SINUMERIK 840D NCU Box Display Stuck at 8: Fan Fault Diagnosis

1. Problem Statement

A SINUMERIK 840D NCU-Box (catalog number 6FC5247-0AA00-0AA2) is energized from a SIMODRIVE E/R module (6SN1145-1BA01-0DA1), and the seven-segment display on the front of the NCU remains frozen on the digit 8. All status LEDs on the NCU box (PWR, SF, BF, RUN, STOP, etc., depending on firmware revision) are illuminated simultaneously and the controller never reaches the operating / NC-Ready state. The PLC startup is blocked and the operator panel (HMI) never finishes the splash screen.

This condition is not a random PLC fault. According to the Siemens CNC Commissioning Manual, Part 1 (NCK, PLC, drive), the digit 8 displayed on the NCU seven-segment indicator, combined with all front-panel LEDs on, is the dedicated machine code for fan monitoring: either the internal NCU fan has failed or the unit is being operated without the mandatory cooling fan installed. Until the fan fault is cleared, the NCU will not continue past the boot self-test.

Siemens CNC Commissioning Manual, Part 1 (PDF)

2. Affected Hardware and Part Numbers

The following Siemens and OEM part numbers are relevant to the fault. Confirm the exact MLFB / serial on the rating plate of the unit in the cabinet before ordering spares.

Original Siemens documentation requires the NCU 710 / 720 to be operated with the rear fan installed. Operating the NCU without a fan causes overtemperature shutdown on most firmware versions, and the firmware reports the fan failure with the seven-segment code 8 on cold start, before the NCK has finished initialization.

3. Display Code 8 — Decoded

The NCU 7-segment display is a multi-function state indicator. The mapping relevant to this fault is summarized below.

The combination "all LEDs on, display locked at 8" therefore points directly at the fan-monitor watchdog, which is implemented in the NCU baseboard hardware and firmware. The NCU checks the tachometer (FG) signal of the fan continuously. If the FG pulse-train is absent, the firmware latches the 8 condition and refuses to advance the boot sequence.

4. Root Cause Analysis

On this specific machine, three root causes must be ruled out in the order listed below. The discussion in the source case terminates with the fan-stopped observation; field experience shows that on 6FC5247 NCUs the most common triggers are:

1. Mechanical fan failure — seized bearing on the Sanyo Denki 109P0424H316 (debris ingress, end-of-life, oil loss). Winding measures intact but rotation is zero.
2. Open FG (tachometer) signal path — yellow wire broken, oxidized connector at the fan module header, or a dry-joint on the 570038.0034.03 PCB that prevents the FG pulse reaching the NCU baseboard monitor. Voltage to the fan is correct but the firmware sees "fan stopped".
3. Fan-connector unplugged or wrong fan installed — service error. The NCU can detect a missing rotor current and will report the same code 8.

A secondary root cause that produces identical symptoms but is more expensive is a faulty NC system card (the PC-card / CompactFlash that carries the NCK software and the system serial). If the NCU is powered but cannot read its boot media, the firmware can latch on the same code on early-revision bootloaders. The Siemens NCU manual recommends verifying the fan first, and only then the system card.

5. Preconditions for Diagnosis

Before opening the cabinet, confirm the following safety and operational preconditions. SINUMERIK 840D cabinets contain 600 V DC link energy stored in the SIMODRIVE modules even after the main breaker is opened.

• Main disconnect opened and locked-out / tagged-out (LOTO).
• Wait a minimum of 5 minutes after power-off for the DC link to discharge below 50 V (verify at the SIMODRIVE E/R test points).
• 24 V logic supply isolated at the E/R module output terminals.
• ESD wrist-strap connected to the cabinet PE bar.
• Latest Siemens NCU backup / archive (.ARC) available, either on the CF card or uploaded via WinSCP / HMI.
• Documentation set available: CNC Commissioning Manual Part 1 and the NCU-specific Equipment Manual.

6. Step-by-Step Diagnostic Procedure

Step 6.1 — Visual Inspection

1. With the NCU powered, look at the rear of the NCU box and verify whether the fan blades are turning. The 109P0424H316 is a sleeve-bearing unit, so a stopped rotor is normally obvious within a few seconds.
2. Inspect the fan guard for accumulated dust, oil mist, or swarf. NCUs mounted near hydraulic or coolant enclosures fail this way first.
3. Verify the fan connector is fully seated on the PCB header. The 3-pin JST-style connector on the 570038.0034.03 module can work loose after thermal cycling.

Step 6.2 — DC Voltage Verification at the Fan

With the cabinet safely energized and the NCU display showing 8, measure the supply at the fan connector (do not disconnect the fan with power applied). Use a back-probe or T-pin on the connector housing.

