JoeGuardian Automation

6. Troubleshooting One Device That Does Not Work

---

How to Diagnose a Single Failed Load in an Electrical Control Panel

When troubleshooting an electrical control panel, not every problem means the whole system is down. Sometimes the panel has power, the PLC is running, the HMI is working, and most devices operate normally — but one device does not work.

Examples:

One pilot light does not turn on.
One relay does not energize.
One solenoid valve does not actuate.
One contactor coil does not pull in.
One sensor input does not change state.
One PLC output device does not energize.

This type of problem requires a focused troubleshooting method.

When only one device fails, do not start at the main disconnect or replace random parts. Instead, focus on the specific branch circuit for that device.

Control circuits are generally designed so that an individual circuit controls one function of a machine or process, such as starting a motor, stopping a motor, controlling a solenoid valve, or responding to a limit switch or proximity switch.

---

1. First Question: Is It One Device or a System Problem?

Before testing, determine the scope of the failure.

Ask:

Is only this device not working?
Are similar devices working?
Is control power available?
Is the PLC running?
Are other outputs energizing?
Are other lights, relays, or solenoids working?

This matters because the troubleshooting path changes depending on the answer.

Condition	Troubleshooting Direction
Nothing works	Start at incoming power or control power
One section is down	Check common power for that section
One device is down	Troubleshoot that device branch circuit
PLC output LED is ON but device is OFF	Check output power, field wiring, and load
Device works manually but not automatically	Check logic, permissives, commands, and interlocks

A strong technician always defines the size of the problem first.

---

2. Use the Branch Circuit Method

When one device fails, isolate the branch circuit.

A branch circuit is the part of the control circuit dedicated to that device.

Example:

L1 → Fuse → Relay Contact → Pilot Light → Neutral

Or for a solenoid:

+24VDC → PLC Output → Terminal Block → Solenoid Coil → 0VDC

Or for a contactor coil:

L1 → Stop PB → Start PB → Overload Contact → M1 Coil → Neutral

The goal is to prove where the expected voltage or signal disappears inside that specific branch.

---

3. Start at the Failed Device

The best place to start is usually the device that is not working.

Example:

Pilot light does not turn on.

Start by checking the pilot light circuit.

Example:

Solenoid valve does not energize.

Start by checking voltage at the solenoid coil.

Example:

Contactor does not pull in.

Start by checking voltage across the contactor coil.

This follows the backtracking method:

Failed Device → Field Wiring → Terminal Block → Control Contact / PLC Output → Fuse / Source

The manual emphasizes using the circuit diagram as a guide and beginning troubleshooting from the point of failure, then working backward toward the source.

---

4. Check Voltage at the Device

The first major test is:

Is the device receiving the correct voltage when it is supposed to operate?

Examples:

Device	Expected Test
120 VAC pilot light	Measure across the lamp terminals
24 VDC solenoid	Measure +24VDC to 0VDC at the coil
Contactor coil	Measure across A1 and A2
Relay coil	Measure across coil terminals
PLC input sensor	Measure sensor output to 0VDC
PLC output load	Measure at output terminal and field device

Result interpretation:

Reading	Meaning
Correct voltage present	Device may be failed
No voltage present	Problem is upstream or return path is open
Low voltage present	Possible voltage drop, loose connection, overloaded supply
Voltage on one side only	Check neutral/common return
Voltage present at panel but not at field device	Field wiring or connector issue

This one test can immediately narrow the problem.

---

5. Do Not Forget the Return Path

A device needs a complete circuit.

For AC:

L1 → Load → Neutral

For DC:

+24VDC → Load → 0VDC

A common mistake is checking only the hot side.

Example:

You measure +24VDC at one side of a solenoid.
The solenoid does not energize.

That does not prove the circuit is complete.

You must also verify:

Is 0VDC common present?
Is the neutral connected?
Is the return wire broken?
Is the terminal loose?
Is the common fuse open?

A missing common or neutral can make one device fail even when power appears to be present.

---

6. Example 1: One Pilot Light Does Not Turn On

Symptom:

The left pilot light does not work.
The right pilot light works correctly.
The relay responds to the pushbuttons.

This tells us the whole panel is not dead. The problem is likely related to the failed pilot light branch.

Basic circuit:

L1 → Relay Contact → Pilot Light → Neutral

Troubleshooting steps:

Step 1 — Operate the circuit

Confirm that the relay or controlling device is actually changing state.

