400E Bulb-out light comes on and off! when I am stepping on the brake

[werminghausen]

I have a strange problem with my 400E. The dash light comes on when I step on the brake with maybe 1 second delay. When I get off the brake the light goes out. I checked the bulbs but they were all fine (2 x 21W bulbs work and the LEDs work) I exchanged the 21 W bulbs with 2 new ones (Sylvana) and it went away for a minute or two and then the same game started...no success.
One little detail: When I step on the brake the small lights of the seat memory (white dots with numbers 1 and 2) starts to light up slightly and flicker.
Does anyone know what my problem is and how to solve it?

Martin

 

[nocfn]

I would make sure each of the light sockets are clean and the bulbs are tight. Some dielectric grease on the contact points may help. You did stand back in the dark to be sure all lights work, and have a helper step on the brakes? License plate lights are a problem.

 

[2phast]

I have a strange problem with my 400E. The dash light comes on when I step on the brake with maybe 1 second delay. When I get off the brake the light goes out. I checked the bulbs but they were all fine (2 x 21W bulbs work and the LEDs work) I exchanged the 21 W bulbs with 2 new ones (Sylvana) and it went away for a minute or two and then the same game started...no success.
One little detail: When I step on the brake the small lights of the seat memory (white dots with numbers 1 and 2) starts to light up slightly and flicker.
Does anyone know what my problem is and how to solve it?

Martin

You mentioned LED's, are you using these in your tail lights?

 

[werminghausen]

Sorry...no LED but LCDs? The original lights in behind the rear window.

 

[nocfn]

Third brake light?

 

[werminghausen]

I carefully checked each socket...all clean and tight. I check several time with a helper the function of the lights. Very positive all bulbs go on when I step on the gas. .... my license plate lights seem to work... I guess. What is the problem there?
Martin

 

[werminghausen]

yes, third brake light. Martin

 

[nocfn]

That's the problem. Resistance tripping the light out indication.

 

[werminghausen]

Is a faulty bulb out relay a common problem?

The weird thing is that the bulb out light goes on and off when I use the brake...but I guess it does this (going on and off) also when another bulb is dead?
I tried to have the relay sense a missing brake bulb...and it does. But then the light stays on...even if I install a new bulb... light goes off only after restart of engine.

Martin

 

[werminghausen]

That's the problem. Resistance tripping the light out indication.

Are you saying that the new brake bulbs or the socket are the problem- too much resistance?
How can I solve this?

 

[Ascension]

Had this problem when I switched over to a LED in the 3rd brake light and corrected it by going back to a normal bulb. It's a current flow/resistance thing some where in that circuit that is tripping the bulb out light. Check the socked and bulb then work back.

 

[2phast]

Are you saying that the new brake bulbs or the socket are the problem- too much resistance?
How can I solve this?

If your using anything but a regular bulb, this will happen. The electrical system expects a certain amount of resistance, you only get this with standard bulbs. Your problem tells me your using something other than a filament bulb in one of your brake light circuits, that is why the "bulb out" comes one when you step on the brakes. Replace the bulb(s) with standard filament bulbs and your problem should go away. Either that or you would need to install some resistors inline to trick the electrical system into believing your LED bulbs are drawing more power than they actually are.

 

[nocfn]

Correct info above! I use the led on the puddles, in the domes and the trunk.

 

[gsxr]

yes, third brake light. Martin

Martin, just to clarify... you have a non-standard bulb in the 3rd brake light assembly? If so, that may be the root cause of your issue.

If you have a standard bulb in the 3rd brake light, we still have a mystery here.

 

[werminghausen]

I did not change the 3rd brake light and I am assuming it is the standard bulb.
I did not have the issue since I own the car. But now it annoys me. I have tried to replace rear bulbs with at least 5 different standard bulbs with no change.

