Car Wash Troubleshooting Quick Reference #3 – PLC Control

Stop blaming the PLC. Follow the signal and find where it stops.

When car wash equipment fails to operate, the PLC or controller is often suspected first. But many apparent PLC problems are actually caused by missing inputs, field wiring, control voltage, relays, outputs, or the device being controlled.

This quick-reference flowchart provides a systematic method for tracing a control problem through the PLC system.

Use this guide to help diagnose:

• Missing PLC inputs

• Inputs that fail to change state

• PLC output problems

• Control-voltage problems

• Failed relays and interface circuits

• Wiring and connection problems

• Sensors and switches

• Missing commons

• Field-device failures

• Sequence-of-operation problems

Follow the control path:

COMMAND → INPUT → PLC LOGIC → OUTPUT → RELAY/INTERFACE → FIELD DEVICE → PHYSICAL RESULT

Use the PLC indicators as diagnostic tools. Determine what should happen, observe what actually happens, and find where the sequence stops.

Do not replace an expensive PLC until you have proven the PLC is actually the problem.

Designed as a fast field reference for car-wash technicians, owners, operators, and maintenance personnel.

Keep it on your phone or tablet, or print a copy for the equipment room.

Part of the Practical Car Wash Troubleshooting Series by Loyd Smith.