Bordon tube: Closed tube bent into a coil. The tube is filled with a liquid. When heated the pressure inside goes up and the tube tries to get straight. When it cools, pressure goes down, tube re coils. Simple. One end sits on the engine, other end is the gauge. Works until it don't. Why does it stop? Leak, crimp, and stuck. They last a long time but are not readily repaired. No voltage required. I just replace when they quit. Some one with more time and patience might try to fix or repair. But I don't see any advantage when there are replacements available. I hope this helps some.
