Abstract: A motor vehicle electrical system disconnect switch is disclosed which operates to disconnect the positive lead of the battery of a motor vehicle from its electrical system and to ground the positive lead of the vehicle alternator upon either the application of a decelerating impact force to the vehicle or a tilting of the vehicle by at least a minimum predetermined angle with the horizontal. The device includes a housing containing at least three hollow chambers, a first one of which normally carries an electroconductive liquid such as mercury. A pair of electrical terminals are mounted on opposing housing end walls which partially form the first chamber so that the terminals are normally electrically connected together through the liquid in the first chamber. When the terminals are so connected, the positive lead from the battery to the vehicle electrical circuit is complete.