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Premium Member
1999 500SL, 1988 SEC
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1,462 Posts
Discussion Starter #1
This one will take some thinking. I have euro headlights and when I put them in I installed them using the Daniel Stern wiring plan. Essentially this has you install two relays. One for low beam and one for high beam. What triggers the relay is the wire that would normally feed the headlights. Using relays provides more power to the lights, which increases the brightness. Ok, here is the issue. Occasionally the low beam shuts off. The high beam works when that happens but not the low. Sometimes if you shut the car off and restart it you can get them to work. Shutting the lights on and off with headlight switch does not help. Something must be weak and shutting power off. Has a new headlight switch. Could it be the stalk, is there a headlight relay? So far I am stumped on this one. FYI each relay is separate with separate feed lines from the batery. FYI it also triggers that light out warning light
 

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Registered
1981 300SD W126
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148 Posts
Interesting! Are you able to test the relay for the low beam? Or try to switch relays and see if the issue follows the relay. Keep us posted.
 

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Moderator
1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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4,989 Posts
John,

The added relay or triggering has the issue,

It ain’t the MB engineers.

Your friend

M
 

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Registered
1985 300 TD 1981 300 SD
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470 Posts
Interesting, I had a similar issue years ago with a 79 Grand Marquis the lights would kick out after being on for a while, it turned out the headlamp relay was over heating and causing a built in breaker to kick out which reset once the relay cooled enough. The cause in my case was pulling to many amps through the relay but a weak relay or breaker can cause the same issue.
 

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Too many to list
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9,405 Posts
Try the $9 relay kit on amazon and if it works then return Daniel sterns kit

 

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Premium Member
1999 500SL, 1988 SEC
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1,462 Posts
Discussion Starter #6
No it can't be the relays or the wiring to them. There is one relay each, one for the low, and one for the high. The power side of the relay comes right from the battery, and the other side of the power circuit goes right to the headlights. The relay ground side is grouned right to a MB ground lug. Yes I have tried switching relays and that does not work. What triggers the relays is the original wire that powers the headlight now supplies power to the power side of the magnet in the relay. When you turn on the headlight the relay simply closes, which bring full battery power to the headlighs. Yes I have a fuse in each power line. Similar thing happens when you hit the high beam. A relay closes, which again brings full power to the headlights. There is much less current needed to close the relay than run the headlight so you save on wear on the switch, stalk and other items. Supposedly the lights are much brighter as well. So, there must be a relay, a computer, or something that goes bonkers every now and then. Do you suppose MB in its Mars engineering has a headlight computer somewhere that if it does not sense the correct load to the headlight circuit and simply shuts that circuit off? So the question I guess I am asking is:
1) Is there a headlight relay somewhere
2) Is there a headlight computer
3) Could a stalk cause this

I am going to see if I can do some sleuth work today but whenever I try to find the issue, the headlights work as they are supposed to.

Actually, the relay thing is kinda neat. Cost about $10.00 and some time. Since I used 12 ga wire from the battery to the relay, and then from the relay to the headlights, you can imagine the amount of amperage available. Relays are rated at 30 amp
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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4,989 Posts
In addition to the above, for reference here is the MB headlights schematic too.

HEADLAMPS.png
 

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Premium Member
1999 500SL, 1988 SEC
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1,462 Posts
Discussion Starter #9
ok so where is this outside lamp failure unit?
 

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SuperModerator
1986/1990 W126
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14,926 Posts
I think it might be the big long relay in the fuse box, from memory?
 

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Premium Member
1999 500SL, 1988 SEC
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1,462 Posts
Discussion Starter #11
Thanks, I will look. One more little quirk that is a fact and not necessarily part of the issue but occassionally, the right turn signal will begin flashing very quickly and the idiot bulb out light will come on. When you shut if off and turn it back on it can go back to normal and work correctly. I have replaced all the turn bulbs with the correct ones and they all work so its not that. Nor is the headlight relays connected to them
 

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SuperModerator
1986/1990 W126
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14,926 Posts
Yeah indicators are nothing to do with the headlamp relay I don't think.

You might check the hazard warning switch for that problem. Even just clicking it a few times can help. Otherwise a grounding problem. Or dirty fuses etc.
 

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Premium Member
1999 500SL, 1988 SEC
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1,462 Posts
Discussion Starter #13
The very large black box in the fuse box under the hood is the light failure lamp module. I guess its prone to weak solder joints. I don't know if that is the issue yet but that's where it is
 
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