Step-by-Step Procedure to Troubleshooting Solenoid Valves in PLC Digital Output Loops

Solenoid valves (SOVs) are important parts of automated control systems because they connect electronic control signals from Programmable Logic Controllers (PLCs) to the actual movement of gases or liquids in industrial operations. If a solenoid valve doesn’t work right, it can cause problems with the process, the quality of the product, and even safety risks. This detailed guide shows a step-by-step method for fixing problems with solenoid valves in PLC digital output loops. It uses the schematic that goes from the PLC to the final control element to show the whole process.

Understanding the Complete Digital Output Loop

The diagram shows the main parts of a typical solenoid valve control loop:

1. PLC Rack: This is where the controller and I/O modules are housed.
2. PLC DO (Digital Output) Module: Changes PLC signals into electrical outputs
3. Output Relay Module: allows you switch and isolate the outputs.
4. Fuse Module: Fuse Module protects individual circuits from too much current.
5. Marshalling Panel: organizes and sends field wire to the right places.
6. Local Junction Box: This is where connections are made between devices that are close to the field.
7. Solenoid Valve (SOV): changes electrical energy into mechanical action.
8. Power Supply: Gives the right voltage (110/230 VAC or 24V DC, as shown)
9. Pneumatic Parts: These include tubing, regulators, and vent plugs (sometimes called silencers).
10. Process Valve: The process valve is the Finalcontrol element that changes the flow of the process.

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Initial Observations Before Troubleshooting

Before you start any physical troubleshooting:

• Check to see if the valve is stuck in the open or closed position.
• Check the last command the PLC gave to the SOV.
• Look at HMI alarms or SCADA events to find out what’s going on. Make a note of if the problem is happening all the time or just sometimes.
• Write down the present state of all indicators and operational circumstances.
• See if the problem is with the electrical circuit (from the PLC to the SOV) or the pneumatic circuit (from the air supply to the valve actuator).

Tools you Need for Troubleshooting

To effectively troubleshoot solenoid valve issues, ensure you have:

• Digital multimeter (DMM)
• Loop calibrator or signal generator to verify the loop.
• Appropriate screwdrivers and tools
• Manufacturer’s specifications and documentation
• Loop diagrams and electrical schematics
• Communication interface for PLC (laptop with programming software)
• Spare components (fuses, terminal blocks, solenoid coils)
• Personal protective equipment (PPE)

How to Troubleshoot Solenoid Valves in PLC-Controlled Digital Output Loop

Step 1. Field-Level Troubleshooting: Solenoid Valve and Pneumatic Components

Check the SOV coil:

• Measure coil resistance with multimeter (de-energized):
  • Normal values: 200 to 2000 Ω (see datasheet)
  • Open circuit then coil is burnt
  • 0 Ω/very low resistance then Coil is shorted
• Check the voltage at the coil terminals when it is turned on:
  • No voltage then issue is upstream
  • Correct voltage, but no actuation then the faulty coil
• To distinguish between mechanical and electrical problems, give the SOV direct power.

Check mechanical movement:

• Remove vent plug and listen for the solenoid clicking
• Check to see if the pneumatic actuator reacts: If not look for a stuck actuator, a clogged pneumatic tube, or a broken pressure regulator.

Inspect the pneumatic circuit:

• Check the reading to make sure the air supply pressure is correct at the regulator.
• Make that the regulator is set to the right pressure for the valve.
• Look for leaks in the tubing and fittings that let air in.
• Check that the vent plug (silencer) isn’t obstructed.

Examine the process valve:

• Check to see if the valve stem can move freely.
• Make sure the valve isn’t stuck mechanically. Check for too much wear or damage to the valve parts.
• Make sure the valve is the right size for the job.

Step 2. Local Junction Box Inspection

Check terminal tightness:

• Intermittent malfunctions are caused by loose terminals.
• Check for water getting in or contaminants
• Check to see sure the grounding is correct.

Check polarity and continuity:

• Check the integrity of the cable with a continuity tester.
• Make sure that the +ve and -ve wires are not crossed or shorted.

Test electrical connections:

• Check the voltage at the terminals of the junction box
• Check if the voltage levels are what they should be (24V DC or 110/230V AC).

