I read your post if you are getting 12volts at the s post at all times the starter would crank as soon as the key is turned on this should not be. How this should be wired there should be a wire at the back of the key switch marked start should be center post there should be 12 volts there only in the crank position.after that if c500 Clark on trans valve body is natural safety switch wire from keyswitch goes there than on other side of natural safety switch goes to s post on starter when only in neutral will truck start. Check wiring also the r post should have no bearing on the starter cranking it is sometime used to get voltage to coil when cranking