The 27 V DC reading on the red/black pair in the source case confirms that the E/R module is delivering the 24 V rail correctly, and the wiring to the fan is intact. The fault is therefore either:

• The fan itself is not rotating despite 27 V being applied, or
• The fan is rotating but the yellow (FG) wire is open-circuit and the NCU cannot detect the tach pulses.

Step 6.3 — FG Tachometer Test

1. With the NCU still powered and the fan running (or stopped), connect an oscilloscope or frequency counter to the yellow wire referenced to the black (GND) wire.
2. A working 109P0424H316 at 24 V / 2700 rpm produces a square wave on the yellow line at approximately 90 Hz (2 pulses per revolution × 2700 rpm / 60 s = 90 Hz).
3. If the line is held at 0 V or 24 V with no edges, the fan is either not spinning or the FG output transistor inside the fan has failed.

Step 6.4 — Fan Current Measurement

Clamp the red wire with a DC current clamp. Expected running current for the 109P0424H316 at 24 V is approximately 0.10 A to 0.16 A (100 mA to 160 mA). A reading near 0 mA with 27 V applied confirms an open winding or open connector; a reading at the stall current of approximately 0.35 A confirms a mechanically locked rotor.

Step 6.5 — NC System Card Verification

Only proceed to this step if the fan replacement does not clear the 8 code. Siemens historically does not publish the NC system software for public download, so do not attempt to re-image the system card from a generic CF card.

1. Power down, wait for the DC link to discharge, and remove the lower PC-card slot cover of the NCU.
2. Eject the NC system card (typically a PCMCIA / PC-card type II or CompactFlash adapter).
3. Inspect the connector for bent pins, oxidation, or contamination.
4. If a legacy laptop with Siemens CardWare toolchain is available, attempt a low-level read of the card to confirm it is responding to ATA / PCMCIA commands. A card that is unreadable at this level must be replaced by Siemens Service.

7. Fan Module Specifications (Sanyo Denki 109P0424H316)

8. Replacement Procedure

1. Isolate the 24 V supply and wait 5 minutes for the SIMODRIVE DC link to discharge.
2. Remove the NCU rear cover (4 x Torx T10 screws on the 6FC5247 chassis).
3. Disconnect the 3-pin fan connector from the 570038.0034.03 PCB.
4. Release the four fan-mounting rubber vibration isolators. The 109P0424H316 is held by the isolators, not direct screws — pull the fan body straight out of the mounts.
5. Install the replacement fan. Pin-compatible direct substitutes are Sanyo Denki 109P0424H3024 and 9GA0412P2H003 (verify voltage, dimensions, and FG function before substitution).
6. Reconnect the 3-pin connector. Confirm a positive click on the JST latch.
7. Reinstall the rear cover. Tighten the Torx screws to 0.6 N·m to avoid stripping the brass standoffs in the NCU chassis.
8. Restore 24 V supply. Power up the E/R module, then the NCU.

9. Verification

1. Watch the NCU 7-segment display during power-up. The display must sequence through 1 → 2 → 3 within approximately 30 s to 90 s, depending on the size of the NC archive, and then go blank. The 8 must not reappear.
2. Verify that the SF (red) and BF (red) LEDs on the NCU are OFF in the steady state. RUN (green) should be ON or flashing, depending on PLC mode.
3. Confirm the rear fan is audibly running and that airflow at the rear vent is normal. A 109P0424H316 at 24 V is very quiet (~22 dB(A)) — verify by feel, not sound, if the cabinet has high ambient noise.
4. From the HMI, navigate to Diagnostics > Hardware > Fan (if exposed in the HMI sl image) and verify the reported fan state is OK.
5. Run an NC reference axis (e.g., X axis handwheel jog at low feed) for 5 minutes. Monitor the NCU internal temperature via Diagnostics > Temperature. A working fan should hold the NCU internal sensor below 60 °C in a 25 °C cabinet.
6. If the NC system card was disturbed, restore the NC archive from the latest backup via the HMI Commissioning > Series commissioning > Read in archive and accept the NCK reset prompt.

10. Troubleshooting Matrix

11. Preventive Maintenance

• Replace the NCU fan as a planned maintenance item every 5 years or 35,000 operating hours, whichever comes first. Sanyo Denki specifies a service life of 40,000 h at 40 °C for the 109P0424H316 sleeve-bearing version.
• Clean the fan guard and the NCU rear vents every 12 months using ESD-safe vacuum tools. Do not use compressed air — it can force swarf into the NCU enclosure.
• Maintain cabinet ambient at or below 35 °C. For every 10 °C rise above 40 °C, fan bearing life halves (Arrhenius rule of thumb for sleeve bearings).
• Keep the HMI image, NC archive, and CF card image versioned in a documented backup plan. A failed NCU replacement that does not have a matching archive is a multi-day outage.

12. Frequently Asked Questions