TM 10-3930-671-10 Table 2-1.  Controls and Indicators - CONT. Key Control/Indicator Function 20 Master Electrical Switch Used to control electrical current to all electrical components on truck and as emergency shutdown.  Switch has two positions: ON and OFF. 21 Air Restriction Indicator Indicates  restriction  of  air  intake  system.    When  yellow  flag  is  raised,    air intake filter should be serviced. 2-5