FANUC Servo Amplifier Replacement: Power-Off and Discharge, Teardown Order, Motor ID No.2020 Write-Back, Three Post-Swap Checks-CNCShare

The servo amplifier is the most expensive — and least forgiving — component on a CNC machine: swap it right and the machine is back in an hour; swap it wrong and it greets you with a string of alarms, or worse, a crash. This article follows the official FANUC αi / βi parameter wording through a complete replacement: deciding whether to swap at all, power-off and discharge, teardown order, motor ID and parameter write-back, FSSB setup, and the three verification checks that decide whether the job is actually done. Two reference tables (alarms and key parameters) close the article.

Related reading: the repair-or-replace decision is in Should you repair or replace a FANUC servo amplifier?; the mandatory data backup is in FANUC Data Backup and Restore: ALL I/O and SRAM; the full servo alarm list is in FANUC Servo Alarm Code Reference SV0301-SV0441.

1. Before you start: should this be a swap at all?

“Amplifier alarming” and “the amplifier is broken” are two different findings. Spend ten minutes with the repair-or-replace decision table first: repair versus replace is decided by four cost items — part price and lead time, downtime cost, the repair shop’s test capability and warranty, and the history of the same fault recurring. A quotation only covers the first item, and signing off on a quotation alone is the most expensive habit in maintenance.

Once the decision is made, two things must happen before the power goes off:

Back up the data. SRAM (including servo and spindle parameters), PMC program, ladder, macro variables and pitch error compensation — run the full ALL I/O backup procedure. Initialization after the swap can disturb the parameter area; without a backup you are betting the machine.
Record the original parameters. Photograph or write down the key items for the axis: No.2020 (motor ID), No.2001 (AMR), No.1820 (CMR), No.2022 (direction), No.2023 / No.2024 (speed/position pulse numbers), No.1821 (reference counter capacity), No.2084 / No.2085 (FFG). These must be restored or verified after initialization; without the record you will be reverse-engineering at midnight.

Verify the replacement part three ways: model and rated current match (compare the amplifier nameplate motor type code A06B-XXXX against the motor manual), axis count matches (single- versus dual-axis modules assign axes differently), and interface version (FSSB fiber interface type). The wrong spare part makes everything after it wasted work.

🚨 One hard rule: every parameter number in this article follows the FANUC αi / βi parameter manuals and your own machine’s manual; numbers and writable ranges vary by series and version. Field measurement and the machine’s own manual are the final authority.

2. Power-off and discharge: five minutes is a step, not a suggestion

The first step of an amplifier swap is not unscrewing — it is making the power genuinely dead:

Cut the machine’s main power (at the distribution panel), not the panel E-stop — the E-stop only drops servo enable; the main supply and the amplifier DC bus stay live.
Wait about five minutes. The DC bus capacitors need time to discharge; the exact wait follows your machine’s manual, and most models specify five minutes or more.
Verify with a meter. Measure the P–N bus voltage with a multimeter and confirm it has fallen below the safe level. Skipping this step is handing your life to “it should be discharged by now”.
Lock out / tag out. Hang a “do not close” tag on the isolation point and keep the key yourself; with multiple workers, log each one. The isolation point must stay visible.
ESD control. Wear an anti-static wrist strap (or touch grounded metal first) before touching amplifier boards; recap modules and fiber connectors the moment they come off.

FANUC servo amplifier replacement safety flow: cut main power not the E-stop, wait about 5 minutes for capacitor discharge, verify P-N bus voltage with a multimeter, lock out and tag out, wear an ESD wrist strap before teardown

3. Teardown order: mark first, pull lines second, screws last

Reverse the order and you pay for it as guesswork during reassembly. The standard order:

Photograph everything first. Wide shots of the terminal box, fiber routing and module positions before touching a single screw — this halves the recall cost at reassembly.
Signal lines and fiber first. Pull FSSB fibers straight out by the connector root, never bend them (an over-tight bend radius damages the fiber and you inherit an intermittent comms fault), and cap them immediately.
Power lines second. Mark every terminal: L1/L2/L3 incoming, U/V/W outgoing per motor; on dual-axis modules do not mix up the two motors’ outgoing lines.
Mounting screws last. Support the module with both hands; mind the removal path where obstructions exist, and never let the module’s weight rest on its terminals.
Keep the old module. Record its model, serial number and the alarm number at failure time — that is your evidence when you talk to the supplier.

