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Discussion Starter #1
My 1986 560sl. The headlights stay on with the ignition switch is in the off position. With the switch in the on position or the car is running, the headlights work fine and can be truned ON and OFF using the headlight switch. The fog lamps work as they should. However, when the car if OFF, the headlights will come ON no matter where the position the headlight switch is at. I'm constantly removing the battery terminal after parking to shut off the lights and prevent battery drained.

Anyone have any idea what the problem might be? Anyone know if there is a relay and if so, where it is???

Please help.
 

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1983 380 SL
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Might mean nothing but the ignition switch makes an breaks numerous electrical connections other than just providing electric power to the engine. Might be worth checking. :unsure:
 

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Discussion Starter #3
I thought about it too. But it didn't make sense if a faulty ignition switch would turn on the headlights all time, as for a normal operating car you can turn on headlights even when the ignition switch is off.

I am kind of thinking if this is something related to the alarm system, as on 560sl when the alarm is triggered when the ignition is off, the two headlights will flash. But I have already disconnected the alarm module and disable the alarm system and it didn't help.

Wondering if there's any relay I can check on. If a relay is stuck on, even with the alarm system diabled, the headlights could be still on when ignition is off.

Any thoughts/suggestions?
 

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560SL '88 Suzuki GS1000E '78
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Hi, this is a really sexy issue! I don't have any idea how this effect can be triggered, but maybe we can get to the secret :)
But at first: Which headlights are on. Low beam or high beam?

The idea with the alarm system was great, but it didn‘t helped. Did you disconnected the controlunit behind the glovebox or something different?

It may be related to the combination-switch, but I have absolutely no idea how this should work with the described effect. Maybe a first step will be to check the wiring of the main lightswitch (is every cable connected to the correct terminal). https://www.startekinfo.com/StarTek/outside/11883/PROGRAM/ETM/86etm.pdf
Regards Martin
 

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Discussion Starter #6
Found the following discussion about the exact headlight problem with 560sl, but no solution was posted.

 

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1987 560SL, 2000 Kawasaki W650
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Sounds like the first place to start is the headlight switch. The headlight switch is independent from the ignition/key switch, the lights don’t turn off with the key in the off position or removed. They’re on an “always hot” circuit. See page 110 in the manual using Martin’s link above. I left my headlights on a few times when I first got my 87 560SL until I got into the habit of turning them off.

You’ll have to remove the knee bolster to get to the back of that switch. Could just be crud in it. Easiest fix is to spray some contact cleaner into the switch (with the battery disconnected) and blow a little compressed air into it. If that doesn’t work, next would be to disconnect the wiring to the switch and check it with a continuity tester or a DMM to see if there’s a short in the switch.

Electrical problems take a little detective work and you have to track down the fault by a process of elimination.
 

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Discussion Starter #8
If it's the headlight switch, why when the ignition key is on ON position the headlights can be turned OFF by the headlight switch?
 

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1983 380 SL
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This might be from left field but newer cars are designed to work that way. My wife's 2017 Honda does that by design... when you turn off the car (at night) the headlights and interior lights come on to give you time to get in the house. After a minute they turn off. Maybe your car is just ahead of it's time. (y)

OK... seriously, pull the headlight switch and check voltages. Something is supplying voltage to the headlights when the ignition switch is off. My '83 has some half-assed wiring scheme in the warning flasher circuit with voltage being supplied to the flasher itself from 2 different places depending the position of the warning flasher switch. I'm not saying your flasher has anything to do wioth your problem, only that Mercedes can do some pretty screwing things with the source of 12 volts to various circuits. It's a German engineering thing. Why make things simple when you can make them overly complex?
 

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560SL '88 Suzuki GS1000E '78
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Yes, the answer will be found somewhere between ignition switch and headlight switch. To access the headlight-switch it will be a bit easier to do this trough the dashboard by removing the instrument cluster.

At my car I installed an assembly from 3 relays to prevent the headlights soaking the battery empty when parking the car. The first thing I killed after buying this car was the warning buzzer, next I forgot several times to turn the lights off and needed to jump-start the car...
I solved this problem with a simple relay-cirquit and have now the option to use a daytime runnig light mode. The car turns on the headlights (and only the headlights!) automatically when the engine is running.

Very nice.
I will posts this information this evening when Im back at home, it's a very simple hack. For using the DRL-feature it requires to change the alternator-warninglight in the instrument cluster from a standard bulb to a LED.
Martin
 

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560SL '88 Suzuki GS1000E '78
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Hi Gents, here is the little cirquit. It will solve @hriant88 's problem as well, but won't give an answer what the cause was...
But the battery discharge protection and the DRL-mode are nice features as well.
I hope you can understand my translation on the attachment.
Regards Martin
 

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1987 560SL, 2000 Kawasaki W650
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978 Posts
If it's the headlight switch, why when the ignition key is on ON position the headlights can be turned OFF by the headlight switch?
Good point, although when you turn the key to on or the car is running, all the voltages change and your problem could be one that occurs at a particular voltage level. It’s just better, in my experience, to first rule out the parts of a circuit that are easier to get to, so that you’ve removed one variable from the experiment.

Also try a shot of contact cleaner into the ignition switch. You can get CRC brand at any hardware store, though some people have their preferred brand. My go-to product for anything electrical that moves is Deoxit F5 Fader Lube. More expensive, but it’s what the guys with EE degrees use on frighteningly expensive mixing consoles and such, so it’s got to be good stuff.
 

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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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Check to see if your anti theft alarm is plugged in, two plugs in the module behind the glove box. If its plugged in, unplug it. Is there any aftermarket alarm on this car?
 

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Discussion Starter #16
No, the anti theft alarm module has been unplugged already, but didnt do anything to the headlights issue. No other aftermarket alarm was equipped.

I will recheck the headlight switches and the main ignition switch over the weekend following Martin's suggestion, and let you guys know if I find anything

Thanks,
 
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