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Technical Resources & Knowledge Base
Is it the supply, the input power, or a short downstream?
Step 1: Visual & Smell Check
Smell: A strong "burnt electronics" smell usually indicates a blown capacitor or MOS-FET. The unit is likely dead.
Visual: Look for charred marks, bulging capacitors, or a blown internal fuse (if visible).
LED Status: Is the "DC OK" or "Power" LED on? If it's off or blinking, proceed to testing.
Step 2: Check Input Voltage (Source)
Set your multimeter to AC Volts.
Measure across the input terminals (L and N, or L1/L2/L3).
Result: You should see your standard facility voltage (e.g., 120VAC, 240VAC, 480VAC).
If 0V: The problem is upstream (breaker, fuse, or wiring to the panel).
Step 3: Check Output Voltage (Load Disconnected)
Crucial Step: Turn off power, disconnect the output wires (+V and -V), and then turn power back on. You must test it "unloaded" to rule out a short circuit in your machine dragging the voltage down.
Set multimeter to DC Volts. Measure across the output terminals.
Result: You should see a steady voltage (e.g., 24VDC).
If voltage is bouncing or 0V: The power supply is dead.
If voltage is steady: The power supply is good; you have a short circuit in a sensor, valve, or wire downstream.
Is the "brain" dead or just confused?
Step 1: Check Status LEDs
Power LED: If off, check the power supply feeding the PLC (see above).
RUN Light: Should be solid Green. If flashing, it may be in program mode or stopped.
FAULT / SF (System Fault) Light:
Flashing Red: Often a recoverable error (battery low, I/O mismatch, memory card missing). Try power cycling.
Solid Red: Usually a fatal hardware error or "watchdog timer" fault. The CPU may be dead.
Step 2: Isolate the CPU
If the power supply is good but the PLC won't power up (or stays in fault), power down and remove all I/O modules.
Power up with just the Power Supply and CPU.
If it boots up: One of your I/O cards is faulty and dragging down the backplane. Add them back one by one to find the bad one.
If it still fails: The CPU or rack/chassis is faulty.
Step 3: Communication Basics
Ping Test: Can you "ping" the PLC IP address from a laptop?
Port Check: Look at the Ethernet port. Are the Link/Activity lights blinking? No lights usually means a bad cable, dead switch port, or dead PLC port.
Is it the drive, the motor, or the mechanical load?
Step 1: Check the Display/Fault Codes
Most drives have a display. A blank display (with good input power) usually means the internal power supply is blown.
Common Codes:
Overcurrent: Often a short in the motor or cable.
Overvoltage: Often a regeneration issue (braking too fast) or input power spike.
Feedback Error: The cable connecting the motor encoder to the drive is loose or broken.
Step 2: The "Disconnect" Test (For Overcurrent Faults)
If the drive trips immediately upon enable:
Power down and disconnect the motor cables (U, V, W) from the drive.
Power up and try to enable the drive.
If it still trips: The drive is blown (shorted internal IGBTs).
If it does NOT trip: The drive is likely fine; the problem is in the motor or the power cables running to the motor.
Step 3: Mechanical Binding Check
With power off, try to turn the motor shaft by hand (if it doesn't have a brake).
If it won't turn, your machine is mechanically jammed. The servo drive is just doing its job by tripping out to save the motor.
Why isn't the machine seeing the part?
Step 1: The "Wrench Test" (Inductive Proximity Sensors)
Hold a metal object (wrench/screwdriver) in front of the sensor face.
Does the light on the sensor body turn on?
No Light: Check the cable for breaks. If 24V is present at the connector but no light, the sensor is dead.
Light turns on but PLC doesn't see it: The sensor is switching, but the signal isn't reaching the PLC. The break is in the wire running back to the panel, or the PLC input card is dead.
Step 2: Cleanliness Check (Photoeyes)
90% of photoelectric sensor failures are just dirty lenses. Wipe them off with a clean cloth.
Check alignment: Is the reflector bent or knocked out of position?
Is the screen broken or is the system frozen?
Step 1: The Flashlight Test (Black Screen)
If the screen is black but the power light is on: Shine a flashlight at an angle against the glass.
If you can faintly see the graphics: The Backlight has failed. The HMI brain is fine, but it needs a repair (bulb/inverter replacement).
If it is pitch black: The LCD or internal power board has failed.
Step 2: Touch Responsiveness
If the touch is off-center, look for a "Calibration" menu in the system settings (usually accessed by holding a corner of the screen during boot-up).
If one specific area never works (dead spot), the Digitizer (glass touch overlay) is physically damaged and needs replacement.
Is the fuse actually blown, or is it something else?
Step 1: Visual Inspection
Glass fuses: Look for a broken filament or black scorch marks inside.
Ceramic/Industrial fuses: You usually can't see inside. Do not rely on visual checks alone.
Step 2: Continuity Test (Power OFF)
Remove the fuse from the holder (crucial to avoid false readings).
Set multimeter to Continuity/Ohms (Ω).
Touch probes to ends of the fuse.
Result: It should beep or show near 0.0 Ohms.
If it shows OL (Open Line) or no beep: The fuse is blown.