Does the bulb out light come one with A) too high resistance or B) with too low resistance or C) both?
If I remove one of the bulbs there is fundamental difference: if I step on the brake... the light comes on (just like with my 'issue' but the light would not go out any more after releasing the brake. Any explanation here?
Thanks for you help, Martin

 

[samiam44]

Not sure on the 3rd brake light. Make sure the bulbs are the silver bases. I had weirdness w mine.. Jon replaced all the b ulbs w European replacements. All new and matching... problem solved.

M

 

[gsxr]

Does the bulb out light come one with A) too high resistance or B) with too low resistance or C) both?

It may vary depending on the specific bulbs. For example, on the USA rear corner marker bulbs, if one is out, the warning light comes on... but if both are dead (or removed) there is no warning indicator. So that one must compare the two and look for a match. On the brake lights, I'm not sure how it handles things since there are 3 bulbs in the circuit.

If I remove one of the bulbs there is fundamental difference: if I step on the brake... the light comes on (just like with my 'issue' but the light would not go out any more after releasing the brake. Any explanation here?

Now THAT is interesting. I really don't know what's going on, but I'd sure like to try a different (preferably known-good) module.

 

[gerryvz]

I recommend Osram bulbs (preferably the European made ones, if possible) for replacements as conventional bulbs.

Unlike some folks here on the forum, I also do not recommend using LED bulbs, as they can cause anomalies and wreak havoc with the "bulb-out" warning system, which is extremely sensitive to system resistance.

One thing I would also recommend, would be to test the bulb warning module with a spare by changing it temporarily, just to eliminate it as a potential issue. These modules are readily available at wrecking yards for very cheap, or from forum members here. They are common on W124 and W126 models.

These "bulb out" modules are not an uncommon failure, particularly of the solder joints because of simple age and heat cycles.

The module is part number 126 545 01 32, and is illustrated in the attached images.

Cheers,
Gerry

 

[emerydc8]

http://www.ebay.com/itm/1265450132-...ash=item23519c685a:g:RDYAAOxyyF5ROgGO&vxp=mtr

Where is this module located?

 

[JC220]

http://www.ebay.com/itm/1265450132-...ash=item23519c685a:g:RDYAAOxyyF5ROgGO&vxp=mtr

Where is this module located?

In the main fuse box under the hood. Remove the hinged cover then also the main top which is secured with screws. Then you will see the Module + some other relays

 

[gerryvz]

I have nine of these modules in my parts stock, and would be willing to send one at my expense, if needed. Just PM me with your address.

 

[werminghausen]

Hi Gerry, very nice of you.
I'll send you a note.

 

[werminghausen]

More weird stuff.
I figured out when I turn the lights on (can be position 1 or normal lights in position 2) the bulb out light will not come on at all when I brake.
When I turn the lights off bulb out will go on again if I step on the brakes.

 

[werminghausen]

Where can I buy the 'Osram' bulbs Gerry mentioned for the stop lights. Is this the Osram #7506 bulb? I think I bought Sylvania bulbs at O-Reilly but I think I have also seen 'Osram' on the package...

Martin

 

[gerryvz]

It used to be that bulbs purchased in the US at common auto parts stores were branded "Osram Sylvania," and in recent years they have just been branded "Sylvania." I believe that any red- or green- (now light-blue) packaged (regular or long-life) "Sylvania" bulb that is purchased at O'Reilly, Checker/Shucks/Kragen, AutoZone or Advance Auto Parts is of fine quality and totally usable. I've certainly purchased plenty of them over the past 5 years with no problems whatsoever.

I'd stay away from anything that isn't branded "Sylvania."

Cheers,
Gerry

 

[nocfn]

May be unrelated, but I had a bad brake light switch on the c126, causing other things to be cattywhompus. It cost a few bucks and 3 minutes to replace. I did this to the E500 as preventative when I replaced the tumbler just after I bought the car. I have not been able to repeat my electrical issues since I made the change.

