Air bags and memory keepers

[Stephen Tashiro]

Instructions for many jobs on modern cars say to disconnect the battery to avoid accidentally triggering the air bags. Instructions for other tasks show putting a small battery on the cigarette lighter (or "auxilliary connector") before disconnecting the battery in order to keep the computer's memory from being lost. Does attaching the memory keeper keep the air bag circuit "live" too?

[Dan Hintz]

The idea behind disconnecting the airbag is to avoid it blowing if a transient surge shoots through your electrical system while disconnected, or jarring the sensor while still connected. If the electrical is disconnected and you will not be connecting electrical in any way (this includes test equipment, too), I'd leave it connected... offhand, I do not know how much of a charge those system retain, but I can't imagine it's for more than a few minutes, if at all.

EDIT: To answer your question... if you've hooked power to the cigarette lighter, you've applied power to everything normally attached to the battery, which includes the airbag. The only reason to keep power alive is to retain stereo stations, ECU settings (for smart ECUs that learn over time... pretty common these days), and ECU fault codes.