In the previous June 7, 2014 posting, the SRS (Supplemental Restraint Sensor) was relocated away from an area where body repair welding was planned. Previous research of the literature implied that, if the sensor were to be shocked while powered by 12 volts, then the sensor might act like a fuse at the same time that the airbags deployed. This suggested that it might be a one time, one use device, and it was wondered how the sensor actually functioned. Subsequent dissection indicated that the sensor is not a one use device, and what was meant by the literature was only that if powered with 12 volts while being connected to the car and then shocked, that the obvious result would be that the airbags would deploy and thus the airbag would require full replacement !
To learn more about the sensors, some were obtained from the body of a salvaged 09/91 BMW 535i. There are two sensors located under the hood, one on the drivers side and one on the passenger side directly over the wheel wells. Each sensor is connected to a wiring harness by way of a 3 wire weather tight plastic plug (Picture DSC01853).
Picture DSC01853 of the 3 wire weather tight connector.
Picture DSC01854 of the sensor after removal from the 535i. Note the black O-ring on the male portion of the plug. Very firm force was required to separate the plug.
Picture DSC01858 after the removal of the orange plastic coating and internal black "epoxy". A heat gun was used to soften the plastic and epoxy to aid in its removal.
Picture DSC01863 showing the top view of the internal device after cutting open the protective steel housing. The sensor is a plastic tube (center of the device) that is wrapped in a thin layer of spring steel. As the sensor is rolled from the right to the left (what would be the front of the car), electrical contact is made between the silver spring steel, and a pair of small copper electrodes that can be seen in the center of the black circle in the middle of the device.
Picture DSC01867 demonstrating the cylinder shaped detector mechanism while at rest.
Picture DSC01866 of the side view of the sensor when rolled to the left and fully deflected (what would be toward the front of the car) during a simulated crash or impact.
Picture DSC01865 showing a second view of the cylinder shaped mechanism when fully deflected (what would be toward the front of the car) during a simulated crash or impact. In this position the silver spring steel is shorted to the small copper electrodes that are now beneath the spring steel. When released, the tube immediately snaps back and to the right.
So the spring is acting as an inertial switch?
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