The front impact sensors are located on the right and left wheel-housing walls, with cables running from each sensor to the control unit. The front sensors function mechanically but are monitored electronically. Each sensor has a hollow roller that allows for the insertion of calibrating weights held in place by a tensioned spring band (wrapped around the roller) and mounted to the base plate of the sensor. An isolated electrical contact sits next to the roller and, in a frontal impact resulting in sufficient deceleration to overcome the spring tension, the roller moves forward to the contact and closes the circuit to the control unit. This type of sensor switch, called a Rolamite switch and developed by Sandia National Laboratories, was commonly used for airbag impact sensors until the mid-1990s.
Airbags are made of nylon cloth with an internal coating of Neoprene. Straps within are arranged to form the desired shape when inflated. The internal volume of a driver’s airbag is approximately 60 liters while a passenger-side airbag is approximately 150 liters. The airbags are mounted on the housing of a gas generator filled with solid fuel in a closed combustion chamber. An ignition pill is fitted in the center of the gas generator. When the pill receives an electric pulse, it ignites, which in turn ignites the main pyrotechnic propellant fuel. The exploding fuel process produces nitrogen gas that flows into the airbag through outlet ports that filter combustion materials and help to cool the hot expanding gases.
Porsche Airbags Evolve
By the 1990 model year, all Porsches destined for the U.S. were equipped with driver and front-passenger airbags. The 944 S2, 968, 964 Carrera 2 and 4, 928 S4/GT, and 964 Turbo all utilized the basic design first released in the 1987 944 Turbo. For 1991, all models got a new control unit, which allowed the Porsche System Tester 9288 to download stored fault memory and clear it.
The new control unit also provided a means of identifying whether the airbag control unit had been replaced. From the factory, the control unit’s power connector came attached to the chassis by an orange securing bracket. If the control unit must be replaced, this bracket must be destroyed to release the connector. Since the replacement bracket is available only in green, a look at this connector will shed some light on the history of a used Porsche from this era.
The 1995 993-based Carrera saw the removal of the right and left front impact sensors. The Rolamite type switches were replaced by the Micro Electro Mechanical System (MEMS) Accelerometer — allowing complete crash detection for airbag actuation within the airbag system control unit. The MEMS Accelerometer is a very small integrated circuit with an internal mechanical element that moves in response to rapid deceleration. Such a motion causes a change in capacitance that is detected by the control unit, prompting it to send a signal to fire the airbags. While this control unit does not need to be replaced following a single airbag deployment, it must be replaced after the system has been triggered three times. Faults that cannot be erased require control unit replacement. Internal (backup) power remains in these control units for one minute after turning off the ignition or disconnecting the battery.
While some features of the 993 were incorporated in the 1997 Boxster, several important changes were made. First, the passenger-side airbag no longer used a breakaway cover; the airbag itself is visible in the dash. Next, the passenger-side solid fuel gas generator was replaced by a pressure canister filled with 95-percent argon and five-percent helium pressurized to approximately 2,940 psi.