faulty plc

i run a stone processing factory in calabar nigeria,the machines are fully automated(using a plc system).one of the two limit switches that controls the haudraulic powered trolley is not indicating on the plc.we tried a new limit switch,yet the problem persist.meamwhile we can move the trolley manually by pressing the contactor that control it in the panel.when you press the control switch on the machine panel,its indicates on one of the pls cards but the contactor is not engaged to move the trolley.what may be the solution to this problem.

The wires connecting the limit switch to the PLC may be broken at some point. The limit switch may also have been wired to be an absolute cut off of the contactor. This would have been through normally closed contacts. Please inspect the cable run carefully.

Just a wild guess, but it sounds to me like a control circuit fuse may be open (blown). Have you taken voltage measurements at the appropriate places?

The light at the PLC responding to the push-button seems to say that control voltages TO the PLC are good. But from the PLC to the motor contactor you get no response, so that control voltage source is highly suspect.

Bill