Step 3: Voltage Drop Test (Power ON - Advanced)
If you cannot remove the fuse, measure Voltage across the fuse terminals while the circuit is live.
Result: You should read 0V across a good fuse (electricity flows through it, no potential difference).
If you read full voltage (e.g., 24V or 120V) across the fuse: The fuse is blown (it is acting as an open switch).
Why isn't the air/fluid turning on?
Step 1: The Magnetic Test
With the solenoid powered ON, hold a steel screwdriver tip near the top of the coil stem.
Result: You should feel a magnetic pull.
No pull: The coil is not energized (check wiring) or the coil is burnt out (open circuit).
Step 2: Resistance Check
Turn power off and disconnect the plug/wires from the coil.
Measure Resistance (Ohms) across the coil pins.
Result: You should see a specific resistance value (check manufacturer specs, usually 20-100 Ohms).
If OL (Open): The internal winding is broken. Replace the coil.
If 0.0 Ohms: Short circuit. This likely blew your fuse or PLC output card.
The PLC light is on, but the motor starter isn't engaging.
Step 1: The "Click" Test
Toggle the input signal. Can you hear or feel the relay "click"?
No Click: The coil isn't getting power or is burnt out.
Click heard, but nothing happens: The contacts are likely pitted, carbon-covered, or welded.
Step 2: Inspect the Contacts
Look through the clear plastic case. Are the contacts black or melted?
If they look okay, perform a continuity test across the Normally Open (NO) contacts while manually pushing the actuator button (if equipped).
The motor stopped and the drive is dark.
Step 1: Check Incoming Power
Measure Phase-to-Phase voltage at the drive input terminals (L1, L2, L3).
Result: Should be balanced (e.g., 480V across all legs). If one leg is dead (0V), you have a blown main fuse or tripped breaker upstream ("single-phasing").
Step 2: Check the DC Bus
If the display is blank but input AC is good, the internal DC power supply might be dead.
Measure DC Voltage at the DC Bus terminals (+ and -).
Result: Should be roughly 1.41 x AC Input Voltage (e.g., ~678VDC for a 480VAC input).
If 0V or very low: The drive's pre-charge circuit or rectifier is blown.
Why is my temperature reading "9999" or "Error"?
Step 1: Check for "Open Wire"
Most controllers display a high-limit error if the sensor breaks.
Disconnect the thermocouple from the controller.
Measure Resistance (Ohms) across the two wires.
Result: Should be low resistance (a few Ohms).
If OL (Open Line): The sensor tip is broken internally. Replace the probe.
Step 2: The "Jumper" Test (Controller Check)
If the sensor tests fine, jumper the two input terminals on the temperature controller with a piece of wire.
Result: The display should read "Ambient Temperature" (room temp).
If it still says Error: The controller input card is faulty.
Is it the cable or the port?
Step 1: Continuity & Shorts
For Serial (DB9) or simple cables, use a multimeter to check continuity from pin to pin.
Also check for shorts between pins (e.g., Pin 2 touching Pin 3).
Step 2: The "Wiggle" Test
With the system running, gently wiggle the connector at the device end.
If communication drops out or lights flicker, the solder joints on the device's port or the crimp on the cable are bad.
The machine won't reset.
Step 1: Cycle the E-Stops
Push and pull every E-Stop button on the line. Sometimes a contact block comes loose on the back of the button.
Step 2: Check Contact Blocks
Safety circuits are "Normally Closed" (NC). They open when you hit the button.
With power off, measure continuity across the contact block on the back of the button.
Result: With the button out (run position), you should have continuity.
If Open (OL): The contact block is broken mechanically.
The coil pulls in, but the motor doesn't run.
Step 1: Check Voltage Through the Contacts
With the contactor engaged (pulled in), measure voltage across the top and bottom of each pole (L1 to T1).
Result: Should be near 0V (indicating a closed switch).
If you measure full voltage (e.g. 480V): The contact is burnt out inside and not conducting electricity.
Step 2: Smell Check
A distinct "burnt plastic" smell usually means the coil has overheated and melted the casing, preventing movement.
The sensor light is on, but the PLC doesn't see it.
Step 1: Identify PNP vs NPN
PNP (Sourcing): Sends +24V to the PLC when active.
NPN (Sinking): Sends 0V (Ground) to the PLC when active.
Step 2: Measure at the PLC Card
Place your black meter probe on 0V (Common). Place red probe on the PLC input screw terminal.
Trigger the sensor.
Result (PNP): Meter should jump from 0V to 24V.
If 0V stays 0V: There is a break in the wire between the sensor and the PLC.
Is it tripped or failed?
Step 1: Check the Handle Position
ON: Up/Right.
OFF: Down/Left.
TRIPPED: Floating in the middle / loose feel.
To Reset: You must firmly push it fully to OFF before you can push it to ON.
Step 2: Voltage Drop Check
With breaker ON, measure voltage from the Line side (Top) to the Load side (Bottom) of the same pole.
Result: Should be 0V.
If you measure voltage (e.g. 120V): The internal contacts are burnt open, even if the handle says "ON." Replace the breaker.