Air Bags

[ahhughes]

I have a potential problem with air bags. The srs light is on and I am sure that it is due one of the air bags. On my 124, the easiest bag to access is the passenger bag. I assume that one way to check the bag is to measure its impedance( resistance). Where can one find what the impedance is? I assume that it has to be measured without passing current through it.

All help and suggestions will be appreciated.

[Strife]

A modern DVM will pass almost no current, and, SUPPOSEDLY, it takes an amp to se these off, BUT.... without any "safe" specs available, I would NOT try it.

I think that it's highly unlikely that an air bag would fail in this manner. Failures from age would happen in other ways, charge gone bad, dry rot, etc. This would be the last place I'd look.

[whunter]

Due to legal liability and the potential for you to be KILLED...

I refuse to answer any SRS/airbag questions.

This is where your only safe option is the MB dealer or independent shop...

Even after a battery disconnect, it is possible for static electricity to deploy the air bag.

Static electricity is a real, and deadly danger...

http://www.pei.org/static/

http://en.wikipedia.org/wiki/Electrostatics

http://www.electrostatics.com/page2.html

Static electricity can be generated by the use of tools, repair person sliding across the seat, cutting of safety belts, etc.
It is not possible to determine how much static electricity is present around the vehicle and specifically what wires you or your tools may contact.

This is why you must always treat every air bag system as if it were “live.”

[whunter]

Mercedes Benz AIR BAG RESTRAINT SYSTEM

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IR BAG RESTRAINT SYSTEM
1985-87 MERCEDES-BENZ AIR BAGS
IDENTIFICATION

WARNING: To avoid injury from accidental air bag deployment, read and carefully follow all WARNINGS and SERVICE PRECAUTIONS.

All models equipped with an air bag have the letter "B" or "D" in the eighth position of the Vehicle Identification Number (VIN). The words AIRBAG or S.R.S. will be embossed on the steering wheel hub pad. Also, all models equipped with Supplemental Restraint System (SRS) can be identified by the SRS labels provided at different locations of vehicle, such as driver's door latch post.
DESCRIPTION & OPERATION

The following are main components of Supplemental Restraint System (SRS): inflatable air bag/gas generator module, slip ring, carbon brushes, SRS sensor/control unit, energy accumulator, voltage converter (or voltage transformer), SRS warning light and driver's knee bolster. See Fig. 1 . Emergency Tensioning Retractor (ETR) is incorporated within passenger's seat belt assembly. For SRS to operate properly, 3-point seat belts must be worn.

If vehicle's deceleration rate and "G" force recorded by SRS sensor/control unit is sufficiently high, SRS sensor/control unit generates a voltage signal. This voltage signal triggers gas generator, igniting a solid propellant to produce a quantity of non-toxic nitrogen gas, which fills the air bag in milliseconds. In about 6 milliseconds, after the air bag has inflated to maximum expansion, the gas is released via lateral ventilation slots in air bag, and the air bag collapses. The entire sequence of air bag inflation and collapse takes about 25-30 milliseconds.

Fig. 1: Locating SRS Components
Courtesy of MERCEDES-BENZ OF NORTH AMERICA
AIR BAG MODULE

Air bag module consists of a signal receiver, gas generator, and an inflatable air bag. The complete air bag module is attached to the steering wheel hub. The steering wheel-to-air bag module wiring connector contains a special plug to fit module. When steering wheel-to-air bag module wiring plug is disconnected, module is automatically short circuited by a shorting bridge wire inside module connector. This prevents accidental operation of system by static electricity or careless handling. The shorting bridge wire is automatically opened when wiring plug is inserted into connector.

When an electrical impulse is transmitted from SRS sensor/control unit to ignition pellet in the gas generator, the solid propellant will then burn off within milliseconds to produce a given quantity of nitrogen gas at a given pressure. During this process, gas is guided through a filter into the air bag. The folded nylon air bag on the gas generator will then rip open the vinyl of steering wheel hub pad. The air bag is tightly inflated by the gas, and will prevent head/facial impact of driver to steering wheel during a frontal collision.
EMERGENCY TENSIONING RETRACTOR (ETR)

The ETR passenger's seat belt assembly consists of gas generator, cable pulley, clutch, guide tube, automatic belt retractor reel, and piston with cable. The complete unit is installed in the right center door pillar, in place of standard belt reel unit. The ETR unit is electrically connected to SRS system by a double plug connector.

When ETR unit wiring connector is disconnected, unit is automatically short circuited by a shorting bridge wire inside connector. This prevents accidental operation of ETR system by static electricity or careless handling. The shorting bridge wire is automatically opened when wiring plug is inserted into connector.

The moment an electrical impulse is transmitted from SRS sensor/control unit to ETR capsule containing solid propellant, a high pressure burn-off gas is generated. This gas accelerates piston and cable in upward direction. The cable transmits a turning torque to pulley, causing a shear pin to break off, and pulley is then positively connected to wind-up shaft. The wind-up shaft is turned back to eliminate any slack. This pulls the seat belt snugly to front passenger in milliseconds.

In other words, DO NOT manually test, alter the airbag, alter the wiring, alter their function, or even look at them with a funny face unless you are certified to do so with the correct equipment.

Working with a possible defective airbag or airbag electrical system is one of the few things that scare the ***** out of me. I've had to change a few due to a bad integrated horn switch and it's one of the most nerve racking things I've ever done.

EDIT: Testing an electrical circuit for OHMs with an OHM-meter uses a very small current to measure the resistance. It's not much but it may be enough to cause an accident.
You don't want this to happen: http://www.youtube.com/watch?v=N8NwyzVuMy8

[whunter]

Copied from an e-mail request.

I have a 1995 SL500.
I am looking to remove the bolts from this airbag face panel, but the bolts are QUITE odd.
Is this a special mercedes tool that I need?
Where can I can this tool?

Screw head... types
Screw head... types

[whunter]

Warning: Shipping Airbags and explosive seatbelt retractors


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