Air bag warning light intermittent operation
Some time ago the air bag warning light began to operate intermittently. After some research this was found to be a common fault on the 3 series and caused by the seat sensor failing. This is what the sensor looks like. (The top of the picture is the lifted seat fabric. The view is from the back of the seat looking down).
Emulating the sensor pad
Confirmation that it was a sensor fault was obtained by using a simple device documented on other sites and forums. This device mimics the sensor by means of a zener diode and resistor. In fact the mat itself contains a diode but the resistor in the mat is in the form of a resistive grid that changes resistance when stretched i.e. when someone sits in the seat. The diode provides a means of creating a reference voltage from which to measure the resistance changes. (Well that's how I think it works).The parts to make the seat sensor emulator cost pence, can be fitted in minutes and can be obtained from Maplin. (I haven't got them the details to hand but will try and post them later).
Although the emulator could be left in place this is probably a dangerous solution long term. The function of the sensor is to allow the air bag system to determine which air bags to fire in the case of an accident. Therefore, the emulator will probably make the system think the passenger seat is unoccupied and therefore not fire the relevant air bags.
Sensor pad replacement
I couldn't find any documentation on the web on replacement of the sensor and so I had to proceed by trial and error. In fact it took me longer than a day to do what in retrospect was a couple of hours work so I thought this post was needed.Key Facts
- The seat is a large complicated assembly but the seat cushion containing the sensor is easily removable. There are 2 screws at the front and a plate at the back to remove.
- Removal of the cushion is possible with the seat in-situ but replacing it would be difficult/impossible. So it will be necessary to take the seat out.
- Removal of the complete seat is easy.
- The electrical loom connector for the seat is a little puzzling to disconnect. It doesn't just pull apart. Instead look for a slider mechanism. This moves perpendicular to the body of the connector and separates it into two halves.
- The fabric on the seat cushion can be prised off the rim of the metal seat pad shell. (It's not glued etc). This will nearly reveal the sensor but first it is necessary to cut a number of attachments rings. These rings hold stiffening wires in the fabric to the seat cushion. These can be replaced on reassembly with wire ties.
Tools
- Torx wrench bits for screws
- 16mm hex socket
- side cutters/pliers
- quantity of small cable ties (20-ish)
