Troubleshooting Dome Light in 1988 560SEC - Mercedes-Benz Forum
 
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post #1 of 10 (permalink) Old 02-27-2019, 06:30 AM Thread Starter
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Troubleshooting Dome Light in 1988 560SEC

Over the past week I believe I have read every post I could find related to dome light issues. From my understanding the dome light, seat belt flash/strobe, and key in buzzer are all related. Everything functioned 100% when I got the car but at some point they stopped working several months ago. I didn't even notice it until it was dark enough outside to warrant needing the light from the dome lights.

What works - key in/on buzzer and the reading light when switched on.

What does not work - both dome lights front & rear and the seat belt flash/strobe.

What I have done - checked bulbs in both F&R dome lights and they are good. I could not figure out how to get in to the strobe bulb. Tried dome light switch in all positions. Upper door switch on each side works. Relay in the under-hood fuse box clicks on when either door is opened and and clicks off when closed. Swapped with another relay to test, no change.

Questions - Is the buzzer box also the box that controls power to the dome lights and strobe? Does anyone have a good picture of it? Some people have said the dome lights just go out on these cars, has anyone successfully repaired one?
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post #2 of 10 (permalink) Old 02-27-2019, 08:50 AM
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Hi there,

Here are schematics for your courtesy lamps & the warning systems too..

PDFs, if the not so fine print eludes...

M
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post #3 of 10 (permalink) Old 02-27-2019, 08:29 PM Thread Starter
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Thank you for the diagrams @MBL87560SEC I have confirmed that fuses 2 and 6 are getting power when they are supposed to. The door switches are activating the relay in the fuse box. I pulled the dome light assembly down again and got it on the bench. I checked all the bulbs again including the three 2W mini bulbs for the seat belt flasher. All were good. With the switch on the bench you can see LK, 31, 15, 30, and 30. On the swtich harness you can see 1,2,3,4,5 with nothing in position 6. As I took my photos of everything I noticed the connector for pin #5 was pushed through. I popped the back of the connector off and corrected the issue. I thought "this could be it" but after some tests and confirmed the connector stayed put, no change.

The harness to switch goes as follows: 2 to LK, 4 to 31, 1 to 15, 3 to 30, and 5 to 30

Working on the black harness, using a test light connected directly to the negative battery terminal, #1 has power with the door opened or closed but only with the key on.

I get half power at #2 and can hear a relay click but only with the door closed. If the door is open, no power is at #2. The same result is when the key is on or off.

I get full power at #3 regardless if the door is open or closed or the key is on or off.

No other connectors offer any power with the key on or off or the door is opened or closed.

Moving on to the switch and checking for continuity. With the dome light switch in the ON position, the bulb out, I get tone on the bottom square contact and pin 31. If I put the bulb in, I get tone on both contacts with pin 31. No other pin combinations produces a tone with the bulb contacts. If I contact both of the 30 pins, I get a continuity tone.

Moving to the strobes, with one lead in one of the sockets for the strobe bulbs I get no tone contacting any of the switch pins. I repeated this for both contacts for each bulb, and all switch pins, same result.

This leads me to believe there is an issue with the dome light switch. Thoughts?

With the switch
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post #4 of 10 (permalink) Old 03-02-2019, 04:14 AM
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Originally Posted by HardwayMB View Post
Over the past week I believe I have read every post I could find related to dome light issues. From my understanding the dome light, seat belt flash/strobe, and key in buzzer are all related. Everything functioned 100% when I got the car but at some point they stopped working several months ago. I didn't even notice it until it was dark enough outside to warrant needing the light from the dome lights.

What works - key in/on buzzer and the reading light when switched on.

What does not work - both dome lights front & rear and the seat belt flash/strobe.

What I have done - checked bulbs in both F&R dome lights and they are good. I could not figure out how to get in to the strobe bulb. Tried dome light switch in all positions. Upper door switch on each side works. Relay in the under-hood fuse box clicks on when either door is opened and clicks off when closed. Swapped with another relay to test, no change.

Questions - Is the buzzer box also the box that controls power to the dome lights and strobe? Does anyone have a good picture of it? Some people have said the dome lights just go out on these cars, has anyone successfully repaired one?
Hi there,

This is quite a diagnostic journey! You are a VERY thorough man.

You've already answered much of this last question, but I'll underscore the interior light bulbs [Reading, Dome & fasten Seat Belts] & the Sun Visor Vanity lights all get their POWER from the buss [within the Int Light assembly] connected via pin 30 to fuse 2 [hot all the time].

The Reading Light works, ergo ground supply via pin 31 is good.

Moving on, all 5 of those bulbs: Reading, Dome, and the 3 Fasten Seat Belt bulbs receive actual ground through the same pin: 31. The Door controlled Dome light selection & the Fasten Seat Belts warning lamps are triggered through the Time Delay circuit board in the interior light. As you can see, the Time Delay portion is powered by the ignition via fuse 6.

