Zose Clever Germans

[Dangermouse]

One of my other cars is a Mercedes convertible and over the weekend, when I started it I got two “malfunctions” displayed on its info screen

Warning – Left Tail Lamp Out

Crap, a bulb blown, and then:

Warning – Left Tail Back-Up Lamp On

Crap, thought I, some electrical glitch. That will be fun (and expensive) to fix – water in a black box somewhere, I figured. So I walk to the rear of the car expecting to see a tail lamp out and the Reversing Lamp on. I mean who wouldn’t, based on those messages! But no, all is well. I stand at the rear while my wife operates all the signals, and all seems normal.

So, a little investigation in the manual, and it would appear that those clever Germans have engineered a redundant (i.e. back-up) tail-lamp circuit, whereby if a main tail lamp bulb goes out, the second one comes into play and keeps you safe (and legal) – ha- take that, DeLorean Safety Vehicle.

So maybe I am a noob, or maybe I have had American cars all these years, but I have never heard of this before and I am really impressed.

But now the engineer in me wants to know how they do it. Two filaments within the same bulb and some circuitry measures resistance and switches between the two either actively or passively?; two separate bulbs? Electron magic?

Lunchtime will be spent with a multimeter and old and new bulbs. I’m intrigued.

So, I know there are a couple of ex-MB techs on here (Michael and Kyle), so how is it done? But don’t tell me til after lunch.

[skynet]

This is common in zie german cars, at least in expensive ones. I've seen this in a BMW, when a regular tail light went out, it switched to brake lamp automatically. It detects the resistance I suppose.

[DMCVegas]

Or you could of course save several thousand dollars and just periodically get out and walk around the car yourself to check and see what bulbs are out...:biggrin:

How does your Alternator light work? Answer that question, and you might just answer your own on your Mercedes.

[Jacko]

Along this same line I've wondered how my Jaguar displays an 'idiot light' when a bulb 'somewhere' is burned out ... measures resistance somehow, I suppose. I'd like to have this feature on the Delorean.:umm:

[nofear365]

Along this same line I've wondered how my Jaguar displays an 'idiot light' when a bulb 'somewhere' is burned out ... measures resistance somehow, I suppose. I'd like to have this feature on the Delorean.:umm:


Same with my Jaguar XK8. Love it!

[Dangermouse]

Robert, yes I do take regular advantage of the plate glass windows at my office to check the state of all my lights in all the cars. And Yes, I understand how the Alternator light works in the D, but I am not sure it is relevant here. I am less interested in how they detect the blown bulb and more in what they do with that information.

Update: so what my lunchtime testing discovered was:

I only get the warning when the car first calls for the lights to be on. Kinda obvious but it threw me when I went to NAPA and I didn’t get an alarm. Car has automatic lights.
The manual calls “reversing” lights – “back-up” lights . Very confusing
The manual list the wrong part number for the tail lamps
The manual states that ”Back-Up bulbs will be brought into use when lamps malfunction” clearly implying a second set of bulbs. But there aren’t.
My left tail light is out (d’oh)

The lamp orientation in the rear assembly is basically vertical: top to bottom; Brake, Turn, Reverse, Tail

The tail light is a single filament 5W bulb. When I was checking the lamps the other night, I am pretty sure that the top light was lit. So when one of the bottom tail lamps burns out, the system switches to using the top brake lights (21W) for both sides (also single filament), presumably at reduced voltage.

I’ll confirm this tonight when I get the correct bulbs on the way home.

[pezzonovante88]

Same with my Jaguar XK8. Love it!
What year XK8 do you have?

[DMCVegas]

Robert, yes I do take regular advantage of the plate glass windows at my office to check the state of all my lights in all the cars. And Yes, I understand how the Alternator light works in the D, but I am not sure it is relevant here. I am less interested in how they detect the blown bulb and more in what they do with that information.

What I was saying about the Alternator light was that it might be sending a low amount of voltage through the bulbs to check that the circuit is closed. But that was before you posted more info.

My old Ford Freestar work van had a feature where if a bulb went out, it would notify you of a burned out lamp. It was the basic instrument panel with no information center, so it was just a warning light that did not point to a specific bulb.

I'm interested in the answer too, and think that you're probably on the right track here in thinking that the voltage to those rear bulbs is reduced. It's probably all controlled by circuitry to accomplish this rather than just simple circuits that connect everything.

[Shep]

Along this same line I've wondered how my Jaguar displays an 'idiot light' when a bulb 'somewhere' is burned out ... measures resistance somehow, I suppose. I'd like to have this feature on the Delorean.:umm:I hate Jaguar's implementation of this thing. The symbol looks like a propane tank or a stereotypical bomb, circle on the bottom, rectangle on the top. No filament, no threads, hell it's not even oriented the right way. Worse yet is that it's a red light, not a yellow or even orange one, which indicates a critical, can't-drive-without-something-blowing-up problem. I was in our '93 XJ6 when this light came on, my dad at the helm. It confused the hell out of us, and we were thinking there was some tank that was empty. Oil pressure was fine, temp gauge was reading normal, gas was fairly full, we couldn't figure out what the hell was going on. Thank god we still have our owner's manual. I dug it out and looked it up, and it said there was a bulb out. Of course, it doesn't say which bulb is out, and odder still is that no exterior lights were on at the time the idiot light came on. Turns out it was the RR side marker light, which (of course) was the last of the bulbs we checked. Replaced it and the idiot light was fixed.

I'm pretty sure it's a continuity checker that checks the resistance. If there's infinite resistance (i.e. blown-out bulb), it knows something's up and trips the system. I'm not an expert in electronics by any means, but I want to say there's some electrical component that can automatically switch circuits when infinite resistance is encountered. Transistor maybe? A reverse relay of sorts would probably serve the same function (i.e. closed with no current, open with current, opposite of most). I'm sure it's not terribly difficult to rig up, you certainly don't need a microcontroller or anything. That only makes a more failure-prone system if anything.

[Jacko]

After driving a Delorean for 30 years, I guess the most persistent annoyance I've had is ... 'wonder if my brake lights are working'. Of course I have my favorite storefronts I visit to look at the D's reflection but it would be nice to have a simple circuit monitoring the tail lights, brake lights and head lights. I have Special T's reworked boards and I'm already having trouble with one of them.

Dave M, this could be your next product ... light fails, dash light comes on.:yesss::yesss:

[Dangermouse]

I was thinking the same thing. 👍

[Jonathan]

Humour Intermission:

No intent to derail the thread, it was just this YouTube clip came to mind with the German accent...


http://www.youtube.com/watch?v=gmOTpIVxji8

Ok, as you were :)

[Dangermouse]

I work for a German company and absolutely love playing that for them when ever one comes to visit. :thumbup2:

[Dangermouse]

I think I may need to change the title to “Zose Germans are too clever by half” or “Zose Germans are too clever by 0,5” in metric. Let me explain:

My above theory was correct. The system detects a burnt bulb in the lower tail lamps and then switches to the upper brake lights at reduced voltage (as the brake lamp bulb is 21W and the tail lamp bulb is 5W, I will surmise that it reduces the voltage to around 3V. Dead battery on my tester so I couldn’t measure this) somewhere below the threshold voltage for the LEDs in the CHMSL anyway.

Normal tail lights on (after bulb replaced)

http://dmctalk.org/attachment.php?attachmentid=12769&d=1346159451

Brake light acting as a Back-up tail lamp

http://dmctalk.org/attachment.php?attachmentid=12767&d=1346159450

Brake light acting as a brake light:

http://dmctalk.org/attachment.php?attachmentid=12768&d=1346159451

I don’t think the detection is done by simple continuity testing, looks like it is current based. The tail lamp circuit actually lights up, detects a blown bulb and then switches over to the brake lamp system. Here it is; this is just turning the lights on but not pressing any pedals.


http://www.youtube.com/watch?v=JYRye2OhTeo


The more observant of you may have noticed something that I had missed first time around. On the bottom row of the lamp cluster, alongside the tail lamp is the fog lamp. Tail lamp is the outer one, fog lamp is the inner one. I had assumed that the fog was just a dedicated single filament lamp. But the actual bulb is a dual filament 21/4W that lights at full power for fog (on drivers side only, which is a little odd) but also lights at low power as an additional tail lamp. So turning on the tail lamps lights the two outer 5W tails and the two inner 4W fog/tail combos – 4 lamps in all. However, when one of these four bulbs fails, leaving 3 bulbs lit, this activates the “back-up” system of only 2 bulbs, which seems a little obtuse. Surely these clever German electronics could detect one bulb failing and give a warning only. One bulb out on either side would give two warnings only. Finally detecting two bulbs out on the same side would switch to the back-up system. I hope Dave M is paying attention.

Interestingly there isn’t a back-up system for a failed brake lamp, but there is for a failed fog lamp, or a failed indicator.

[skynet]

0,5” in metric is 12,7 mm ;)

[Shep]

Isn't it much more reliable (and perhaps even cheaper) to just use LED taillights rather than rigging up a complicated system of sensors, circuitry, and redundant bulbs? It uses less power, heat generation is minimal, and it's designed to last the life of the car. Especially in a Mercedes, who pride themselves on innovation. Sounds more like overkill than a solution.


0,5” in metric is 12,7 mm ;):rolleyes1:


:hihi2: