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Discussion Starter #1
My 1986 560sl headlights turned funny recently. They won't turned off after I shut off the car. But it works just fine when the car is in running mode. So each time I after to disconnect the battery in order to prevent draining out the battery after shut down the car.

Would this be due to the headlight switch or any of the relays?

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1989 560SL
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So the headlights will turn off if the key is on or when the car is running?
If so, that would sound like a relay problem.
Does it have an aftermarket alarm? Some of those back in the day were wired into the lead light circuits so that the headlights would flash or come on.


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Discussion Starter #3
True. If the key is on or when the car is running, the headlight works just find on and off. Only when I take the key out then the headlights become always on, doesn't matter where the headlight switch position is.

The car has a factory alarm system equipped. It was found once when I unlocked the trunk that triggered the alarm system that the headlights were flashing. I ended up disable the alarm system (unplugged the alarm module under the glove box). But the headlights were working just fine after till now.
 

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There are no relays in the headlight system other than the ones possibly built into the exterior lamp monitoring module.
You’ll need a current tester/voltmeter and start to check fuses. You notice that Fuse 5 should only have current when the key is on Run or Start positions. Confirm that one first.
Then check if fuses 17 through 20 are live when the key and headlight switch are both off. Is it low or high beams on? Are they on brightly or just dimly?

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Discussion Starter #5
It sounds logical to me if this is related to the alarm system. When the alarm system was triggered, the headlights was flashing on and off. I wondered how that worked. Was it kind of pulse signal sent from the alarm system to a relay that switches the headlight ON and OFF. If it is, then a stuck relay could keep the headlights ON. But I just couldn't figure out where that relay is.
 

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The relay is built into the factory alarm module which then feeds the low beam fuses as per the diagram. Unplugging the alarm module should eliminate the issue.


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Discussion Starter #7
But I unplugged the alarm module already. This headlight problem came in after the alarm system was disabled. There's no other aftermarket alarm system equipped in this car.
 

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Remove the low beam fuses (and high beam fuses, if necessary) and see if the lights stay on. The factory alarm feeds the input side of the fuses. If they are still on with the fuses removed, then there is likely some aftermarket wiring connected to the wires between the fuse box and the headlights themselves.
As per the diagram, there is no other components in that part of the circuit except the bulb monitoring module. Some voltmeter diagnostics with the circuit diagram would help narrow the problem down. Very unusual, for sure.


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