A crash sensor for a motor vehicle comprising: a housing defining a path of movement between a first and a second position; a mass disposed in said housing for movement along said path; biasing means for biasing said mass toward said first position; crash indication means for indicating a crash when said mass is at said second positions; blocking means for blocking said crash indication means, said blocking means including a blocking element movable between an armed and a disarmed position; and arming means for automatically arming said sensor by deactivating said blocking means, said arming means including an arming member detached from said housing and attached to said motor vehicle and a rotatable member extending into said housing for selectively moving said blocking member between said armed and disarmed positions when said housing is installed in the motor vehicle.