Does the relay energize?
Does another light respond?
Does the control circuit appear active?

If yes, continue with the failed light branch.

Step 2 — Measure across the pilot light

Expected: 120 VAC when the light should be ON

If correct voltage is present:

Suspect bad bulb, LED module, lamp holder, or loose lamp connection.

If voltage is missing:

Move backward in the circuit.

Step 3 — Measure before the pilot light

Measure hot side of the pilot light to neutral.
Expected: 120 VAC when commanded.

If hot side is present, check neutral.

If hot side is missing, move backward toward the relay contact.

Step 4 — Check before and after the relay contact

Before relay contact: should have source voltage.
After relay contact: should have voltage when relay is energized.

If voltage is present before the contact but not after it, the contact may not be closing.

The manual gives a similar example in Basic Controls Problem 2, where one light is not operating. The procedure checks whether the other light and relay respond, then measures voltage at the failed light and works backward toward the relay contact and wiring.

---

7. Example 2: One Solenoid Valve Does Not Energize

Symptom:

One pneumatic valve does not actuate.
Other valves on the machine work.
PLC output LED turns ON.

Possible causes:

Bad solenoid coil
Missing 24 VDC at valve
Open 0VDC common
Broken field cable
Bad connector
Blown output fuse
Bad PLC output channel
Loose terminal block
Wrong output command
Mechanical/pneumatic valve issue

Basic circuit:

PLC Output → Terminal Block → Field Cable → Solenoid Coil → 0VDC Common

Troubleshooting steps:

Step 1 — Check PLC output status

Is the output LED ON?
Is the PLC logic commanding the output?

If the output is not commanded, the issue may be logic-related.

If the output is commanded, continue with electrical testing.

Step 2 — Measure voltage at the solenoid coil

Expected: approximately 24 VDC when commanded.

If 24 VDC is present and the valve does not actuate:

Possible bad solenoid coil or valve mechanical issue.

If 24 VDC is missing:

Move backward toward terminal block and PLC output.

Step 3 — Measure at the terminal block

Panel side: Is 24 VDC leaving the panel?
Field side: Is 24 VDC going out to the cable?

If voltage exists at the panel but not at the valve, suspect field wiring, connector, or junction box.

Step 4 — Check the common

Verify 0VDC common at the solenoid.

If +24VDC is present but 0VDC is missing, the solenoid will not energize.

---

8. Example 3: One Contactor Does Not Pull In

Symptom:

Motor M1 does not start.
Other motors work.
Control power is available.
M1 contactor does not pull in.

Basic control circuit:

L1 → Stop PB NC → Start PB NO → Overload NC → M1 Coil → Neutral

Troubleshooting steps:

Step 1 — Measure across the contactor coil

Expected: coil-rated voltage when Start is pressed or command is active.

If voltage is present:

Possible failed coil, wrong coil voltage, loose coil terminal, or mechanical contactor issue.

If voltage is not present:

Move backward through the overload, start command, stop circuit, fuse, and source.

Step 2 — Check overload auxiliary contact

Is the overload tripped?
Is the NC overload contact closed?
Is voltage present after the overload contact?

Step 3 — Check the start command

Does the Start PB close?
Does the PLC output or relay contact close?
Is the command reaching the coil circuit?

Step 4 — Check neutral

Is A2 connected to neutral or common?
Is the return path open?

A contactor coil cannot energize without both voltage supply and return path.

---

9. Example 4: PLC Input Does Not Turn ON

Sometimes the failed “device” is not an output. It may be an input signal.

Symptom:

A photo eye detects a box, but the PLC input does not turn ON.

Possible causes:

Sensor not powered
Sensor misaligned
Wrong sensor type
Broken wire
Bad connector
Input common missing
Wrong input terminal
PLC input card issue
Sensor output failed

Troubleshooting steps:

Step 1 — Check sensor power

Measure +24VDC and 0VDC at the sensor.

Step 2 — Check sensor output

Measure sensor output to 0 VDC while actuating the sensor.

Expected:

Output should change state when the target is present.

Step 3 — Check signal at panel terminal

Does the signal arrive at the terminal block?

Step 4 — Check PLC input LED

Does the input LED turn ON?

If the signal reaches the PLC terminal but the input LED does not change, suspect input wiring, common, configuration, or input module issue.

