PLC failure. The cargo crane does not start. Troubleshooting

In previous articles we looked at various problems with cargo cranes, this time a PLC failure error occurred and cargo crane does not start.

What to do in such situation and how to restore the operation of the cargo crane will be discussed in this article.

The "PLC FAILURE ALARM" light is on

As usual, the first step is to refer to the Troubleshooting manual.

In this case, we are working with the automation of a PLC module, so troubleshooting will help us; there is no PLC circuit.

PLC failure troubleshooting

The instructions offer the following solutions to the problem:

• 1. If the "PLC FAILURE ALARM" light is on:
• 2. Check the POWER LED on the PLC CPU block. If ON, then go to the 5th point, if OFF, then go to the 3rd - 4th.
• 3. Check fuses.
• 4. Check the PLC supply voltage.
• 5. Check the ERROR LED on the PLC CPU block. If OFF, then go to the 6th point, if ON or ON/OFF, then go to the 7th point.
• 6. Check the operation of the relay 58K3. If the relay does not work, replace the relay.
• 7. Check the error history using a PC (unfortunately, there was no special computer on board).

• 1. If the "PLC FAILURE ALARM" light is flashing:
• 2 - 4. Check and replace the battery in the PLC CPU unit.
• 5. Replace the PLC CPU unit itself.

Relay 58K3 in the circuit

Relay 58K3 in the circuit has only one normally closed contact, which is responsible for the operation of the “PLC FAILURE ALARM” light on the control panel. Those. If the relay receives power, the light does not light, otherwise it does. In this case, the light was on, but the relay was not receiving power.

Solving the problem turned out to be easier and faster than described in the instructions, because the 5th step of replacing the PLC CPU unit with a new one was immediately completed. All LED lights on the unit blinked, indicating an internal module error.

PLC and CPU

The fact is that the inspection procedure requires increased attention, concentration and a lot of time, but when working in a ship environment during cargo operations, you have to make quick decisions. 

The only thing I was immediately convinced of was that the problem was not with the internal battery and it did not require replacement. Although such a problem should not affect the operation of the crane.

New PLC CPU block

The new PLC CPU unit was replaced and the cargo crane was put into operation.

Crane model: DMC DDKC-3026-00 (Dongnam Marine Crane)