air bag removal affects dash light?

[sandsmuseum]

Of course I am just trying to understand how my air bag works and would never consider defeating it.

It appears the Sensor and Diagnostic Module (SDM) is attached to a bracket on the scuttle beam. This module provides several functions:

1. It has accelerometer functions that determine the direction and force of the car's impact
2. It is able to respond to diagnostic functions.
3. It has sufficient power to trigger the air bags and seat belt pre-tensioners even if the progressing accident cuts off the power from the battery.

It looks like there are only two wires to the air bag module in the steering wheel. If I disconnect those two wires, will the SDM be able to see that and cause a diagnostic to be raised, and the air bag telltail to be lit?

Just curious...

Michael

[EdHahn]

For most cars, yes. In general, airbag diagnostic circuits can measure continuity through the airbag leads - a completely open circuit will usually trigger a fault condition.

You can complete the circuit in the absence of the airbag to get the diagnostic light to turn off. Most of the time simply shorting the two wires together will work, but you might need to put a small resistance in series with the bag.

ed

[rob13572468]

Originally Posted by sandsmuseum

Of course I am just trying to understand how my air bag works and would never consider defeating it.

It appears the Sensor and Diagnostic Module (SDM) is attached to a bracket on the scuttle beam. This module provides several functions:

1. It has accelerometer functions that determine the direction and force of the car's impact
2. It is able to respond to diagnostic functions.
3. It has sufficient power to trigger the air bags and seat belt pre-tensioners even if the progressing accident cuts off the power from the battery.

It looks like there are only two wires to the air bag module in the steering wheel. If I disconnect those two wires, will the SDM be able to see that and cause a diagnostic to be raised, and the air bag telltail to be lit?

Just curious...

Michael

yes, both of the airbag units have electronic ignitors that allow the SRS mosule to perform integrity checks. the ignitor circuit is really just a set of very small coils of very fine wire. when the module wants to ignite the airbag, it simply sends 12 volts (though as low as 7 volts will work) to the the airbag terminals which instantaneously creates enough heat to ignite the chemicals in the gas-generator (usually natriumazide). the SRS module has a set of internal capacitors that allow it to retain enough voltage to still ignite the airbags 100 milliseconds after power is lost or if the battery voltage drops below the minimum 7 volts.

consequently, you will notice that there are a number of shorting clips designed into the system; the wiring diagrams list 3 at the module itself, one that will trip the SRS light if the module is unplugged and the other two short across the ignitor terminals. this is important since the low voltage design is actually susceptible to ignition from static discharge. there are usually an additional set of shorting clips on the airbag modules themselves that engage when the plug is pulled at the airbag itself instead of the module.

to answer your question; the way the SRS module detects taht the airbags are connected is via the same two wires that are used for ignition. the module performs integrity checks continuously by applying a very small (usually 0.5 to 1.5 volts) voltage across one of the terminals and then looks for it across the other terminal. by measuring the voltage drop across the terminals it can notonly detect that the ignitor coil is present but also check if it has the proper resistance. disconnect the airbag and the module will sense the open circuit and turn on the SRS fault indicator. Also note that the seatbely pre-tensioners work the same way, typically using chemical ignitors to lock the ratchet mechanism in place during an impact. the SRS module also performs the checks on these as well.

[Chococar]

I had a similar issue. I learned that the seat belt pretensioning device looks like a 2.2 ohm resistor to the rest of the circuit. I chose 2.2 ohms by using a resistance substitution box to determine the range of acceptable values, then choosing one near the middle. I was fortunate that Fry's had that value in stock.

Michael, I need to know the answer to your question too, so if you don't have a resistance substitution box, drive over to my house and we'll do this science project together.

[DesignIt]

Originally Posted by Chococar

I had a similar issue. I learned that the seat belt pretensioning device looks like a 2.2 ohm resistor to the rest of the circuit. I chose 2.2 ohms by using a resistance substitution box to determine the range of acceptable values, then choosing one near the middle. I was fortunate that Fry's had that value in stock.

Michael, I need to know the answer to your question too, so if you don't have a resistance substitution box, drive over to my house and we'll do this science project together.

Did you ever finish this particular science project? I was wondering if you replace the airbags (in your science project) with the appropriate resistors, the pretensioners should still be functional, correct?

John

[Chococar]

I didn't. Yet.