Resolving Siemens 840D SL S120 Z2 Axis Drive Faults 231885 207016

Problem Overview

On Siemens 840D SL controlled 5-axis machining centers, intermittent drive and motor alarms concentrated on a single axis — most commonly reported as Z2 — are frequently triggered by a 24 V DC supply disturbance, a degraded DRIVE-CLiQ link, or a contaminated motor temperature sensor circuit. On DMG-built machines such as the DMU 210 P (built 2011) equipped with SINUMERIK 840D SL plus SINAMICS S120 drive line, the same symptom set (motor overtemperature, cyclic DRIVE-CLiQ transfer error, NCK-SI axis communication failure, NCK-PLC cross-check stop) can reappear minutes or days apart and remains latent during commissioning. The fault cluster must be treated as a system-level electrical and topology problem, not a single component swap.

Alarm Manifest and Interpretation

System Architecture Reference

The 840D SL with SINAMICS S120 is a distributed drive system. Each axis has a drive object (DO) on a Motor Module (e.g., AXZ2_01 = drive object 1 of the Z2 Motor Module). Encoder feedback is read by a Sensor Module (SMC20 for incremental, SME20/SME25 for absolute) and tunneled to the Control Unit (CU320-2) over a dedicated DRIVE-CLiQ ring. Temperature feedback is provided by:

• Motor temperature model in the drive firmware (thermal i²t for 1FK7/1FT7/1PH8 motors)
• Direct sensor input on the Motor Module X21/T1-T2 terminals (KTY84-1C130 or PTC triplet)
• Encoder-module-derived temperature on DRIVE-CLiQ encoders with integrated electronics (DQ encoder)

All three channels are evaluated in parallel. A failure in any one of them is sufficient to raise alarm 207016, and a thermal model trip is sufficient to raise 700000 / 700134 even if the physical sensor reads normal — which is why supply noise and DRIVE-CLiQ corruption can produce an apparent overtemperature event.

Root Cause Hypotheses (Ranked by Field Frequency)

1. 24 V DC supply dip on DMG-branded power supplies. DMG (MORI SEIKI) historically used proprietary green-cased 24 V supplies. Unlike Siemens SITOP units, those supplies do not always latch off on a short-circuit event; they self-recover, which masks the fault. The control cabinet can read 24 V with a handheld meter while a transient dip of 4–8 V is occurring for several milliseconds, enough to corrupt DRIVE-CLiQ and sensor sampling.
2. Short or partial short in the 24 V field wiring. A chafed I/O cable, a coolant-soaked limit-switch harness, or a feedthrough penetration can produce a momentary short that drops the bus. The fault then disappears once the short clears.
3. DRIVE-CLiQ cable degradation on the Z2 axis. Vibration, oil ingress, or a half-latched RJ45 connector on the Motor Module or Sensor Module produces cyclic transfer errors (231885) that the drive interprets as encoder fault, which then corrupts the thermal model.
4. Motor temperature sensor failure. KTY84 sensor opens or shorts at the motor terminal box due to coolant ingress through the motor seal (a documented failure mode for 1FK7 motors in live-tool and Z-axis service with high-pressure coolant). Alarm 207016 directly indicates this.
5. Motor bearing failure pulling encoder rotor into misalignment. Coolant travels back through the bearing into the encoder cavity, contaminating the optical disk. The encoder then reports CRC or position errors to the CU.
6. Auxiliary 30.5 V supply (AS-i/Profibus gateway) failure. Already observed on the affected machine, this can pull the gateway bus and the safe-speed bus simultaneously.

Pre-Diagnostic Inventory

Capture the following data before any component swap, so the root cause is not lost in the rebuild:

• Topology image from Commissioning → Topology in the SINUMERIK HMI. Record DRIVE-CLiQ port number, component number, and serial number of the Z2 Motor Module and Sensor Module.
• Service log of HMI alarm history with timestamps, including 231897 component ID 14 and F-net ID 24708310|DT011.
• Drive firmware version of the CU320-2 (read at p0018 on the Control Unit) and Motor Module (read via p0018 of the Z2 DO).
• Motor article number and order number from the motor nameplate; the Z2 motor is most likely a 1FK7 (synchronous) or 1PH8 (asynchronous) Siemens servo motor.
• 24 V supply model number, rating, and how the supplies are paralleled (DMG machines typically use two 30 A or 40 A units in parallel for the I/O ring).

Diagnostic Procedure — 24 V Power Branch

1. Disconnect both DMG-branded 24 V supplies from the I/O ring. Measure open-circuit voltage at the supply terminals; record under no-load.
2. Reconnect one supply only. With a storage oscilloscope or a power-quality recorder (e.g., a Fluke 173x or Hioki PW3198) on the 24 V rail, run the machine in a known fault-producing mode for at least 4 hours. Set trigger to capture any excursion > ±2 V from 24.0 V.
3. Repeat with the second supply, then with both supplies in parallel.
4. Replace both supplies with Siemens SITOP PSU8200 or SITOP PSU100S rated at least 40 A, 3-phase 400–500 V AC input. The 6EP1334-3BA10 (40 A, 3AC 400 V) is the conventional cross-reference for DMG's 30 A DMG-branded unit.
5. Insert branch protection: a 10 A glass fuse link (e.g., 5×20 mm slow-blow) on every I/O sub-circuit. Never share a fuse between two I/O groups. The objective is to make a single short circuit localized instead of dragging the entire 24 V bus down.

