From impact to full deployment, a driver’s airbag takes approximately 30 to 40 milliseconds (0.030 to 0.040 second) — a time span comparable to an upshift in a modern F1 race car’s transmission and much faster than the blink of an eye (about 0.2 second). A passenger airbag is larger and requires approximately 40 to 80 milliseconds for gasses to fill it.
Side-impact airbags must deploy far quicker, as there is no crumple zone and little space between the occupant and the door. Side-impact detection must take place within three milliseconds and complete deployment of the thorax and/ or side-curtain airbags must be accomplished within 10 to 15 milliseconds.
In the real world, crashes take place at various angles and with other vehicles at various relative speeds. An airbag will only deploy if the set deceleration threshold is met or exceeded. Hit a sizeable object or an animal on a highway and your car will sustain moderate to serious damage, but the airbags won’t pop unless the car decelerates abruptly enough to trigger a deployment.
Early Porsche Airbag Systems
The airbag system starts with a control unit, which provides diagnostic/fault monitoring with memory. It has a self-contained power supply with backup via a charging converter, a safety (triggering) sensor, and airbag firing circuitry. When the car’s ignition is turned on, the system powers up and the diagnostic unit interrogates all airbag system circuits and components for resistance, capacitance, shorted or open circuits, power, and grounds. If operational status is confirmed, the airbag light turns off. Normally, the airbag light goes out in three to five seconds. Even so, a full diagnostic check continues for approximately 70 seconds.
The diagnostic unit monitors the system continuously, switching the airbag lamp on if a fault is detected. The airbag control unit utilized in the 944, 944 Turbo, and 944 S2 coupe (part number 944 618 217 00) stores faults in its memory and can be played back as blink-code sequences on the airbag light. Accessing fault memory requires activating the diagnosis circuit (pin 2) per the instructions in the 944 repair manual, section 68, pages 14–23. The 944 S2 Cabriolet utilized a new control unit (part number 944 618 217 01). With this unit, the fault memory could be downloaded using the Porsche System Tester 9288 (also known as the Hammer or Bosch KTS-300/301). In the course of the 1991 model year, all models were equipped with this unit.
The charging converter has a large capacitor that builds up internal power to 35 volts; this capacitor acts as backup power supply by holding the voltage needed to operate the system and ignite the airbags, even if power to the system is interrupted. The airbag system remains charged and completely operational for approximately 20 minutes after the car is turned off as voltage dissipates from the capacitor. Thus, before working on early airbag systems, wait a minimum of 20 minutes after disconnecting the battery to ensure airbag ignition power has dissipated. On later control units, starting with the 1995 993, the backup electrical charge is held for approximately one minute after turning the car off.
The control unit’s internal safety sensor functions like the front impact sensors, by closing a switch at a specific threshold of vehicle deceleration. The airbags will deploy only when the unit’s internal safety sensor and at least one front impact sensor switch have closed. The logic behind the internal safety sensor is to prevent accidental airbag deployment caused by something like a hard hammer blow near a front impact sensor while service work is performed.