Umbrella seal A hard plastic shield that’s slipped over a valve stem to form an umbrella-like shield over a valve guide. Underinflation A term used to describe a tire with too little tire pressure (less than minimum allowable pressure).
© PENN FOSTER, INC. 2016 PAGE 126 AUTOMOTIVE REPAIR TECHNICIAN Glossary Undersquared engine This term is used to describe an engine in which the cylinder bore diameter is smaller than the stroke dimension. Understeer The difference between where a wheel is actually steered and the direction it takes. Understeer occurs when the front end moves less than desired in relation to the actual turning position of the front tires. Term used to describe the handling of a vehicle where the driver must turn the steering wheel more and more while turning a corner. UNF Unified national fine. Unit-body A type of vehicle construction, first used by the Budd Company of Troy, Michigan, that does not use a separate frame. The body is built strongly enough to support the engine and the power train as well as the suspension and steering system. The outside body panels are part of the structure. Also see space-frame construction. Units of measurement A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Universal asynchronous receive and transmit (UART) Electronic communications medium used between the ECM, off-board diagnostic equipment, and other control modules. UART is a data line that varies voltage between 0-5 volts at a fixed pulse width rate. Universal joint, or U-joint Consists of two yokes with a steel crosspiece joining them together. Unsprung weight The parts of a vehicle not supported by the suspension system. Examples of items that are typical unsprung weight include wheels, tires, and brakes. V-ref Reference voltage from the controller. V-type engine An engine whose cylinders are are arranged in two separate rows, with an equal number of cylinders in each row. The two cylinder rows are placed together at an angle. V-type intake manifold A manifold used in V-type engines which fits between, and is attached to, the two cylinder heads. Vacuum Negative pressure (below atmospheric); measured in units of inches or cen- timeters of mercury (Hg). The low pressure within the engine is called vacuum. Vacuum causes the higher- pressure air on the outside to flow into the low-pressure area inside the cylinder. Vacuum bleeding A brake bleeding process that uses a special suction pump that attaches to the bleeder screw. The pump creates a low-pressure area at the bleeder screw, which allows atmospheric pressure to force braked fluid through the system when the bleeder screw is opened. It requires only one technician.
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