W124 Seat Belt Warning Lamp Stays On - PeachParts Mercedes-Benz Forum

Oldwolf · #1 12-20-2010, 07:40 PM

Just today my seat belt warning lamp will not shut off normally. Here is what it does:

Start Engine:
Dome light goes out
Seat belt light flashes continuously

Engine turned off:
Seat belt light goes out
Dome light still out

Open door:
Dome light on
Seat belt light still out

Close door:
Dome light stays on
Seat belt light keeps flashing.

Door switches appear to work OK, they still turn the puddle lamps on and off.

I can turn the dome light off with the manual switch, but the flashing seat belt light follows the same pattern (i.e. only is off after turning the engine off and driver’s door not opened yet).

Fuses OK.

I have the N2/4 control circuit diagram, shown below. Anybody encountered this?

What can I do to prevent my battery from running down other than pull the dome light switch?

Jeremy5848 · #2 12-20-2010, 08:11 PM

Thank you for the complete symptomatic description. Have not seen your exact problem but that's not unusual, this circuit is so interconnected that it's hard to figure out the failure modes. Three possible culprits: the warning unit N2/4 as in your diagram, the seat belt flashing circuitry in the dome light, and possibly (but less likely) one of the seat belt switches. What is the seat belt status in each of your four modes?

Unfortunately, the only practical way to find a solution is to replace the suspected units and see what happens. I have had failures in both N2/4 and the dome light module.

Jeremy

Oldwolf · #3 12-20-2010, 08:37 PM

Thanks Jeremy.

It behaved the same with the belts on or off.

Must be the dome light circuitry. I just pulled the dome switch thinking I would remove the seat belt bulb. I disconnected the dome light plug, reattached it and now things are working as they should, for now. Wierd. Must have "rebooted" the dome light.

I opened and closed the front doors multiple times and the dome light auto/off'd OK and the seat belt light turned off too.

This is a good thing because now I can help my wifey wrap presents ...she was worried I'd be working on my MB all night instead!

compu_85 · #4 12-20-2010, 09:27 PM

My guess is the dome lamp module is simply bad. It has no connection to the seatbelt switches, just the door switches, constant, and switched power. If the dome light and entry lights are working properly the problem likely is the dome lamp.

-J

Oldwolf · #5 12-20-2010, 10:17 PM

Well, I'll keep an eye on it. Guess I'll start looking around for a replacement in case I need one.

Oldwolf · #6 12-20-2010, 10:19 PM

Quote:

Originally Posted by compu_85

My guess is the dome lamp module is simply bad. It has no connection to the seatbelt switches, just the door switches, constant, and switched power. If the dome light and entry lights are working properly the problem likely is the dome lamp.

-J

The dome light would stay on no matter what. That is not proper operation for sure. The timer and logic circuitry for the dome light and door/seat belt inputs are all on the dome/light switch module?

compu_85 · #7 12-20-2010, 11:19 PM

Yes. The flashing seatbelt warning is 100% independent of the seatbelt buzzer. When working correctly it flashes all the time a front door is open and for a short time thereafter, and for a short time after the key is turned on (which it knows from switched power coming on).

I believe there's a section in the FSM about this. I'll try to dig it up.

-J

compress ignite · #8 12-21-2010, 01:12 AM

That'll keep the Battery Alive until you can fool with what's wrong.

file:///E:/W124v4.3.2/W124/w124CD2/Program/Chassis/82-0010.pdf

Follow directions in procedure above.

Also the Schematic shows the "Signal" from the Seatbelt lock is sent through
the Instrument Cluster itself to the Dome Light/Seatbelt Warning array.

jfikentscher · #9 12-21-2010, 11:45 AM

Must be in the dome light assembly. I went to the recycler and got a used one for $10. problem fixed.

hotwingcooker · #10 06-28-2014, 12:32 AM

I have a 1987 300td Wagon and this is almost like my car symptoms CAN SOMEBODY HELP ME the car is off and the Front Door lights Stay on the car is on and the seat belt light Flashes Constantly and the Door lights still on when the car is turned off the Seat belt light goes off but door lights stay on i removed the Bulbs on the Door to keep from running battery down

vstech · #11 06-28-2014, 07:20 AM

Have you put the light control switch in the full forward position?

hotwingcooker · #12 06-28-2014, 09:26 AM

Quote:

Originally Posted by vstech

Have you put the light control switch in the full forward position?

you mean the switch to turn off and on the dome light i have tried it in every position there is ....

AusExp · #13 09-18-2017, 04:28 PM

I have a 92 300te, the dome light does not work as intended.

Position 4: open or close door "dome" light does not illuminate. Puddle lamps will go on and off.

Positions 1,2,3 work as intended (interior and reading lamp, off, reading lamp)

Seat belt lights are always illuminated.
2 are dimly lit, 1 brighter (lights stay on all the time even if car is shut off)

has anyone run into this issue?