CesarBartz

Folks,

I am new to 4x4. my 1996ZJ "apparently" has airbags (like a match, you can only "test" it once!).

I had seen 3 sensors on the manual - one in the middle of the car, inside (believe is an accelerometer) and 2 behind the front bumper, one each side. Manual does not tell in detail - are these 2 accelerometers or some sort of "crash sensor", i.e., if they crush, will tell the airbag central control car has just crashed and to deploy bags?

Reason for my question - most of my use will be in paved road with regular traffic, including highway. If possible I want to keep the peace of mind in having airbags. But I will modify the front bumper - put a steel one (DIY), tougher than the stock bumper + fascia. So, if the sensors are "chush sensors", then it means I need to crash it harder but still has the airbag to work... if it is however accelerometers, probably does not matter - they will deploy if I manage to hit harder enough to achieve the same acceleration that the "standard design crash" would produce...

Another bit - what if I go to trail and hit small stuff? stuff that does not damage the car? I dont want booming airbags for any ordinary stick I break. A friend that has a Suzuki told me trail cars should have airbags disabled or they are deployed on the bumpy trail.

Thanks in advance...

Big David

I just always pulled the fuses in my old TJ & XJ whilst wheeling. Luckily, my '89 doesn't have them.

dmill89

95-96 XJs have "mechanical" airbags (97+ xjs got electronic airbags), there are no control modules, or sensors. There is only a mechanical accelerometer inside the steering column/airbag itself, which triggers a firing pin and percussion cap igniting the propellant. Any modifications to the bumper will have no effect on this system (aside from possibly transmitting more force to the chassis in the case of an impact). The mechanical airbags are known for being difficult to trip (often not deploying in 30 mph+ crashes) and are unlikely to deploy accidently off-road. If you do want to disable them there is a screw on top of the steering column (under a plastic cover) that you loosen.

Bustedback

No, that's wrong information. There is in fact a module and crash sensors on the 95-96 vehicles. The module is mounted on the tranny hump under the dash, and the crash sensors are on both sides of the front unit frame. Those sensors are accelerometers. There isn't such s thing as mechanical air bags.

dmill89

Oops, missed that it was a ZJ, you are correct, 95-96 XJs have mechanical air-bags, but ZJs do have electronic air-bags.


Yes, there is, many older vehicles have them, rather than an electronic control module with sensors that ignites the propellant with an electric charge, there is a mechanical accelerometer which trips a firing pin that strikes a percussion cap igniting the propellant.

http://www.carsdirect.com/car-safety...ag-system-work


http://papers.sae.org/856014/


http://www.sciencedirect.com/science...22460X98917515


http://www.firehouse.com/forums/t124055/


Here's a pic of the back of a 95-96 XJ airbag. Note the label "All Mechanical Air Bag System" and the lack of any electrical connectors:






VS a ZJ electronic Airbag, note the electrical connector in the center:

Bustedback

Every air equipped vehicle that I have worked on over the last twenty five years as a mechanic has had a control module and crash sensors. I'm gonna have to do some research on this.

Bustedback

Just did some research and found out you're correct. There is such s thing as mechanical air bags. Learn something new everyday.

CesarBartz

Thanks, dmill89. I did not knew about the "all mechanical air bag"! Learned something new.

Thanks for the photos also.

But the sensors my ZJ has on the bumper, anyone would know if they are accelerometers or a king of a "crash sensor"? I can imagine being both valuable and easy to build a sensor with some sort of breaking device that would open the contacts.

If this is the case, then a stiffer bumper would require more speed to trigger airbag. If they are regular accelerometers, SHOULD not affect.

Anyway, installing stiffer bumpers does not cut out the "car deformation" part of the passengers integrity - it just "transfer" the deformation to the other vehicle!

dmill89

As far as I'm aware they are essentially accelerometers/force sensors that measure impact forces. There is a possibility that a stiffer bumper could transmit more force to the sensors (vs. the stock foam/plastic bumper that will absorb some, but not much, impact), but the difference is likely to be minimal. As far as off-roading it is possible to trip airbags on the trail, bur relatively rare (and generally more common on newer vehicles with more sensitive airbags), but if you are worried you can always pull the fuse before any heavy off-roading and put it back in when on pavement. While state laws/inspection requirements could potentially be an issue with no/disabled airbags (though may states do not require functional airbags, even on newer vehicles, and as long as you just pull the fuse they are easy enough to "re-enable" for inspection if needed), the actual safety aspect is minimal assuming you're wearing your seatbelt. Frontal airbags, especially older ones don't offer that much more protection than a seatbelt alone, their primary advantage is for unrestrained occupants, if the crash is bad enough to push the dash/firewall into you airbags won't help much.