The terminator, as Kyle Reese explained in the first movie, is an infiltration unit. It is an armored combat chassis covered by living tissue. As such it has sweat, bad breath, everything to pass as a human. Yet it cannot produce tears. The terminator in the second movie, who serves as a protector for John Connor, says he now understands why John cries but that its something he will never be able to do. If a terminator can produce sweat, bad breath, and pass as a human it would make sense it should be able to produce tears too but it can't, something I just thought about.