A Class W169 SRS seat sensor

[DaveLA180]

I have a 2008 A180 CDI W169 with an SRS fault that has been haunting me since the day we bought it second hand from a dealer.

The fault presents itself by the passenger seatbelt symbol on the dash blinking on for a while followed by the addition of the SRS symbol and eventually an audible beeping similar to when you haven’t connected your seat belt or a door is left open.

I have taken the car back to the dealer and had the Star diagnosis identify the error code as a passenger seat sensor fault. According to the dealer he “fixed” the issue but a week or so later the issue occurred again.

To cut a long story (slightly shorter) we have failed to resolve the issue with the dealer and I have since found a 2 piece sensor Matt emulator which I have now removed and I purchased a new sensor Matt from MB and painstakingly removed the old one and carefully installed the new sensor. This didn’t fix the issue..

I then purchased a second hand passenger seat belt housing with associated wire in case this was at fault but the fault persists.

I have recently given in and took the car to a MB specialist who removed the flooring and found some water collecting near some of the wiring. He cleared this and apparently cleaned the connectors but the fault appeared again. He has now replaced one of the relays that provides volts to the ECU that controls the SRS system advising that he identified there was a fluctuation in voltage on the sensor which could be caused by the relay or possibly the SAS unit.

After replacing the relay the fault is still present but I am hoping someone here can advise whether this fault could indeed be caused by a faulty SAS unit which I’m told could be expensive to replace.

I have read that a faulty ECU could cause this issue but your thoughts/experience would be appreciated as this is becoming an expensive problem..

 

[BlackC55]

Have you replaced the seat belt buckle?

 

[DaveLA180]

Have you replaced the seat belt buckle?

Yep but with a used item from eBay..according to the seller it had been tested before being listed.

 

[BlackC55]

What fault codes are there?

 

[DaveLA180]

What fault codes are there?

I haven’t managed to get the code from either of the mechanics I’ve had this checked by which is frustrating to say the least. I’m pretty sure I was told there weren’t any codes present when this was initially happening.

I will try to get in touch with the last mechanic I to and ask him for the code but they have all pointed to the passenger sensor mat.

 

[DaveLA180]

Does anyone know the resistance values for a working seat belt buckle from the connector?

 

[BlackC55]

It shouldn't be open circuit. From memory its 3-5 ohms

 

[DaveLA180]

Okay thanks. (The only thing that might rule this out as a cause is that the alarm goes off with or without a passenger.)

Either way I’ll check the resistances.

 

[BlackC55]

Be careful with the meter. You can set off the tensioners if you induce a voltage. It takes less that 1v to set one off.

 

[DaveLA180]

I’m just measuring the original buckle unit which I replaced. So the resistance is 0.6 ohms when circuit is closed ie buckle fastened. Am guessing that is correct.

The only other measurement i could do is the sensor and loom that connects to the red connector on the floor. I imagine the loom should have little or no resistance.

 

[ioksuz]

Hello DaveLA180

Have you fixed your problem? I'm having the same in my W176. I fitted emulator for seat belt and occupancy sensors and now getting those error codes below and airbag light still flashing red on my dash

U1008-55 Lin Bus 1 has a malfunction - The component is not configured
U1008-87 Lin Bus 1 has a malfunction - The message is missing

 

[DaveLA180]

Hi ioksuz

Unfortunately not. We’ve managed to get by with it up till now but it has become more and more frequent. An independent Mercedes service told us the error code on ours relates to a drop in voltage which could come from one of the relays in the front SAM module. He replaced that and fault persists. Strangely seems to go off when we go over a bump or do a tight turn.

Thinking about swapping out the SAM module next then possibly the Airbag ECU if that doesn’t resolve it. Really disappointed none of the service centres have managed to resolve this and c’è coming too expensive to pay for fault diagnosis that don’t actually get anywhere.