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

The symptom

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

The signal and the air pass along a chain: output → wire → coil → valve spool → air → cylinder → load. Your job is to find the first link that does not do its part.

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

Step 1: measure the voltage at the coil

Measure at the valve connector, not at the PLC terminal. You should see 24 VDC, and many valve connectors have their own LED that shows it.

If you have 24 V at the PLC and 0 V at the valve, look for a broken wire, a loose terminal, a blown fuse or an open safety relay contact in series. A low reading, for example 15 V, points to an undersized supply or a bad connection that only shows up under load.

Also check that a transistor output can supply enough current for the coil, and that the contact on a relay output has not welded or burnt.

Step 2: test the coil

Disconnect it and measure the coil resistance. An open circuit (∞) means the coil has burnt out. A quicker test with the coil connected: hold a screwdriver near the solenoid while it is energised. A working coil pulls it magnetically.

Step 3: use the manual override

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

Step 4: check the air supply

Read the pressure gauge at the maintenance unit; it is typically 5–6 bar. Pilot-operated valves need a minimum pilot pressure to shift, often 1.5–2 bar. A closed isolation valve, an empty soft-start valve or a blocked filter will each stop the motion.

Step 5: check the valve and the flow controls

A flow control (speed control) valve that is closed completely stops the cylinder dead. Contamination can also make a spool stick. On a valve island, confirm the coil address matches the output; two connectors swapped over is a common commissioning error.

Step 6: check the cylinder and the load

Disconnect the cylinder from the load. If it then moves freely, the load is jammed, or it is too heavy for the bore at that pressure. Worn piston seals let air pass through the piston, so 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. The next person gets a message on the HMI instead of a mystery.

For output types and the loads they can drive, read the hardware chapter on signals.