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Step 3. Marshalling Panel Checks

Inspect terminal strips:

• Check for broken wires, loose connections, or rust.
• Check that the wiring matches the loop diagrams and the termination schedules.
• Make sure that any loose screws or cables are tight.

Cross-reference tags:

• Make sure that the mapping between JB and I/O cards is proper.
• Check that the wires are numbered and connected correctly according to the loop diagrams.
• Wrong terminals lead to wrong DO assignments.
• Check for continuity along the whole length of the wire
• Look for dips in voltage across long cable lines.
• Look for things that could cause electrical interference close to signal cables.

Step 4. Fuse Module Troubleshooting

Visual inspection:

• Check for blown fuse lights (LEDs or color changes)
• Check to see if the fuses are rated correctly.

Test with multimeter:

• The fuse should not have any resistance.
• Check the voltage on both sides of the fuses, at the input and output.
• Check that the voltage is right on both sides of the fuses’ input and output.

If fuses keep blowing:

• Look into possible short circuits
• Look for wire or coils that are shorted.
• Check for damage to the insulation

Verify power supply stability:

• Check for changes in voltage when the device is on.
• Check to see if the power source can handle the demand.

Verify correct voltage levels throughout the circuit:

• 24V DC for control circuits with low voltage
• For high-voltage control circuits, use 110/230V AC.

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Step 5. Output Relay Module Diagnostics

Verify LED indications:

• Most relays have status LEDs that show when they are working.
• No LED means the relay isn’t energized. Check the PLC logic or DO module.

Check relay contact output:

• Check to see if there is voltage at the relay output when the PLC sends DO command
• No voltage then relay failure or not triggered
• Voltage present then problem is downstream

Test relay operation:

• When the relay works, listen for a clicking sound.
• Look for signs of wear or arcing on the contacts.
• Check to see if the relay is working with the command

Note: If you need to, switch the relay with one that you know works to find the problem.

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Step 6. PLC DO Module and Rack Inspection

Check DO channel health:

• Check the DO point with PLC programming software.
• Only with safety authorization, use the Force ON command to test actuation.

Observe module status LEDs:

• Check to see whether there is an issue at the channel or module level.
• Check to see if the output module is getting power.
• Check that the module is properly seated in the rack.

Measure voltage at the output terminals during an ON command:

• For DC solenoids: Usually 24VDC (±10%)
• For AC solenoids, the voltage is usually 110/230VAC (±10%).

Note: If a hardware problem is found, replace the module if needed.

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Step 7. PLC Logic Troubleshooting

Check the DO logic conditions:

• Is the DO coil (like O:1/3) really getting power?
• Are all of the interlocks met, such as permissives or trip conditions?

Look for overrides or forces:

• Someone might have caused the DO to forced OFF in the software. 
• Make sure that the HMI controls or shutdown logic aren’t stopping the output.

Check for scan time delays:

• Large scan cycles could slow down DO execution. This is rare, but it’s worth testing.

Review program structure:

• Check the logic conditions that should turn on the output.
• Look for any safety conditions or interlocks stopping activation
• Check any timer or counter problems that are influencing the cycling of the valve.
• Look over the program version history to see if anything has changed.

Note: Test forcing the output ON and OFF in the program (if safe to do so)

Step 8. Power Supply Verification

Check for stable voltage across the entire loop:

• Check the voltage at the SOV coil
• Look at the terminals on the output relay
• Check the voltage at the fuse terminals.
• Check the PLC DO terminals
• Look for fluctuations or undervoltage that may cause solenoid misbehavior

Verify proper voltage levels at all junction points:

• Check that the voltage drop across long cable lines is within acceptable norms.
• Make sure the polarity is right in DC circuits.

Measure incoming power to solenoid:

• Check that the voltage is proper with respect to ground and that the power source can handle the load.

