docmirror

I worked on the sensors for the first series of airbags. The sensors are good forever, but that's not the issue.

I've asked the same kind of questions. It used to be that the explosive material was pretty sensitive to humidity, age, etc. I know the DOT required all kinds of safety specs for the charge, so I'm concerned about 20 YO airbags more for their ability to deploy when needed rather than an inadvertent deployment.

The sensors are mounted at the inside base of the A pillar against the wall of the junction of the door support and just inside the firewall. They are mounted horizontally and have a three wire connection that comes off the sensor, through a pigtail that has a waterproof rubber connector boot. The reference resistance is 10Ohms, and it can often throw a fault when the resistance goes up with age.

There are two sensors, one left and right. For the airbag to deploy, both sensors must show a dead short from the reference resistance within a few milliseconds of each other. It takes a jolt of about +8Gs for the bags to deploy. This eliminates false deployments in a front end crash where the likelyhood of injury is pretty low. You really have to knock someone hard for them to go off.

I'd like to think the airbags are available new, but I'm pretty sure the only thing you would get from Porsche now is NOS. It would be interesting to know the date of last mfg. I'm sure if you could find newer ones they would be $$$$$,$$$$$$$.

mickster

If you want, put a call into PCNA tomorrow: 800-767-7243 and see what they say. They might refer you back to a dealer or they might get a solid answer.

I am glad to read what "blown 87" wrote as your concern has been mine as I've been researching not only 928 but also other older vehicles.

While I never go anywhere front or back seat without a seatbelt, I am a short person. The airbags in older vehicles are not usually the multi-stage type found in today's vehicles and fully deploy which can sometimes cause more harm than an actual accident.

I would assume these are single-stage type.

Michael

Erik N

That's interesting that they are mounted inboard of the crumple zones.

docmirror

Early tests had them mounted inside the bumper. Way too many false deployments. Then they were moved back to the inner fender well near the front suspension, still to rough.

Erik N

Do you know if the 8G's required to trip them is a common figure for other cars? Or is it less because they're away from the crush zones? As in, the crumple zones sustain higher G's (as they are being deformed) than the passenger compartment. Just curious.

blown 87

Every one I have ever seen has been sealed from what I could tell, as far as the generators.

Have you ever heard of one that did not go off when commanded to?

tveltman

I don't really know, and I could be way off here, but I thought they use accelerometers now to measure the G-force and trip the circuit if necessary.

GregBBRD

All of the airbag problems I've had with 928's have been with the control units, except for a couple of steering wheel contact problems with the "winding spring".

I did have a customer with a 1990 that deployed the passenger airbag (interesting that it only set off one side), but he had been ignoring the airbag warning light for months, before this happened.

Model year 1990 used a system that had the loom hard wired to the brain and the sensors. When anything goes wrong, it needs to be updated to the later system, which has two separate looms with a separate brain and crash sensors. Major dollars to update. Model year 1991 and later used this style system.

We seem to replace quite a few of the control units on the later systems...I always keep my ear open for good used control units, as they do seem a bit hard to find, at times.

I've never seen any information from Porsche about replacing the airbags after a certain age.

mickster

We had a 2000 VW New Beetle with a winding spring problem-3 times. Probably the same bloody supplier...

docmirror

I'm sorry I don't know. I worked in the vibration analysis part of the sensor design, and we were surprised that we had to get it that high to limit the number of false positives. If you recall back then the media made a big deal about airbags blowing up in peoples faces without provocation. This led to a higher than expected G load rating for the sensors, and also led to them being moved closer to the occupants. I've been in a car that decelerated at about 6Gs and while it was abrupt and a bit painful, it wasn't life threatening.

I'm sure each mfg and each design is a bit different. You wouldn't use the same settings for a Corolla as you would for a Bentley.

docmirror

Exactly. The 1990 used the last of the 'series one' airbag systems in the 928. After that, they were all series two style which incorporated a lot of design updates. Although these two series are significantly different, they do the same job. Each mfg also made running changes to the airbags of their cars.

The earliest cars in the late 70s and early 80s did have a problem with false deployment. It lead to a few fatalities, one notable in the US that put back our development several years. Mercedes went ahead with development and was ahead of the US mfg for a while.

The 928 got the last of the series one systems for 1990 and went with S2 type bags in 1991.

c_span

I remember a few years ago, various European manufacturers did some testing as airbags were approaching their 10 year recommended service life and there was concern this could be a "ticking time bomb" esp for the used car market.

Testing showed no noticeable degradation in performance, and it seemed to be pretty much across the board (for the makes that concerned me at the time, anyway!) that service bulletins were being updated to extend to 20 years and they envisaged this could be extended yet further when the time rolled around.

I wish I could find the original link, it came complete with a list of manufacturers and their new recommendations, but here's an example..

axis of evil

Most moving parts (and a lot that don't) in Land/Range Rovers need replacing in less time than that!

docmirror

Thomas, this is correct on more modern systems. There are airbags now that have staged deployment. The accelerometers determine the strength of the hit, and deploy enough charge to cushion the body while not blowing the person backward. These are systems I've never worked on but I believe they do use piezio type accelerometers to determine the impact.

Older airbag sensors just use a weighted toggle with a mass setting. There were two and the signals had to arrive at the receiver within a few milliseconds of each other or no blow.

Jim M.

There is one other concern with our 928's. Someone told me (Jim Bailey???) that when the passenger airbag deploys, it bends the "A" pillar. When that happens, your insurance company will declare the car a total loss! The cost of replacing the "A" pillar along with the other repairs necessary will far exceed the current value of your beloved 928.

Just food for thought.