PLC Repair Case Study

Contact Us

PLC repair case study 1Late last year SAC was contacted by RT Technical regarding a PLC issue they were attempting to address at the Brookeland Fresh Water Supply District. In the pictures provided, the SAC engineering staff was able to determine the line of products currently installed was obsolete.

These parts included Moeller, now owned by EATON, PLC MFD CP8-ME, Display MFD-80-B, Relay module RA-17, Serial Interface Easy200, and Remote Relay Unit 618. With this information, SAC knew it was going to be difficult to find and replace defective items.

Download a PDF Version of this Case Study

During our first site visit, SAC was introduced to the treatment facility’s process and the key external sensors/controls feeding into or out of the PLC. The sensors and controls included an ultrasonic wet well level sensor, on/off for wet well pumps 1 and 2, pump fail sense, loss of prime sense, an equalizer (EQ) tank level sensor, EQ tank fill valve control, EQ tank drain valve control and EQ tank aerator control.

The complaint by the customer was the PLC would not enter RUN to activate the embedded program to allow the expected control functions. Part numbers of other devices associated with the PLC were recorded and photographed for further investigation.

As we powered on the system the LCD did not display the expected screens of Wet Well Level and EQ Tank Level. It appeared the program was not running, so we quickly determined the end-user had replaced the PLC and LCD, but the technician had not programmed the PLC, and there was no obvious way to extract the program from the old PLC and update the new.

At this point, it was decided SAC would take the damaged PLC and Display back to our facility to search for a way to extract the program and load it on the new PLC.

PLC repair case study 2Upon inspection, it was seen the old PLC and Display were so severely damaged there was no easy way to connect them together. Normally the two devices connect at mating 24 pin surface mount connectors, but enough force was used, in an attempt to remove the Display, to pull the PLC’s 24 pin connector and gold pins off the board, when the release lock was not properly activated.

The SAC technical staff used its surface mount skills to carefully scrape away solder mask from usable traces formally connected to the 24 pin connector and soldered in place 24 small gauge wires ultimately allowing the PLC and Display to be hard-wired together and energized.

With the help of discovered online manuals, the keypad combinations were determined and the embedded ladder logic code was revealed. The code was extracted line by line and hand programmed into the new device. However, the Display screens and button control definitions were not contained in the ladder logic, and the only way to gain access to this code is with a special EATON development software and interface cable.

Both of these items are obsolete, but EATON said they could provide the software if SAC could find a cable. The cable was finally located in Germany, purchased and EATON provided the full development version of the software.

Download a PDF Version of this Case Study

While waiting on the cable, SAC returned to Brookeland to test the old PLC and Display to verify the water treatment system would work. However, the depth signals from the Wet Well and EQ Tank were not present on the LCD, so we started troubleshooting these signal loops.

We never saw a good reading at the Wet Well level sensor’s LCD, so we determined the problem could be this sensor or the 4 to 20mA converters adjacent to the PLC. A third working 4 to 20mA converter could have been used in place of one of the suspect devices, but we decided it was too risky to do this test and possibly damage the only good converter.

It was decided the end-user would order two new converters, and SAC would take the Wet Well level sensor back to our lab for testing. The Wet Well sensor worked fine, so this confirmed our analysis, the two converters were bad.

When the interface programming cable arrived it was finally possible to fully extract the program from the old PLC and transfer it to the new. The EATON software allowed SAC to adjust limits and run simulations to test for correct outcomes, and comments were added to the program to make it easier for the next PLC programmer.

The simulations gave us a high degree of confidence the PLC would work properly once installed, but we did not have all the external items i.e. RA-17 Relay Module to fully test the PLC, so another trip was scheduled to Brookeland for installation of the two 4 to 20mA converters and PLC in-system testing.

SAC and RT Technical returned to Brookeland to install the new converters and new PLC. While the Wet Well level sensor worked and the correct voltage was presented to the RA-17 Relay module, there was no indication on the PLC’s LCD module meaning the RA-17 was defective.

There was no replacement module, so we had to wait for the end-user to purchase a refurbished module. Over two months elapsed but the RA-17 finally arrived in Brookeland on 1/4/22, so SAC made a trip to the site to retrieve the module for lab testing.

An acceptance test was created and all voltages simulated which proved every module functioned properly, so the equipment was now ready for full installation. SAC completed the assignment by traveling back to Brookeland to install all equipment, updating critical wiring with more reliable ferrule terminations, and verifying every available input and output signal was correct.

It was a pleasure working with RT Technical and the end-user on this project, and we feel confident we added value to the customer’s operation.

Download a PDF Version of this Case Study


   Southern Avionics Company

P.O. Box 5345
Beaumont, TX 77726


Direct: 409-842-1717
Fax: 409-842-2987


Sales Inquiries: 
Technical Inquiries:


Direct: 409-842-1717
Fax: 409-842-2987


Sales Inquries: 

Technical Inquries: