Seat belt stay on
Thread Starter
Newbie
Joined: Feb 2022
Posts: 5
Likes: 1
From: Sofia, Bulgaria
s203
Seat belt stay on
Hello,
Sorry for my English
My car is S203 2.2 cdi facelift. I have problem with my seat belt light. When all seat belt are locked the seat belt light on my dashboard stays on. I think the seat buckle is bad but, when the car is on and no one is in it, the lamp is still on. i check the passenger seat buckle with multimeter and when the belt is unlocked, multimeter show 400 Ohms but when i lock the seat belt it show 100 Ohms then i switch to the driver's seat buckle and multimeter show 400 Ohm when belt is locked and 400 Ohm when is unlocked. OK that means the driver's buckle is bad, but why the light stay on when the car is empty?
Did driver's seat have occupansy sensor? there is no driver's seat occupansy in the mercedes spare parts catalog.
Thank you!
Sorry for my English
My car is S203 2.2 cdi facelift. I have problem with my seat belt light. When all seat belt are locked the seat belt light on my dashboard stays on. I think the seat buckle is bad but, when the car is on and no one is in it, the lamp is still on. i check the passenger seat buckle with multimeter and when the belt is unlocked, multimeter show 400 Ohms but when i lock the seat belt it show 100 Ohms then i switch to the driver's seat buckle and multimeter show 400 Ohm when belt is locked and 400 Ohm when is unlocked. OK that means the driver's buckle is bad, but why the light stay on when the car is empty?
Did driver's seat have occupansy sensor? there is no driver's seat occupansy in the mercedes spare parts catalog.
Thank you!
MBWorld Fanatic!
Joined: Dec 2003
Posts: 3,275
Likes: 236
From: BC, Canada
2003 C230K Coupe Orion Blue
The car probably doesn't check if the driver's seat is occupied (weight sensor) because the brake pedal has to be depressed before the car can be shifted out of gear. 
The car is checking whether or not the driver's seat belt is buckled (which assumes there is a driver in the seat), and not checking if somebody is poking the brake pedal with a stick from the passenger side. The car also doesn't check if someone is attacking it with a sledgehammer or setting it on fire.
I imagine the seat belt light will stay on if a) the driver's seat belt isn't buckled, and b) the passenger seat belt is buckled if there is enough weight in the seat to trigger the weight sensor (I think the required weight is least some 10kg). Since your driver's side buckle sensor is broken, a) is not being met.
The car is checking whether or not the driver's seat belt is buckled (which assumes there is a driver in the seat), and not checking if somebody is poking the brake pedal with a stick from the passenger side. The car also doesn't check if someone is attacking it with a sledgehammer or setting it on fire.I imagine the seat belt light will stay on if a) the driver's seat belt isn't buckled, and b) the passenger seat belt is buckled if there is enough weight in the seat to trigger the weight sensor (I think the required weight is least some 10kg). Since your driver's side buckle sensor is broken, a) is not being met.
Last edited by slammer111; Feb 17, 2022 at 04:49 AM.
Thread Starter
Newbie
Joined: Feb 2022
Posts: 5
Likes: 1
From: Sofia, Bulgaria
s203
I disassembled the buckle and began to follow this scheme. I measure the resistance of the two pins and it shows 400 ohms when the belt is not fastened. This means that both resistors are strong. When I fasten the belt, the circuit must isolate the resistor, which is 300 ohms, and at the end of the circuit it must show 100 ohms. Yes, but for me it still shows 400, which means that the key that closes the circuit is broken or does not make contact. Has anyone repaired the key and if so please help me with the repair. I sprayed with contact spray, but it didn't help.
MBWorld Fanatic!
Joined: Dec 2003
Posts: 3,275
Likes: 236
From: BC, Canada
2003 C230K Coupe Orion Blue
The seat belt buckle cannot be disassembled without destroying it. I think the outer plastic casing is melted together during assembly.
Just buy a new one from the dealer, or a used one from a local wrecker.
Sounds like you got really unlucky. This is the first time I have ever heard of a seat belt buckle failing this way.
Lastly, I strongly do not recommend defeating this safety feature by adding resistors or otherwise modifying the wiring. This can have fatal consequences in the event of an accident.
Just buy a new one from the dealer, or a used one from a local wrecker.
Sounds like you got really unlucky. This is the first time I have ever heard of a seat belt buckle failing this way.
Lastly, I strongly do not recommend defeating this safety feature by adding resistors or otherwise modifying the wiring. This can have fatal consequences in the event of an accident.
Last edited by slammer111; Feb 17, 2022 at 08:28 AM.
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MBWorld Fanatic!
Joined: Dec 2003
Posts: 3,275
Likes: 236
From: BC, Canada
2003 C230K Coupe Orion Blue
Wondering if you can cut open the old buckle and post some photos after the job is done. I’m curious to see how the mechanism works internally. Thanks in advance. 
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Newbie
Joined: Apr 2003
Posts: 6
Likes: 1
From: Milwaukee
C280
Thanks for posting details of the ohms locked and unlocked. 100 ohms does work on a 2014 BTW to tell computer that seat belt is "locked". Downside of putting a resistor in is that the seatbelt retractor doesn't offer up the belt without the resistance at 400 ohm and then retract when locked. Upside is that the warning light is off and airbag and pre-safe will work normally. I pick up new buckle receptacle at Mercedes tomorrow. I always wear wear my seat belt and I suggest everyone else should do the same.