Diagnostic Procedure — DRIVE-CLiQ Link

1. Open Commissioning → Drive system → Topology and verify the DRIVE-CLiQ topology matches the wiring. Pay attention to port-to-port continuity at the Z2 Motor Module.
2. With the control de-energized, remove the DRIVE-CLiQ cable between the Z2 Motor Module and its Sensor Module. Inspect both RJ45 connectors for oil, fretting, or a half-latch position. Replace the cable with a Siemens pre-assembled DRIVE-CLiQ signal cable of the same length (6SL3060-4Axx-0AA0 family, xx = length code in meters). Do not use third-party Cat5/6 cable; the impedance is not characterized for the drive ring.
3. Check the Sensor Module (SMC20 / SME20 / SME25) on the motor side. A known failure mode is internal electrolytic capacitor degradation on early SMC20 units; the part number is on the housing label. Cross-check with the Siemens product support database.
4. Run the drive object test routine from Commissioning → Drive system → Drive objects → AXZ2_01 → Commissioning → Drive functions. Read parameters r0190, r0191, and r0192 on the Z2 DO to confirm the component is still seen as "OK" in the topology after the cable replacement.

Diagnostic Procedure — Motor Temperature Channel

1. At the motor terminal box, isolate the temperature sensor wires (typically labelled 1U2/1V2/1W2 plus TP1/TP2 for KTY84). Disconnect at the Motor Module terminal X21 and measure resistance at room temperature: a healthy KTY84 reads approximately 350 Ω at 20 °C and rises by ~10 Ω/°C.
2. If resistance is < 50 Ω or > 30 kΩ, the sensor or the cable is shorted or open. Check the cable for insulation breakdown along the cable carrier, especially at the bend where the cable enters the motor gland.
3. If the sensor and cable are healthy, evaluate the motor's internal winding temperature through parameter r0035 on the Z2 DO. If r0035 stays close to ambient (25–35 °C) but the drive still raises 207016, the fault is in the evaluation path of the Motor Module, not the motor.
4. Compare with the model temperature: read r0034 (model temperature) and r0035 (sensor temperature) over a 30-minute cutting cycle. A divergence > 15 K between model and sensor typically indicates a sensor or wiring issue; both rising together at > 110 °C indicates a real overload or brake-binding.

Diagnostic Procedure — Motor and Encoder Swap Test

If the Z2 motor and another feed axis motor are the same article number (typical on DMG machines — both Z and Y axes use 1FK7xxx with brake), perform a swap test to localize the fault:

1. Move both axes to a safe service position; lock the mechanical brakes mechanically.
2. Swap the motor assemblies (motor + encoder, leave the cable in place if the cable length is acceptable).
3. Re-commission Z2 with the donor motor. Run the machine. If alarms follow the motor → motor/encoder is the cause. If alarms stay with the Z2 axis → cable, Motor Module, or DRIVE-CLiQ link is the cause.
4. Re-apply the manufacturer's Z2 axis parameter set (drive parameters live in the encoder's electronic nameplate, so the swap is largely transparent to the application).

Parameter and Topology Walk-Through

The Z2 drive object uses the standard Siemens 1FK7/1FT7/1PH8 parameter set. The following drive parameters are critical for this fault pattern:

Firmware and Engineering Notes

DRIVE-CLiQ cyclic transfer errors (alarm 231885) were the subject of multiple Siemens product support and firmware corrections. Older CU320-2 firmware (V2.4 and earlier) has known issues where transient topology faults are not latched cleanly. The current supported SINAMICS S120 firmware branch (V5.x for production machines) is the recommended baseline. Verify on the affected CU320-2:

• CU320-2 firmware version, accessible via Commissioning → Drive system → CU → Version or via parameter r0018 on the Control Unit
• Motor Module firmware version, accessible via the same path on the Z2 DO
• Cross-check the firmware compatibility table in the SINAMICS S120 Function Manual (FH1) for the installed CU firmware

Recommended Spare Parts and Cross-References

Verification After Repair

1. Clear the alarm buffer at the HMI (Diagnostics → Alarm log → Clear).
2. Run the Z2 axis through a full reference traverse, then a circularity test, then a 30-minute cutting cycle on a known program.
3. Capture the 24 V rail trace during the test; a clean trace should stay within ±1 % of 24.0 V (23.76–24.24 V) for the full cycle.
4. Log r0034 and r0035 on the Z2 DO for the cycle. The two curves should track within 10 K of each other at steady state.
5. Verify alarm buffer is empty after the cycle.
6. Re-run the production program for a full shift (8 h) before returning the machine to production flow. Keep the oscilloscope file and the topology screenshot attached to the maintenance record.

Troubleshooting Matrix

Safety Considerations

Before any work on the SINAMICS S120 drive line or the 24 V bus, observe the Siemens five safety rules:

1. Disconnect mains at the main isolator
2. Lock out the isolator
3. Verify zero voltage at the line side with a known-good test instrument
4. Ground and short-circuit (if required by local practice)
5. Cover or barrier adjacent live parts

After re-energization, the DC link in the Line Module may carry 600 V DC for several minutes. Wait at least 5 minutes after power-off before touching any drive component. Confirm the DC link voltage has dropped below 50 V (read at the Line Module test sockets) before proceeding.

Documentation to Reference

Keep the following Siemens documentation accessible during this work:

• SINUMERIK 840D sl Operator Components / Commissioning Manual
• SINAMICS S120 Function Manual (FH1) — drive functions and alarm list
• SINAMICS S120 List Manual — parameter descriptions for p0010–p0610, r0034/r0035, r0190
• SINAMICS S120 Function Manual (FH2) — DRIVE-CLiQ topology and component replacement procedure
• SITOP PSU8200 manual for 6EP1334-3BA10 and SITOP UPS module for 6EP1931-2DC21

FAQ