Without LADDER III software, can I troubleshoot PMC on the CNC screen?

Yes. On the CNC screen go to SYSTEM, PMC, LAGDIAG (ladder diagnosis) to view signal on and off in real time, but you cannot do complex signal tracing there. For full tracing and cross-reference you still need LADDER III.

FANUC PMC Troubleshooting: 5 Real Cases Using Signal Tracing-CNCShare

Q: How do I quickly find the ladder network that triggers a given alarm?

Note the alarm number, find the corresponding A address, which is the alarm signal, in the PMC parameters, then search that A address in the ladder. That leads you to the logic network that raises the alarm.

Q: After modifying the ladder, do I need to restart the machine?

In most cases, after writing the ladder and selecting Run mode it takes effect without a restart. But when you modify timer or counter constants, a restart is recommended to ensure safety.

Master FANUC PMC troubleshooting with 5 real-world cases. Learn how to diagnose spindle not rotating, tool changer failure, E-stop issues, and more using LADDER III signal tracing. Step-by-step analysis for CNC maintenance engineers.

Key skills: Signal tracing, interlock analysis, timer/counter checks.

In previous articles we covered PMC principles, the address table, and ladder programming. But what really tests an engineer is: the machine alarmed — how do you quickly find the fault point? Many beginners face hundreds of pages of ladder and have no idea where to start, so they blindly swap parts. In fact, 90% of electrical faults can be located through PMC signal tracing — no parts swapping needed at all.

This article picks 5 of the most classic FANUC PMC fault cases (spindle, tool magazine, E-stop, lubrication, hydraulic) and teaches you the “four-step troubleshooting method” hands-on, so you can upgrade from “parts swapper” to “diagnostic expert.”

1. Core Methodology: The “Four-Step” PMC Troubleshooting Method

Before the cases, master this universal formula:

Define the phenomenon: clarify the fault表现 — e.g. “press start, no response” or “action stops halfway.”
Find the output: in the ladder, locate the output coil that controls that action (Y address or R address).
Trace the condition: from the coil, trace backward to the left; see which contact is open (shown red).
Check the external: based on the open contact’s address (X address), go to the field and inspect the corresponding sensor, button, or wiring.

Tool tip: Get proficient with LADDER III’s “Signal Trace” and “Cross Reference” functions — efficiency improves 10x.

Case 1: Spindle Forward (M03) Does Not Start

Fault phenomenon

In MDI mode, enter M03 S1000 and press cycle start. The spindle does not turn, the CNC shows no alarm, and the status shows “waiting for M-code completion.”

Troubleshooting steps

Step 1: Confirm the CNC output the M code — check system parameters or PMC signal F7.0–F7.3 (M-code decode signals). Found F7.0 = 1 (M01), F7.1 = 0 … meaning the CNC did correctly output the M03 signal.

Step 2: Check the PMC decode logic — open the ladder, search the M03 decode network. Found the decode output coil R10.0 is not conducting. Cause: a series interlock condition X4.5 (spindle coolant pressure low) is 0 (open).

Step 3: Field verification — check the spindle coolant unit; the pressure gauge shows insufficient pressure and the indicator light for X4.5’s sensor is off. Truth: insufficient coolant caused the pressure switch to stay open, so the PMC interlock forbids spindle start.

Solution

Add coolant to the standard level, the pressure switch closes, X4.5 = 1, and the spindle starts normally.

Case 2: Tool Magazine Stuck Mid-Change (ATC Alarm)

Fault phenomenon

Execute T05 M06 tool change; the magazine rotates halfway then suddenly stops, and the CNC alarms “ATC Time Over” (tool change timeout).

Troubleshooting steps

Step 1: Locate the change flow — find the magazine rotation output coil Y5.0 (magazine forward). Monitoring the ladder shows Y5.0 is output but the motor does not turn.