Why your dome light does not operate either way suggests an issue with the switch itself.. As you can see by the schematic [L to R], either Connects GROUND directly [ON], GNDs via the Time Delay Circuit [Delayed Off], or OPEN circuits GND supply [OFF]. It follows that the dome light ground switching [Manual & Delay] is OPEN.
I wonder if this 'wonky' operation could just be a dirty contact in there? Its a guess..... but I think you've said pin 15 gets power with key ON via fuse 6... That's all normal..

Otherwise, you'll probably be replacing the light unit, as you've tested everything else that I could think of at this moment.

M.
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post #5 of 10 (permalink) Old 03-02-2019, 07:41 PM Thread Starter
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Thank you for the info above @MBL87560SEC As you indicated above and I thought as well, the switch itself is bad. The time delay circuit and other related components on the board go out. I took the afternoon off on Friday and visited two salvage yards. I managed to pull a complete dome light assembly from a 1986 560SEL. It had been picked over really good but the dome light was left. I also snagged the cruise control amp and buzzer relay. Someone else had already removed the lower dash panels and steering column so it was a quick grab. All of the more valuable relays were gone. Once I home I tested all the bulbs and all but the reading light was out. I pulled all the good bulbs from my unit, hopped in the car, plugged in just the dome light side of the assemble and wha-la! It works perfectly! Now I needed to swap my sunroof switch as the one in the assembly had a lot of wear and two of the tabs holding it in were broken so it was not sitting flush in the assemble. Unfortunately this required gluing it but once installed it looked and functioned as expected.

The only mystery that still remains is the rear dome light. It is still not lighting up under any circumstances. I never get 12V to it so I am wondering, does the power for it also flow through the dome light switch? Could it be since I pulled this dome light from a different year and model cause it not to work? The switches have the same part number on them but in the middle where the harness plug goes, my original has a 7 molded in to it and the one from the salvage yard has a 2. Even when my original dome light was working, I do not remember if the rear dome light ever came on. I very rarely drive the car at night.

Since I now have my original non-working switch on the bench, I am going to carefully open it up tomorrow and take a look at the circuit board. I will post my findings once I have some to share.
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post #6 of 10 (permalink) Old 03-03-2019, 02:35 AM
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Great news!

As always, your documentation is excellent!

Thanks man.

MBL


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post #7 of 10 (permalink) Old 03-03-2019, 03:00 AM
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Originally Posted by HardwayMB View Post

The only mystery that still remains is the rear dome light. It is still not lighting up under any circumstances. I never get 12V to it so I am wondering, does the power for it also flow through the dome light switch? Could it be since I pulled this dome light from a different year and model cause it not to work? The switches have the same part number on them but in the middle where the harness plug goes, my original has a 7 molded in to it and the one from the salvage yard has a 2. Even when my original dome light was working, I do not remember if the rear dome light ever came on.
.
I just researched your rear dome light and it's nothing at all that was done or left out that's involved.

It turns out it is a different circuit powered by fuse 15 and has a revision in the '88 up wiring..

Here are some tasty schematics for ya..

PDFs too, for da fine print..

MBL
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post #8 of 10 (permalink) Old 03-03-2019, 02:44 PM Thread Starter
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Thank you for the PDFs and info @MBL87560SEC I just confirmed I am getting power at fuse 15 and the fuse itself is good. My driver's side seat belt extender does have a mind of its own more than half the time so this may explain why no power is getting to rear dome light. If I feel up to it I will remove the DS rear panel and see if the red/white wire going from the control box has power. If it does then there is a break somewhere leading up to the dome light.
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post #9 of 10 (permalink) Old 03-04-2019, 10:11 AM
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Originally Posted by HardwayMB View Post
Thank you for the PDFs and info @MBL87560SEC I just confirmed I am getting power at fuse 15 and the fuse itself is good. My driver's side seat belt extender does have a mind of its own more than half the time so this may explain why no power is getting to rear dome light. If I feel up to it I will remove the DS rear panel and see if the red/white wire going from the control box has power. If it does then there is a break somewhere leading up to the dome light.
Hey, it's me again..

How about a circuit UPstream in the diagram? Are you having issues with the Drivers side exit lamp, the one located at the bottom of the driver's door?

Just a thought.

M.


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post #10 of 10 (permalink) Old 03-04-2019, 04:21 PM Thread Starter
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Originally Posted by MBL87560SEC View Post
Hey, it's me again..

How about a circuit UPstream in the diagram? Are you having issues with the Drivers side exit lamp, the one located at the bottom of the driver's door?

Just a thought.

M.
Good news maybe, my driver and passenger side exit lamps light up as expected. May just live without the rear dome for awhile. Got bigger fish to fry with my passenger side mirror. Thank you again for all your help @MBL87560SEC
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