Common Fault Scenarios and Solutions

Fault	Cause	Solution
SOV not actuating	Burnt coil	Replace solenoid
	Low or no supply voltage	Check power supply
	Faulty electrical connections	Check wiring integrity
DO energized but no voltage at SOV	Faulty relay	Replace relay
	Blown fuse	Replace fuse
	Break in wiring	Repair or replace wire
Fuse keeps blowing	Shorted coil or wiring	Replace SOV or check cabling
	Incorrect fuse rating	Install properly rated fuse
	Ground fault	Check for insulation damage
Valve moves slowly or erratically	Low air pressure	Check regulator setting
	Blocked vent plug (silencer)	Clean or replace silencer
	Air leaks	Find and repair leaks
	Worn valve components	Replace valve components
DO not energizing in PLC	Logic condition not met	Debug PLC logic
	Interlock preventing operation	Check permissives
	Forces or overrides active	Remove forces after testing
Voltage at SOV but no motion	Mechanical jam	Inspect and clean actuator
	Blocked air passages	Clear blockages
	Incorrect coil voltage	Install correct voltage coil
Intermittent operation	Loose connections	Tighten all terminals
	Voltage fluctuations	Stabilize power supply
	Environmental interference	Add shielding
Multiple valves malfunctioning	Common power issue	Check main power supply
	PLC rack fault	Check backplane and power
	Master air supply problem	Check main air pressure

Example Troubleshooting Tips for PLC DO Signal to SOV using Actual Loop Diagram

Based on illustrated above Loop Diagram – A1-5000-JY-102 Digital Output Loop, refer the below troubleshooting ,mnbvxzuy points

Key Components from the Loop Diagram

• Solenoid Valve (A1-5000-JY-102) – The final control element.
• Solenoid Valve Terminal Block – Where the solenoid coil is connected.
• Local Junction Box TB2 (5000-JT-00005-PDN) – For cable termination and isolation.
• Field Cable Wires (e.g., A1-5000-JY-102-TB2-1+, TB2-2-)
• Marshalling Cabinet Terminals (1511/1512) – Intermediate terminal block inside cabinet.
• 24V DC Power Supply – Common source for multiple DOs.
• 1756-OB16DK DO Module (Rack-1, Slot-6) – PLC output module with source-type logic.
• Earth/Ground Terminals (CE) – For safety and shielding.
  

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Step-by-Step Troubleshooting Using the DO Loop Diagram

Verify the PLC Output Status:

• Check the output status of Channel OUT-2 (pin 5) on the 1756-OB16DK DO Module in Rack-1, Slot-6.
• Ensure logic “1” (energized) is commanded from the PLC program.

Check Power Supply Availability:

• Confirm the presence of 24V DC at terminals +18 and 18A+.
• Ensure the power supply is healthy, properly grounded, and the voltage is within range (typically 23–25V DC).

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Module Terminal Voltage Test:

• Measure DC voltage between pin 6 (+DC) and pin 5 (OUT-2).
• If output is active, you should see voltage drop across the solenoid; otherwise, check if the output is open or shorted.

Wiring Continuity from PLC to Marshalling Cabinet:

• Trace wire A1-5000-JY-102-5 from the DO module to terminal 1511 in the marshalling panel 5000-JT-00005-PDN.
• Check for any loose, cut, or shorted wires between the PLC rack and this terminal.

Inspect Intermediate Terminations:

• Confirm wire continuity from terminal 1511 to TB2-1, and from 1512 to TB2-2.
• Ensure proper termination at Local Junction Box TB2.

Check Local Junction Box to Field Cable:

• Verify signals from TB2-1 (+ve) and TB2-2 (-ve) are correctly connected to the solenoid valve.

Inspect Field Device (Solenoid Valve):

• At solenoid valve terminal, measure voltage across + and –.
• If 24V is present but valve doesn’t actuate, solenoid coil may be damaged.
• If no voltage is present, trace back wiring or output signal issue.

Check for Ground/Earth Faults:

• Confirm proper earthing at all grounding points labeled CE.
• Ensure no wire is inadvertently grounded or shorted to earth.

Simulate Output Manually (if safe):

• Disconnect output from DO module and simulate 24V DC manually at solenoid terminals to check valve actuation.
• If valve operates, fault is in control loop upstream (PLC/DO/wiring).

Check Module Fuse or Internal Protection:

• Ensure the 1756-OB16DK module is not internally fused or latched due to overcurrent.
• Replace or reset if necessary.

Loop Integrity Check:

• Perform a full loop resistance check from DO module output to solenoid coil.
• Compare with healthy reference loop resistance if available.

Document Findings and Restore System:

• Once fault is found and corrected, verify system operation.
• Log actions taken and validate that the PLC output now energizes the solenoid correctly.

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