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A wig wag is a warning device for low air pressure found in air brake systems on large commercial trucks. This device drops a mechanical arm into view when the pressure in the system drops below the threshold of sufficient pressure to reliably deploy the brakes.
An automatic wig wag will rise out of view when the pressure in the system rises above the threshold. The manual-reset type must be placed in the out-of-view position manually. Neither will stay in place unless the pressure in the system is above the threshold. The photo to the left shows a manual wig wag which the operator swings to the right out of view when the air pressure is above the threshold where it will remain as long as the pressure is sufficient.
Most U.S. state commercial driver's license manuals, published by the states’ Departments of Motor Vehicles or equivalents, describe this term.