What does the trailer air supply control look like? Question What does the trailer air supply control look like? Answers It is a square blue knob. It is a triangular yellow knob. It is an eight-sided red knob. It is a circular orange knob. Explanation On newer vehicles, the trailer air supply control is an eight-sided red knob. It's used to control the tractor protection valve. This questions appears in the following tests: Arkansas Double Colorado Combination District Of Columbia Tank Florida Combination Illinois Combination Massachusetts Combination Massachusetts Combination Mississippi Combination Montana Combination Nebraska Combination Nevada Double New Jersey Combination New York Tank New York Combination Pennsylvania Tank Pennsylvania Combination Tennessee Combination Texas Double Utah Combination Wisconsin Combination