Step 2: Check the drive feedback — the VFD status shows “normal.” But PMC input X8.2 (magazine rotation-in-position) stays 0. Tracing further, X8.0 (magazine origin) signal flickers unstably.

Step 3: Field verification — inspect the magazine origin proximity switch; found the sensing distance is off and the surface has iron chips. Truth: the lost origin signal makes the PMC think the magazine never returned to the initial position, so it aborts the change and alarms timeout.

Solution

Clean the iron chips off the proximity switch, adjust the sensing distance to 2 mm, re-home, and the tool change returns to normal.

Case 3: E-Stop Cannot Be Released After Pressing

Fault phenomenon

After pressing the E-stop button and turning it clockwise to release, the CNC E-stop alarm cannot be cleared and the servo cannot power on.

Troubleshooting steps

Step 1: Check the E-stop input signal — view PMC input address X0.0 (E-stop). Normally, after releasing E-stop, X0.0 should be 1. Monitoring shows X0.0 stays 0.

Step 2: Trace the circuit — the E-stop loop is usually a series circuit. Measure across the E-stop button with a multimeter; found the circuit is still open even with the button released. Truth: the E-stop button’s internal contacts are oxidized or mechanically stuck, so the physical circuit does not conduct.

Step 3: Check other E-stop sources — some machines have multiple E-stop buttons (operator panel, handheld unit, cabinet door). Checking one by one reveals the handheld unit’s E-stop button was not fully popped out.

Solution

Fully pop out the handheld unit E-stop button; X0.0 becomes 1 and reset succeeds.

Case 4: Lubrication Pump Does Not Work (Timer Fault)

Fault phenomenon

Lubrication alarm, but the lubrication pump does not turn.

Troubleshooting steps

Check the ladder: the lubrication timer T10 is not counting. Further found the enable condition R20.0 (lubrication mode) was mistakenly set to 0. Check the PMC holding relay K address; found the parameter was accidentally modified. Reset the K address data and it returns to normal.

Case 5: Hydraulic Chuck Clamp/Release No Action

Fault phenomenon

Step on the foot switch; the chuck does not move and the pressure gauge does not change.

Troubleshooting steps

Monitor the PMC: output Y2.0 (clamp) has a signal, but the solenoid valve does not pull in. Measure Y2.0 output voltage = 0V. Truth: the PMC output module’s relay contact is burned — although the coil is energized, the contact does not conduct. Replace the PMC output relay module.

2. Expert Advice: How to Avoid Recurrence

Back up PMC parameters regularly: after every commissioning, be sure to back up PMC parameters and the ladder.
Use signal trace: for intermittent faults, use LADDER III’s Signal Trace to record the waveform and catch momentary signal changes.
Understand interlock logic: 80% of “no action” is an unmet interlock condition (e.g. low air pressure, door not closed, servo not ready). Check interlocks before hardware.
Mark key signals: in LADDER III, add comments to key X/Y/R addresses for easier future maintenance.

3. Frequently Asked Questions (FAQ)

Q1: Without LADDER III software, can I troubleshoot on the CNC screen?

Yes. On the CNC screen go to [SYSTEM] → [PMC] → [LAGDIAG] (ladder diagnosis) to view signal on/off in real time, but you cannot do complex signal tracing.

Q2: The signal in the ladder is conducting, but the external device does not act — what to do?

It means the PMC logic is fine and the fault is in hardware. Check: 1. whether the output module relay is damaged; 2. whether the 24V supply is normal; 3. whether the cable is broken; 4. whether the solenoid/contacto coil is burned.

Q3: How to quickly find the ladder network for a given alarm?

Note the alarm number, find the corresponding A address (alarm signal) in the PMC parameters, then search that A address in the ladder — you’ll find the logic network that triggers the alarm.

Q4: After modifying the ladder, do I need to restart the machine?

In most cases, after writing the ladder and selecting “Run” mode it takes effect without restart. But when modifying timer/counter constants, a restart is recommended to ensure safety.