO illustrated pictorial.....

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    That sounds like the simple fix for brighter brake lights.

    I would hope some genius here could figure out how to make the 3rd brake light blink/flash when you apply the brakes. Similar to what I have seen on some Subaru WRXTIs. I think that is the ultimate attention getter.
    Terry

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Quote Originally Posted by TerryA View Post
    That sounds like the simple fix for brighter brake lights.

    I would hope some genius here could figure out how to make the 3rd brake light blink/flash when you apply the brakes. Similar to what I have seen on some Subaru WRXTIs. I think that is the ultimate attention getter.
    There are commercial, off-the-shelf brake light flasher modules available. I installed one on my motorcycle about 15 years ago that progressively flashes the brake light (flashing quickly at first, and less frequently and then to a solid brake light as you slow down to a stop). It's a great safety measure.

    On motorcycles, you sometimes see daytime headlight flashers as well, though they are probably not practical (or legal) for cars.

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Quote Originally Posted by gerryvz View Post
    There are commercial, off-the-shelf brake light flasher modules available. I installed one on my motorcycle about 15 years ago that progressively flashes the brake light (flashing quickly at first, and less frequently and then to a solid brake light as you slow down to a stop). It's a great safety measure.

    On motorcycles, you sometimes see daytime headlight flashers as well, though they are probably not practical (or legal) for cars.
    How do you think that would work on our E500Es. I would assume that the dash short light would be flashing when the brake light is flashing. Probably not a bad thing as long as it goes out in the end.

    Can you point out where to get something like that flasher module? I for one would be interested in installing one.

    The only thing wrong with the ones on the Subi is that it's down to low. Below the rear bumper. On our cars it would be at eye level.
    Terry

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Terry,

    Here is the one I used for my motorcycle:

    http://www.signaldynamics.com/backoffxp

    I would think you could wire it into just your third brake lamp wire so it would apply just to that lamp. Since it's a relay, I don't think it would affect the lamp-out circuit, since it's just very momentary flashing and simply an interrupter.

    Cheers,
    Gerry

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Thanks Gerry,

    I'll check it out. It
    Terry

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit





    Working on sharing the recipe...
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Quote Originally Posted by northNH View Post
    I am surprised there is no mention here of a simple elegant way to double the amount of W124 brake light illumination...
    Wire the brake light circuit, inside the tailights at the rear bulb holders, to include both rear fog light locations.

    For LHD the fog bulb holder on the left needs to be tied into the left brake light wire; the right side (where no bulb holder resides) needs an additional easily added MBz bulb holder tied into the right brake light wire.
    With an added pair of standard 32cp/21w bulbs, no bulb warning light triggered, illumination doubled and looks better balanced from the rear when braking.

    And, with a pair of diodes strategically placed on either side of the left sided fog light wiring, you can retain normal left rear fog light function from the headlight switch as usual.

    I have done this with many European cars (MBz, BMW, Audi, Volvo) that have a pair of rear fog light lenses.
    northNH, thanks for this clever idea! I would like to implement this --- and use incandescent bulbs in the fog light receptacles (existing on the L ... must add in the R) ---- and use LEDs bulbs in the brake postion so as not to confuse the N7 bulb out warning circuit. Since the fog/brake lights would be tied in parallel, the N7 circuit would of course see the LED as an open circuit, but see the incandescent fog bulb at the same time, and the voltage drop across the incandescent fog bulb would tell the N7 that everything is "a-ok"

    At the same time, by using a pair of LED/incandescent bulbs for each side, as opposed to using two incandescent bulbs per side, I would seek to preserve the lifespan of the brake-bulb-sensing-N7 circuit by not asking it to handle TWICE the current (2x21W = 42W)

    In order to preserve the function of the L fog light, I can understand using ONE diode to prevent current going to the fog bulb to the brake bulb. Can you tell me why you suggest using TWO diodes --- I am guessing it is the following ---- can I ask you to verify this? Also, how do I select diodes (I've never worked with them before).

    Thanks
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Sorry for the delay getting back to this topic...real life's been creeping in around the edges.

    "I would like to implement this --- and use incandescent bulbs in the fog light receptacles (existing on the L ... must add in the R) ---- and use LEDs bulbs in the brake postion so as not to confuse the N7 bulb out warning circuit."
    << I have no experience with LEDs here>>

    "Since the fog/brake lights would be tied in parallel, the N7 circuit would of course see the LED as an open circuit, but see the incandescent fog bulb at the same time, and the voltage drop across the incandescent fog bulb would tell the N7 that everything is "a-ok"
    <<This ain't what's happening...>>


    "At the same time, by using a pair of LED/incandescent bulbs for each side, as opposed to using two incandescent bulbs per side, I would seek to preserve the lifespan of the brake-bulb-sensing-N7 circuit by not asking it to handle TWICE the current (2x21W = 42W)"
    << This is not significant, bulb sensing circuit "sleeping" i.e. no current draw, when not activated>>


    "In order to preserve the function of the L fog light, I can understand using ONE diode to prevent current going to the fog bulb to the brake bulb. Can you tell me why you suggest using TWO diodes --- I am guessing it is the following ---- can I ask you to verify this? Also, how do I select diodes (I've never worked with them before)."


    As I have things wired (4 x 21W bulbs) there are no issues with the warning light. The brake light sensing unit is electrically unconcerned with the (balanced) additional current draw.
    If one cares naught about retaining rear fog function, you need no diodes, will have no warning light malfunction, and the disconnected fog light supply wire in the left bulb housing remains unconnected/isolated.

    Your interpretation of the role of adding the diodes is somewhat flawed; they have nothing to do with the dash bulb failure warning light, they are only needed to retain the (one-sided) rear fog function.
    Your schematic drawing is entirely correct, including diode polarity orientation.

    Therefore, the diode between the brake and fog light bulbs prevents any of the brake bulbs from lighting when you switch on the rear fog light, and the diode between the left fog bulb and it's supply from the headlight switch prevents the tiny fog light indicator bulb in the switch from lighting when you use the brakes...

    Diodes should be rated at >2 amps...
    For reference:
    http://polevolt.co.uk/cgi-bin/sh0000...html#a10ADIODE

    More, or less clear? Let me know if more info would be useful.
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  41. #23
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    I just wonder WHY, would'nt regular bulbs do? Where I live and drive, they are a bit "itchy" police, car inspection, MOT or what not.
    I Think the "styling era" should be out of fashion by now for these cars but thats me, grown out of that, maybe?
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Quote Originally Posted by northNH View Post

    ...Your interpretation of the role of adding the diodes is somewhat flawed; they have nothing to do with the dash bulb failure warning light, they are only needed to retain the (one-sided) rear fog function.
    Your schematic drawing is entirely correct, including diode polarity orientation.

    Therefore, the diode between the brake and fog light bulbs prevents any of the brake bulbs from lighting when you switch on the rear fog light, and the diode between the left fog bulb and it's supply from the headlight switch prevents the tiny fog light indicator bulb in the switch from lighting when you use the brakes...

    Diodes should be rated at >2 amps...
    For reference:
    http://polevolt.co.uk/cgi-bin/sh0000...html#a10ADIODE

    More, or less clear? Let me know if more info would be useful.

    Thanks for the reply! Indeed my reasoning behind the diodes was to preserve the function of the rear fog light circuit ----- and my intention of using LEDs for just the brake lights but not the fog light is so that:

    a) I'm not presenting N7 with 42 watts (as opposed to 21 watts) --- and yes I understand you have been running @ 42 watts w/o issue.
    b) Because I have an extra pair of red LEDs --- which are WAY brighter than incandescent bulbs.

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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    EDIT - 2017-8-15 --- My original assertion that it is possible to install two LED-bulbs per side (driver / passenger) in order avoid tripping the BW circuit is incorrect. Even though two LED bulbs avoided tripping the driver's side BOW circuit, doing the exact same thing on the passenger's side yielded no such luck ---- therefore I have changed the text below to reflect installing ONE LED Bulb and ONE incandescent bulb per-side to avoid tripping the BOW circuit.


    Thanks for the guidance northNH - Hah! I discovered that it is possible to install LED bulbs in the brake lights and still have a completely functioning bulb-out warning circuit. The way to do it is to install one LED bulb and one Incandescent bulb per side.

    The conundrum was that installing one LED bulb in the brake bulb holder results in a bulb-out warning notification ---- reason being that one LED bulb sunk so little current that the BOW thought that the bulb was out.

    Therefore, if, on each side, you install one LED bulb in the brake light position in parallel with one incandescent bulb in the foglight position, the BOW circuit is happy ---- the BOW circuit sees the incandescent bulb and everything is hunky dory. Note that the BOW circuit will only report though if the incandescent bulb is burnt out. If the LED bulb fails, the BOW circuit won't know about it.

    The below pictures details out how to do the modification on the driver's side, which is a little more involved than the passenger's side because the driver's side requires a pair of diodes to ensure that the rear fog light mechanism still operates correctly.


    IMG_6750.jpgIMG_6758.jpgIMG_6769.jpgIMG_6774.jpgIMG_6775.jpgIMG_6776.jpgIMG_6755.jpgIMG_6756.jpgIMG_6762.jpgIMG_6763.jpgIMG_6764.jpg

    The below picture shows the passenger side wiring --- note that I all I did is harvest a "bulb contact" from a junked w124 and solder the lead wire to the brake light bulb:
    IMG_6791.jpg

    Here is the video showing the completed product --- the outer corners (brake light positions) are the bay LED, and the inner corners (fog light position) are incandescent bulbs. No Bulb Out Warnings!

    Last edited by Jlaa; 1 Day Ago at 04:47 PM. Reason: Edited faulty assumptions
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    1996 Porsche 993 C2

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  46. #26
    Senior Member Jlaa's Avatar
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    Re: LED bulbs in the rear brake lights and fooling the bulb out warning circuit

    Note --- I should mention that I haven't tried the dual-LED-bulb method with the cruise control yet. If the resistance of two LED bulbs in parallel in the left / right brake light circuit (along with the incandescent bulb in the CHMSL) is such that it causes the CC not to function, then I will revert each L/R side to brake/fog LED/incandescent ..... (the second video) ..... which *should* present the right resistance to cause the CC circuit to function again.
    ---
    Instagram Here
    1993 MB 500E
    1996 Porsche 993 C2

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