TM 10-3930-669-20 1-14. ELECTRICAL SYSTEM (CONT). f. Instruments. All instruments are equipped with individual lamps for illumination and share a common circuit for power of these lamps.  A common ground circuit is also shared for all instrument illumination.  All instrumentation is fully operable when the main power switch and engine switch are on. • Ammeter (1) receives voltage from the shunt and shares a common ground circuit with other instruments. • Hour meter (2) receives voltage from the fuse panel and shares a common ground circuit with other instruments. • Fuel level gauge (3) receives voltage from the fuse panel and is grounded at the sending unit mounted on the fuel tank. • Engine oil pressure gauge (4) receives voltage from the fuse panel and is grounded at the sending unit mounted on the engine. • Engine temperature gauge (5) receives voltage from the fuse panel and is grounded at the sending unit mounted on the engine. • Transmission oil temperature gauge (6) receives voltage from the fuse panel and is grounded at the sending unit mounted on the transmission. • Air  restriction  indicator  gauge  (7)  indicates  condition  of  air  filter  in  inches  of  Hg.    Red  button  is  for  resetting indicator. 1-16