Fanuc SVM Alarm 8. / 9. / A. (Axis Overcurrent) — Causes, Diagnosis & Fixes

On a Fanuc αi servo amplifier (SVM), a 7-segment display showing ‘8.’, ‘9.’ or ‘A.’ (with the dot) means overcurrent / abnormal current detected in the module's power stage — the amplifier measured current beyond the safe limit and shut down instantly. The digit tells you which axis of a multi-axis module tripped: 8. = first axis (L), 9. = second axis (M), A. = both axes together on a 2-axis SVM (3-axis modules use further codes for the N-axis combinations). On the CNC screen it appears as a servo alarm such as SV0438 ‘n-axis: inverter abnormal current’. Like every hard overcurrent trip: do not keep resetting into it — each attempt into a real short stresses the IGBT/IPM stage further. Isolate first.

What the display tells you before you touch anything

• 8. or 9. alone → the fault is specific to one axis: that axis's motor, its power cable, or that leg of the module. Your search space just halved.
• A. (both axes) → both legs tripping together points more toward the module itself, the shared DC link side, or two cables damaged in the same event (a crash, coolant flood, or a crushed loom — both cables usually run together).
• Trips at servo-on with no motion → hard short. Trips only during rapids/heavy cuts → marginal insulation under voltage stress, mechanical overload, or a weakening module.

Step 1 — Isolate: module, cable or motor?

Power down, lock out, wait for the charge LED to go out (5+ minutes).

1. Disconnect the affected axis's motor power cable at the amplifier (the axis the code names — L or M). Power up and enable.
   • Alarm still trips with the cable off → the module's power stage for that axis is shorted. Go to Step 2.
   • No alarm → the fault is downstream. Continue.
2. Megger the cable and motor (500 V DC, amplifier disconnected at both ends): each phase to earth, phase to phase. Below ~10 MΩ to earth is suspect, below 1 MΩ is your fault. Usual finds: coolant inside the motor power connector, drag-chain crush damage, and oil-rotted insulation at the motor end.
3. Split cable from motor at the motor connector and test each separately; check winding balance U-V / V-W / W-U — equal within a few percent, or the motor needs rewinding/replacement.
4. For an ‘A.’ alarm, test BOTH axes' cables and motors even if only one seems suspect — shared-loom damage regularly takes out both.

Step 2 — If the module is shorted

• Bench confirmation is a diode test across the output legs — a shorted IGBT/IPM reads near zero ohms. The repair is component-level: replace the module, verify the gate-drive circuits (a failed gate driver destroys the new module at first power-up — the mistake that kills DIY repairs), check current sensors and snubbers, then test under real load. Routine work for us on SVM αi units; exchange modules are the fast path when the machine is down.
• On an ‘A.’ alarm with both legs shorted, check the DC link side and the PSM too — a bus event (see PSM alarm 04) sometimes precedes the double failure.

Step 3 — If cable and motor test healthy

• Mechanical bind on that axis: seized ballscrew, crashed slide, failed Z counterbalance — the drive pushes maximum current into a wall. Jog slowly and watch the load meter; high load at low speed is mechanical, not electrical.
• Wrong parameters after amplifier/motor swap: a motor-code mismatch commutates wrongly and spikes current at the first move — verify parameter 2020 (motor ID) against the motor nameplate before condemning hardware.
• Pulse coder fault: wrong rotor position → wrong commutation → current spike, often with feedback alarms in the history — see identifying a faulty Fanuc encoder.
• Intermittent trips after coolant flooding: megger the motor again warm, after a running period — moisture-degraded insulation often passes cold and fails hot.

Diagnosis checklist (in order)

1. Read the digit: which axis — or both? Note when it trips (servo-on / motion / load).
2. Cable-swap test (2 minutes) → module vs downstream.
3. Cable-off test to confirm; then megger cable and motor separately, winding balance.
4. Check mechanics with the load meter; verify parameter 2020 after any recent swaps.
5. Module implicated → bench test and component-level repair; never just reset and hope.

Related guides

• Fanuc SV0410 excess error alarm
• Fanuc spindle alarm 12 (DC link overcurrent)
• Fanuc αi drive alarms — PSM, SVM & SPM codes explained
• Fanuc servo drive alarm codes