---

10. Device Receives Voltage but Still Does Not Work

If the correct voltage is present across the device and it still does not work, now the device becomes suspicious.

Possible examples:

Device	Possible Fault
Pilot light	Burned lamp, failed LED module, bad holder
Relay	Open coil, stuck mechanism
Contactor	Failed coil, mechanical jam, wrong coil voltage
Solenoid	Open coil, stuck valve, pneumatic issue
Sensor	Failed electronics, misalignment, wrong mode
Motor	Open winding, overload issue, mechanical jam

But do not stop at “bad device.”

Ask:

Why did this device fail?

Possible root causes:

Overvoltage
Water intrusion
Heat
Vibration
Wrong voltage rating
Loose terminal
Shorted field cable
Excessive load
Contamination
Mechanical binding

The manual warns that replacing the failed component may be only a quick fix if the root cause is not identified.

---

11. Device Does Not Receive Voltage

If the device does not receive voltage, the device may not be the problem.

Now move backward.

Typical path:

Device → Field Wiring → Terminal Block → Fuse → Contact / Output → Source

Look for:

Open relay contact
Blown fuse
Loose terminal
Broken wire
Bad connector
Failed PLC output
Missing output common
Open permissive
Open overload contact
Safety circuit not reset

The key question is:

Where is voltage present, and where does it disappear?

That point tells you where the fault is located.

---

12. Troubleshooting Table

Symptom	First Test	If Voltage Is Present	If Voltage Is Missing
Pilot light OFF	Measure across lamp	Check lamp/holder	Check relay contact/wiring
Solenoid OFF	Measure at solenoid coil	Check coil/valve	Check terminal/output/common
Contactor not pulling in	Measure across coil	Check coil/contactor	Check overload/start/stop/source
PLC input not ON	Measure sensor output	Check PLC input/common	Check sensor power/output
Relay not energizing	Measure across coil	Check relay coil	Check command/contact/source
One output dead	Measure output terminal	Check field wiring/load	Check output fuse/common/module

---

13. Practical Technician Report Example

Symptom:
Solenoid valve SV-204 did not energize during automatic cycle.

Expected Operation:
PLC output O:2/4 should energize SV-204 with 24 VDC when the discharge command is active.

Observation:
Other solenoids operated normally. PLC was in Run mode. Output LED O:2/4 turned ON when commanded.

Testing:
Measured 0 VDC at SV-204 coil.
Measured 24 VDC at panel terminal TB3-18.
Measured 0 VDC at field connector near valve.
Verified cable continuity after LOTO.

Finding:
Open conductor in field cable between panel and solenoid connector.

Correction:
Replaced damaged cable and secured it away from machine movement.

Root Cause:
Cable jacket was rubbing against a moving bracket, eventually opening the conductor.

Final Verification:
SV-204 energized correctly in manual mode and automatic cycle.

This type of documentation proves the diagnosis and captures the root cause.

---

14. Final Checklist

[ ] Confirm only one device is affected.
[ ] Identify the exact device or signal that failed.
[ ] Review the electrical diagram.
[ ] Identify the device voltage and return path.
[ ] Measure voltage at the failed device.
[ ] Check both source side and return/common side.
[ ] If voltage is present, inspect the device.
[ ] If voltage is missing, backtrack through the branch circuit.
[ ] Check terminal blocks and connectors.
[ ] Check fuses, contacts, PLC output, or relay output.
[ ] Verify the command or logic is actually active.
[ ] Repair safely using LOTO when required.
[ ] Verify operation after repair.
[ ] Identify and correct the root cause.
[ ] Document findings.

---

Final Thoughts

When one device does not work, troubleshoot the branch — not the whole panel.

The rest of the system gives you important information. If other devices are working, the main source is probably good. That allows you to focus on the failed device, its wiring, its control contact, its output channel, its fuse, and its return path.

The professional method is simple:

Confirm the symptom.
Start at the failed device.
Measure expected voltage.
Check the return path.
Backtrack one step at a time.
Find where the signal disappears.
Repair safely.
Verify.
Find root cause.

A good technician does not guess.
A good technician follows the circuit.

In the next post, we will cover:

Motor Starter Troubleshooting Basics

We will look at contactors, overloads, coils, auxiliary contacts, power circuits, control circuits, and how to diagnose why a motor starter does not operate correctly.