Inside an airbag sensor
 

So I finally got around to replacing the faulty driver side airbag sensor with one from a wrecking yard. So far, it has worked great. I almost don't know what to do with the airbag light staying off while I'm driving! ;)

So I took the opportunity to take a look inside the old bad one (GM pn 16240665) . Below are the pictures. The sensor mechanism itself is in the white plastic body. It along with the electrical connector appear to be dropped into the body and then the volume filled with a rubber/silicone like filler. I was able to get it out with a combination of scoring with a utility knife, and digging out the filler with a pick and a flat blade screwdriver.

In the pictures, the direction towards the front of the vehicle is towards the widest part of the white plastic bit with the opening of the electrical plug facing rearward.

I suspect the fault to be where the electrical connector is soldered onto what looks to be a printed circuit board at the forward facing end of the sensor mechanism.

https://cimg5.ibsrv.net/gimg/blazerf...84f2c0a82d.jpg
Note where the electrical leads from the plug receptacles come in the body and then attach to the PCB.


https://cimg0.ibsrv.net/gimg/blazerf...b208b57b7a.jpg
Looking forward


https://cimg3.ibsrv.net/gimg/blazerf...48f5798d1e.jpg
Looking down into the sensor. Forward is to the top of the picture.

Thanks for the pictures.

How'd you know the wrecking yard part was any good? I'm assuming you have an SRS code reader?

Unfortunately I only know that the SRS light has not come back on. If my memory serves me correctly, that means that the continuity in the main circuit to and between the sensors is good. As for the innards of the sensor I don't know. However I'm not quite prepared to do a "test" that would trigger the sensors to find out. ;)

So I found what I think is a description of how this particular air bag sensor works.

https://cimg2.ibsrv.net/gimg/blazerf...c5ca4d8ac5.png

Figure 1. Structural components to an Inertia sensor. Source: Duffy, J.E. (2001). I-Car Professional Automotive Collision Repair. New York: Delmar, a division of Thomas Learning.During a collision with another moving vehicle, the sensing mass is forced forward into the gold-plated contacts as a result of change in the state of motion. Following movement of this metal ball into the contacts, this metal mass makes contact with electrical terminals at either side of the metal ball which alerts the central unit to a collision (i.e., the electrical contact completes the circuit).

https://www.azosensors.com/article.aspx?ArticleID=40

I checked and sure enough, the darker ring inside the sensor body is a magnet.

Going by the structure of the sensor, I would say that this confirms that if you get your own sensors off of a vehicle at a wrecking yard, do NOT get one from a vehicle in which the airbags have been deployed. I looks like once the ball has moved forward, the internal parts of the sensor could be damaged.

This could be a good example for a physics lecture on Newton's Second Law; If the deceleration is great enough, the force of the magnet on the ball is not sufficient to keep the ball from continuing its forward motion and making electrical contact with the electrodes, completing the circuit and triggering the airbags. The magnitude of the deceleration is only one criteria for deployment according to this paper (http://www.theforensicexaminer.com/2...Deployment.pdf). The range of accelerations are about 5gs to 10 gs.

christine, to replace the airbag sensor, did you have to 'de activate' the a/b system prior to working on it?

No I did not deactivate the airbag system. I just unplugged the sensor from the wiring harness. I have the original GM repair manual and I do not recall any instructions to do so like I noticed with the directions I consulted to learned how to remove the airbag from the steering wheel.

This is great stuff!

I also have the intermittent Air Bag light coming and going.

I will look into this further, and probably grab a few from the salvage yard.

Did you disconnect the battery before you unplugged the sensor's wire harness?

Oh good. When I post things I've learned, I do hope people will find them useful.

So when I did the replacement, I had the ignition turned off but the battery was still connected.

That said, I decided to look at the official 1999 GM repair manual and sure enough, they instruct that the SIR (airbag) system should be disabled first. I'm attaching the instructions for disabling it. Considering the downside of the airbags being accidentally deployed, I'd suggest it is worth the peace of mind to disable the system before doing the replacement.

There is a Tech Article for that. :icon_wink:

Thanks! Good to reminder of the extensive library at the Tech Article page. https://blazerforum.com/forum/tech-a...e-first-30093/