 

[werminghausen]

Quick update: I am still having the bulb out light when I step on the brake... I bought a seemingly good used relay off ebay and this helped for a short while (maybe 10 minutes) and then the dash light was back (the light goes out when I release the brake) . I bought another set of Sylvania bulbs (I have now a real collection of these bulbs- more than I ever can use) but nothing changed. It is something very weird going on.
Maybe not a bad idea to change the brake light switch... at least it can't hurt. I'll try that and report back.

Martin

 

[nocfn]

Maybe not. Be sure you have the same brand, number in both sides.

 

[gerryvz]

Brake light switch, and both of the brake light sockets in the taillights for proper connection.

Don't forget the often-overlooked "third" brake light in the rear window !!! This is often the culprit !!

 

[werminghausen]

Hi Gerry, what can go wrong with the third brake light? Mine is 'working' meaning it goes on and off as it should. I have seen it with my own eyes.
Is there more than the eye can see?

Martin

 

[gerryvz]

I wanted to make sure that the bulb is making proper contact and that there is no corrosion at the bulb-socket. Also that the bulb is correct.


Sent from my iPhone using Tapatalk

 

[werminghausen]

Hi Gerry, I am still dealing with this bulb out light issue. I have changed the relay 2 times now, nothing, Changed the bulbs down various times....nothing... I changed the brake light switch down at the pedal.... nothing. It must be the third brake light!
Third brake light is working for sure but maybe there is something in the contact of that light and it could be brighter?
I am hoping to get closer to this third brake light on the weekend.

 

[werminghausen]

I cleaned contact and changed bulb in 3rd brake light - Nothing!
The dash light goes still on.
Interestingly I left the cover of the third brake light off while testing ad I saw the brake light and the dash light going on. It seem that
there is a two second delay between the two. Does this tell something?
All the efforts with used relays (I have 3 bulb iut relays now with one being identical in my W126) did not work out. A friend of mine did re-solder two of them (and he is a professional electrical engineer).
I guess I'll buy a new bulb out relay and if that does't cure that problem I don't know what to do.
Any comments are helpful.

Martin

 

[werminghausen]

I finally found a solution: All these relays I tried to repair and test were actually okay. It was NOT the relay at all ...but a bad ground.
However it was not the ground of any of the 3 brake light bulbs but of another bulb on the rear left side. Ground are the metal strips the bulbs sit in. These metal strips end up in a brown cable which is clamped to the ground strips and I believe the brown cable goes back to chassis (not 100% sure about that)
How I found the problem.
At some point I found out that there might be an option that the reason for the bulb light coming on was not the brake light . I read in the manual that the brake light fault will cause a permanent dash light. The light will stay on until restart of the engine. This is different with the other bulbs. If another rear light has a fault the dash light will go on but it will go out if the bulb works again (without a restart).
This lead me to the conclusion that another bulb is involved and 'lives' as ghost from the brake light through a bad ground. This means in theory that this parasite bulb is lighting up as well stealing current from the brake light..at least a little bit but not visible next to the much stronger brake light.
How I found it : It was easy and I found the failing ground of the parasite bulb in 2 minutes. I removed all rear bulbs except for the brake light bulbs -rear left and right side.
Then I stepped on the brake.... the dash light did not come on this time- yeah ( I tried 10 times at least because I could not believe that this was the reason- that easy... I knew at this point that I am about to find the problem after 1/2 year and 3 relays.
Then I put the rest of the bulbs back... one by one and tested each time if the dash light came on. After a few bulbs back in the lights the dash light came on and this was the problem bulb...actually the bulb was good .... it is a 'just' bad ground of this bulb that was causing the problem.
I took a screwdriver and pushed the crimping of the ground cable a bit and voila.... problem solved.
At least I have found the problem (and I think I understand what happened) and since two days the dash light did not come on again. That does not means that it will stay this way but at least I know what to do next time if it the dash light comes on again during braking. If the same problem come again I'll think about soldering that contact instead of crimping.

Martin (feeling like Sherlock)

 

[gsxr]

I finally found a solution: All these relays I tried to repair and test were actually okay. It was NOT the relay at all ...but a bad ground.

