Repair of Seat Occupancy Sensor

[aqibi2000]

Looks like a LIN message

[geo999]

it might look like it but there's no digital component on the board that is generating this signal

[Stig997tt]

I stumbled upon this forum whilst looking for information about the passenger occupancy sensor that is also faulty in my 1 series. I know this thread is now nearly 3 years old but wondered if the OP ever got his sensor to work. I looked at the partly drawn schematic that the OP did and it’s fairly obvious to me that the SO8 ic is a microcontroller running off 2.1V on pin 3. The digital output is driven by the open collector transistor BC846 and most of the RHS of the schematic is just voltage regulation. The zener that the OP had blown is only acting as a crowbar for any voltage spikes that manage to get through the TLE4285G 5V regulator so assuming it was not shorted, the circuit should have continued to work albeit without surge suppression. I will be debugging my sensor output on a DSO soon as it works most of the time. I can post results here if anyone is interested.

[Valden]

Hello Stig.

Sorry for the late reply. I only just saw this. Must check my email notification settings.

Yes, I did get the board to work. I replaced two components; the blown zener, and I installed a resistor to simulate the weight of my wife’s petite bum on the seat. This resistor sits in place of a resistance from the seat pad. I had my wife sit on the pad, measured the resistance it generated, installed a resistor of that value and checked the frequency the board was sending to the car’s computer. The car is now hard wired for a light adult passenger. The airbag fault light in the dash is no more.

I’m now waiting for one of my kids, or my wife, to crash the car. I expect to see the passenger airbag, even if they’re driving alone.

Your circuit reading skills are impressive. It’s been ages but I would enjoy further thoughts you might like to share.

Regards, Rick.