Restraint systems (Airbag ECU): Description and operation

Airbag ECU

The airbag ECU controls the operation of the system and also contains the collision detection sensors. The airbag ECU is attached to a bracket on the transmission tunnel, directly below the heater. A vehicle earth output connects to one of the fixings. A 30 pin connector provides the airbag ECU connection with the vehicle harness.

Incorporated into the airbag ECU is a mechanical safing sensor, an electronic single point sensor and integrated circuits for control and diagnostics. The mechanical safing sensor is a normally open switch that closes at the preset deceleration limit. The single point sensor is an accelerometer that produces an output proportional to the vehicle’s deceleration .

Power back-up

The airbag ECU incorporates capacitors to ensure the system will function if the external power supply is disconnected during a collision:

• A hardware capacitor provides power for 200 milliseconds to enable system operation and collision recording.
• Individual capacitors for each fire signal output provide power for 150 milliseconds.

The capacitors are kept charged while the ignition is on by a dc-dc voltage converter incorporated into the airbag ECU. It can take up to 10 minutes from the ignition being switched off for the energy stored in the capacitors to fully dissipate and make the system inert.

Airbag ECU

Airbag ECU pin connections Pin Description

1. RH pretensioner (+ve)
2. RH pretensioner (-ve)
3. LH pretensioner (+ve)
4. LH pretensioner (-ve)
5. Vehicle supply
6. Vehicle earth
7. SRS warning lamp
8. Not used

Pin Description

9. ISO 9141 K line
10. Driver airbag module (+ve)
11. Driver’s airbag module (-ve)
12. Not used
13. Passenger’s airbag (+ve) - where fitted
14. Passenger’s airbag (-ve) - where fitted
15. to 30. Not used