Troubleshooting

PLC Output Is On but the Cylinder Does Not Move: Solenoid Valve Checks

The output LED is lit and the program says extend, but the cylinder sits still. Split the chain into electrical, pneumatic and mechanical and you will find it fast.

· 8 min read · By TryPLC Team

  • Pneumatics
  • Solenoid Valve
  • Digital Output
  • Troubleshooting

The symptom

The PLC output for Clamp_Extend is on — the LED is lit and the tag is TRUE — but the cylinder does not move. No alarm, because nobody wrote one.

The chain is: output → wire → coil → valve spool → air → cylinder → load. Find the first link that does not do its job.

Safety: pneumatic actuators can move suddenly when you restore air or operate a manual override. Keep hands out of the motion path and follow the machine's lockout procedure.

Step 1 — Is there voltage at the coil?

Measure at the valve connector, not at the PLC terminal. Expect 24 VDC. Many valve connectors have their own LED.

  • 24 V at the PLC, 0 V at the valve: broken wire, loose terminal, blown fuse or an open safety relay contact in series.
  • Low voltage (for example 15 V): undersized supply or a bad connection under load.

    • Also check the output is a transistor output with enough current for the coil, and that a relay output's contact has not welded or burnt.

    Step 2 — Is the coil OK?

    Disconnect and measure coil resistance. Open circuit (∞) means a burnt coil. Hold a screwdriver near the solenoid when energised: a working coil pulls it magnetically.

    Step 3 — Manual override

    Press the valve's manual override. If the cylinder moves, air and the cylinder are fine — the problem is electrical or the coil. If it still does not move, the problem is pneumatic or mechanical.

    Step 4 — Air supply

    Check the pressure gauge at the maintenance unit: typically 5–6 bar. Pilot-operated valves need a minimum pilot pressure (often 1.5–2 bar) to shift. A closed isolation valve, empty soft-start valve or blocked filter all stop motion.

    Step 5 — Valve and flow controls

    A completely closed flow control (speed control) valve stops the cylinder dead. Contamination can stick a spool. On a valve island, confirm the coil address matches — swapping two connectors is a common commissioning error.

    Step 6 — The cylinder and the load

    Disconnect the cylinder from the load. If it moves freely, the load is jammed or too heavy for the bore at that pressure. Worn piston seals let air pass through: the cylinder moves slowly or not at all, and you can hear air at the opposite port.

    Make the next fault self-diagnosing

    Fit end-position sensors and write a motion timeout: if Extend is commanded and Extended is not reached within, say, 2 s, raise an alarm that names the cylinder. That turns a mystery into a message.

    Read the hardware chapter on signals for output types and loads.

    Frequently asked questions

    Why does my cylinder not move when the output is on?

    Measure voltage at the valve coil, then try the manual override. If override works, the fault is electrical; if not, check air pressure, flow controls, the valve and the load.

    How do I detect a stuck cylinder in the PLC?

    Use end-position sensors and a motion timeout timer that raises an alarm when the commanded position is not reached in time.