Martin (feeling like Sherlock)

Sherlock, that is great news - thanks for posting the solution! I was thinking there was no way you'd have 3 bad relays, but I also wasn't sure what else would cause this. Hopefully this will help other people in the future who run into similar issues with the phantom bulb out...

 

[gerryvz]

I finally found a solution: All these relays I tried to repair and test were actually okay. It was NOT the relay at all ...but a bad ground.
However it was not the ground of any of the 3 brake light bulbs but of another bulb on the rear left side. Ground are the metal strips the bulbs sit in. These metal strips end up in a brown cable which is clamped to the ground strips and I believe the brown cable goes back to chassis (not 100% sure about that)
How I found the problem.
At some point I found out that there might be an option that the reason for the bulb light coming on was not the brake light . I read in the manual that the brake light fault will cause a permanent dash light. The light will stay on until restart of the engine. This is different with the other bulbs. If another rear light has a fault the dash light will go on but it will go out if the bulb works again (without a restart).
This lead me to the conclusion that another bulb is involved and 'lives' as ghost from the brake light through a bad ground. This means in theory that this parasite bulb is lighting up as well stealing current from the brake light..at least a little bit but not visible next to the much stronger brake light.
How I found it : It was easy and I found the failing ground of the parasite bulb in 2 minutes. I removed all rear bulbs except for the brake light bulbs -rear left and right side.
Then I stepped on the brake.... the dash light did not come on this time- yeah ( I tried 10 times at least because I could not believe that this was the reason- that easy... I knew at this point that I am about to find the problem after 1/2 year and 3 relays.
Then I put the rest of the bulbs back... one by one and tested each time if the dash light came on. After a few bulbs back in the lights the dash light came on and this was the problem bulb...actually the bulb was good .... it is a 'just' bad ground of this bulb that was causing the problem.
I took a screwdriver and pushed the crimping of the ground cable a bit and voila.... problem solved.
At least I have found the problem (and I think I understand what happened) and since two days the dash light did not come on again. That does not means that it will stay this way but at least I know what to do next time if it the dash light comes on again during braking. If the same problem come again I'll think about soldering that contact instead of crimping.

Martin (feeling like Sherlock)

Indeed, very good detective work. I too had a wonky rear side marker (tail-lamp) on my 126 coupe, and it would cause the bulb-out lamp on the dash to go on and then turn off at various times. Interestingly, since moving to Maryland, it's never given me a problem again.

I did not know that a brake lamp out would cause a "sticky" bulb-out lamp until restart (upon which it would promptly light again, because of the brake needing to be pressed after starting, but before moving out of Park, on later cars). You learn something every day.

On my 1995 G-wagen (gray market), it is not required by an ignition/brake switch to hold the brake pedal before moving the vehicle out of "PARK" gear.

Great work!

Cheers,
Gerry

 

[werminghausen]

Thanks Dave and Gerry,

I was too optimistic unfortunatly. Last week the damned thing came back and took out that left rear bulb panel and started investigating with my voltmeter (should have done this long time ago but I was fooled and obsessed with the relay that I just did not get the idea.
Now I knew exactly what it was: bad ground inside this panel.

Not knowing anything I started with removing the bulbs and measured resistance from pin G in the plug to the inside ground of the bulb (I called it pin G (ground in the diagram). Resistance should be almost zero (like 1.5 Ohm)
I saw that the resistance for the low row of bulbs was much higher meaning bad ground connection. So I was clear that I needed to permanently solve the issue.
I started soldering these crimped/pressed connections but this did not work... it is steel and lots of metal...so no.
Then I spot welded the parts that were in question... measured continuity/resistance again and voila I was back in the game.

I installed the bulbs and measured resistance including the bulbs. It was 1.6 to 1.7 Ohm for the bigger bulbs (21W) and 2.5 Ohm for the smaller 2 bulbs.
Put the panel back and the ghost is gone! I hope this is a permanent solution.
See attached pictures with the welds and diagram
Martin