Bulb Out Monitor Questions W124 - PeachParts Mercedes-Benz Forum

87tdwagen · #1 01-05-2021, 04:26 PM

Electrical gurus, here's a question for ya that I could not find a straight answer to in the forum thus far.

Does the bulb outage monitor check for lower than defined spec voltage only, or can it be set off due to more than expected voltage?

The issue at hand, on my 300TD, I changed the third brake light to and LED unit, and also used the (US) unused fog lamp locations in the tail light to add secondary set of brake bulbs wired in series to the existing bulbs. I now have a total of 4x brake bulbs plus the LED 3rd brake light. If anything, I know I dropped a little wattage with going LED, but thought that by adding the extra brake bulbs, the brake circuit should be drawing more wattage now than before the mods. Would this extra wattage set off the bulb outage monitor?

I could not find a clear answer to this, and not much was indicated as this being an issue in their brake light upgrades which many have done. Is there a different wiring approach that I should take?

Edit: I know it is in the brake circuit as the indicator turns only only when activating the bakes initially. Have been on, with or without the LED 3rd brake light connected, so it comes down to the extra brake bulbs added to the tail lamps

Any advice, guidance is appreciated.
Thanks

Diseasel300 · #2 01-05-2021, 04:47 PM

Extra bulbs should be wired in parallel, not series. By wiring them in series, you have reduced their brightness by HALF, essentially negating the entire purpose you're going for. The bulb-out monitor sees this as a poor connection or bad bulb and rightly displays a warning. Every W126 I've owned has gotten the rear fogs wired in parallel with the brake lights and I've had zero issues.

The 3rd brake light is monitored independently from the other brake lights, so the reduced burden of the LED will also trip the bulb out monitor. You must use an LED with a loading resistor or stick with the incandescents if you want the bulb out monitor to be satisfied.

87tdwagen · #3 01-05-2021, 05:15 PM

Thanks Diseasel300

My terminology was off, my bad. I guess they are wired in parallel, I soldered a pig tail off of the existing brake lamp wire and added that to the blank socket, so each brake bulb is wired independently, albeit the 12V to both bulbs is fed off of the same power wire in the tail lamp harness, ground is shared by design.

I hope this is the correct method of wiring, I did not see a output drop, just a gain, but maybe old eyes.

You indicated that the 3rd is monitored independently and that seems to be the main issue, having switched to LED, I tried to stay close in total wattage between the LED unit and the original 3rd brake bulb, but as you indicate maybe adding the loading resistor (if possible) might solve the issue. Something to investigate further

If this is not something I could easily add, please let me know.

Thanks again,

87tdwagen · #4 01-06-2021, 12:38 PM

Diseasel300 you are tops, thanks a bunch. Picked up a Sylvania LED load equalizer at a local car parts shop for $15, actually for a pair of them, usually intended for turn signal conversions, but works equally as well for a 3rd Brake light LED conversion, takes care of the bulb outage warning.

Thanks and happy motoring

Diseasel300 · #5 01-06-2021, 02:33 PM

Good to hear that it's an easy fix!

h3ffe · #6 01-06-2021, 07:17 PM

I eliminated the 3rd brake light and have a permanent monitor light (once I touch the brakes for the first time, that is). Have been meaning to research how to trick the monitor.

Will this work if I have no bulb at all?

Quote:

Originally Posted by 87tdwagen

Diseasel300 you are tops, thanks a bunch. Picked up a Sylvania LED load equalizer at a local car parts shop for $15, actually for a pair of them, usually intended for turn signal conversions, but works equally as well for a 3rd Brake light LED conversion, takes care of the bulb outage warning.

Thanks and happy motoring

87tdwagen · #7 01-06-2021, 08:15 PM

Quote:

Originally Posted by h3ffe

I eliminated the 3rd brake light and have a permanent monitor light (once I touch the brakes for the first time, that is). Have been meaning to research how to trick the monitor.

Will this work if I have no bulb at all?

I'm by no means an expert on this, but it should work for you. Technically it is a resistor, heat sink it is wired across the leads +/- and has no polarity requirements. My 3rd was a 23w unit so a 25w load equalizer should replace the current back to the bulb outage monitor. If for some reason you need to add a bulb to the circuit, just add a single LED bulb and burry it away with the equalizer. At $3-5 per unit, it's a cheap experiment.

h3ffe · #8 01-06-2021, 09:14 PM

Good point. Thanks for talking it through. Ordered!

If it doesn't work, or if I need to add an LED and stuff it away, I'll report back.

h3ffe · #9 01-06-2021, 09:23 PM

Where did you end up installing the load equalizer? Instructions say it gets very hot and should be securely screwed to a metal surface.

87tdwagen · #10 01-06-2021, 09:34 PM

I bolted it to the metal inside skin of the hatch. It is wired right into to plug outlet for the original brake light. I have not yet felt the heat output, since it is only on when brakes are applied, I'm not too concerned. In the end, at 25w I dont think it will put out much more heat than the original bulb did.