TM9-2320-361-10 2-29. OPERATION OF TRACTOR AND FIFTH WHEEL (Contd) CAUTION Do not force electrical cable connector into trailer receptacle or damage to equipment may result. (16) Open caps (9) and (12) on electrical cable connector (10) and trailer receptacle (11), and install connector (10) in receptacle (11). (17) Lift unlock switch (7), move main switch (3) to SERVICE DRIVE position, release unlock switch (7), and check trailer running lights (13) for operation. (18) Depress service brake pedal (8), and check trailer, stoplights (14) for operation. (19) Turn main switch (3) to OFF position. d. Uncoupling Semitrailer. Do not back up trailer without a ground guide, or injury or death to personnel may result. 2-139 WARNING