FANUC servo amplifier teardown order: photograph wiring and fiber routing first, pull the FSSB fiber and cap it, then power lines with marked phase order, mounting screws last with both hands supporting the module

4. Installing the new amplifier: mechanical fit and wiring checks

Reassembly is the reverse of teardown, but every step gains a check:

Mechanical fit: align the mounting holes and tighten the fixing screws evenly in stages to the torque specified in your machine’s manual — no single torque number is given here; it differs per module.
Wiring checks: go through the section-3 photographs item by item — L1/L2/L3, U/V/W (never from memory), P–N brake resistor wiring (where fitted, confirm wiring and jumpers), and a solid ground bond.
Dual-axis modules: confirm which outgoing line belongs to which axis; mixing them guarantees a failure at direction verification.
Fiber reinsertion: fully seated with the latch engaged. FSSB is a serial topology — one loose link in the chain takes out every axis behind it.

5. Parameter write-back: motor ID No.2020 and the init bit No.2000#1

Wiring done, prepare the parameter path before power-on. Two routes by model:

0i-D / 30i-B and newer: PARAMETER SETTING SUPPORT → SERVO SETTING screen, following the guided setup of initialization items including the motor ID.
Classic parameter screens: enable the parameter write switch first, then set No.3111#0 (SVS) to 1 so the servo setting (SV SET) screen appears.

Two actions carry the job:

Write the motor ID: No.2020. The motor ID is the code for the motor model, obtained by looking up the motor nameplate type (A06B-XXXX format) in the motor parameter manual — the amplifier does not “know” it at power-up. A wrong motor ID gives mismatched characteristics at best and an immediate alarm at worst.
Initialize: No.2000#1 to 0 → power off → power on. The system runs automatic initialization of amplifier and motor using the written motor ID; when it finishes, PW0000 “PLEASE TURN OFF POWER” appears. Power off and on as prompted; on success the bit returns to 1 by itself. Seeing PW0000 is not a fault — it means “I’m done, please restart”.

Verify or write back the remaining parameters against the section-1 record: No.2001 (AMR), No.1820 (CMR), No.2022 (direction), No.2023 / No.2024 (speed/position pulse numbers), No.1821 (reference counter capacity), No.2084 / No.2085 (FFG). Check item by item; if anything mismatches, stop and find the cause — never wave it through as “close enough”.

FANUC servo amplifier parameter write-back flow: back up and record original parameters, enable parameter write and the 3111#0 SVS screen, write motor ID No.2020, set No.2000#1 to 0 and cycle power to initialize, PW0000 prompt then power cycle again, verify CMR direction pulse numbers reference counter and FFG

6. FSSB fiber and amplifier count setup

FSSB is the high-speed serial bus between the CNC and the axis amplifiers, and this is where module swaps most often go wrong:

Amplifier count recognition: set No.1902#0 (FMD) to 1 to run FSSB automatic setup, letting the system re-recognize the connected amplifiers and axis assignment; it returns to 0 when done.
What SV5136 means: it is reported when the connected amplifier count is lower than the CNC-side setting. Three field causes, in order of likelihood — fiber reversed (FSSB fibers are directional; swapping TX/RX breaks the link), fiber not fully seated or the latch not engaged, and a mid-chain module dropped out (in a serial topology, one loose link loses every axis behind it). Checking in this order beats rewiring the module.
Treat fiber as a consumable: a kinked fiber or a connector with dust in it goes straight to the bin — do not wipe it and carry on. Intermittent comms faults can cost you an entire weekend.

7. The three verification checks: miss none

Parameters written and alarms cleared is not the same as finished. Run the three checks in order; skip none:

Insulation and grounding (power off). Measure insulation of the power lines to ground and check ground continuity against your machine’s manual — a screw not fully tightened or a pinched cable sheath is exactly what this step catches. Resistance values follow the manual; no single number is quoted here.
Dry run. Power up the servo, jog at low speed in both directions; watch the load meter / current display (no-load should sit well below rating), listen for abnormal noise, feel the case temperature. If any phase’s current is clearly off, stop immediately — most likely a phase order or motor wiring problem.
Load and direction test. Direction must be confirmed before any homing or move: a wrong No.2022 setting or wrong motor phase order means a crash, with no luck involved. With direction confirmed, do the reference point return, spot-check positioning accuracy, and finish with a test cut compared for size and surface finish.

After the test cut passes, review the alarm history: confirm no SV alarm was merely cleared while its root cause lives on — a machine that had a module swapped deserves observation for at least one full shift.

FANUC servo amplifier three post-swap verification checks: insulation and grounding with power off, dry run at low JOG speed in both directions watching the load meter, load and direction test confirming No.2022 before homing, positioning check and test cut

8. Alarm and key parameter reference tables

Table 1 — Alarms and prompts relevant to this procedure

Alarm / prompt	Related to	Field direction
SV0417	Illegal/abnormal initialization parameters (including motor ID No.2020)	Verify No.2020 motor ID and initialization parameters; look up the motor nameplate type in the manual
SV5136	FSSB amplifier count lower than the setting	Check fiber direction / seating / mid-chain module
PW0000 “PLEASE TURN OFF POWER”	Initialization complete, restart required	Normal prompt: power off and on, confirm No.2000#1 returns to 1

Table 2 — Key parameters involved in this procedure

Parameter	Name	Role in this procedure
No.2020	Motor ID	Looked up from the motor nameplate type in the manual; a wrong value alarms
No.2000#1	Initialization bit	Set to 0 → power cycle triggers initialization; returns to 1 on success
No.2001	AMR	Amplifier/motor spec related; verify against the record
No.1820	CMR (command multiplier)	Verify against the record; affects movement per pulse
No.2022	Motor direction	When direction verification fails, check this and phase order first
No.2023 / No.2024	Speed/position pulse numbers	Verify against the record; tied to the feedback type
No.1821	Reference counter capacity	Verify against the record; affects home grid spacing
No.2084 / No.2085	FFG (feed-forward gain)	Verify against the record; affects following error and contour accuracy
No.3111#0	SVS (SV setting screen switch)	Set to 1 to display the servo setting screen
No.1902#0	FMD (FSSB automatic setup)	Set to 1 to auto-recognize amplifier count and axis assignment

The complete servo alarm list is in FANUC Servo Alarm Code Reference SV0301-SV0441; system-class PS/BG/SR alarms are in FANUC PS/BG/SR Alarm Code Reference.

9. Wrap-up: handover checklist and follow-up

Four items before you call it delivered:

Back up parameters again. The post-initialization state differs from pre-swap; run the backup procedure once more and archive the result as the new baseline.
Archive both parameter tables. The section-1 record and the post-initialization values together — next time this fault appears, that pair is the most valuable document you own.
Keep the test-cut record. Size, surface finish, program and parameter version — the acceptance evidence.
Set expectations on gain tuning. A new module’s current-loop characteristics may differ from the old one; if the test cut shows larger following error or light chatter, that is normal follow-up rather than “not fixed” — handle it per FANUC Servo Gain Parameter Tuning; vibration classification is in Servo Vibration, Overshoot and Noise Diagnosis; when the inertia ratio needs re-identification, see FANUC Servo Inertia Ratio: Measurement and Identification.

Frequently Asked Questions

The new amplifier alarms SV5136 the moment it powers on — why?

SV5136 means the amplifier count recognized by the CNC is lower than the setting. Check three places in order: ① the FSSB fiber is reversed (the fiber is directional — swapping TX/RX breaks the link); ② the connector is not fully seated, the latch is not engaged, or dust is in the connector; ③ a mid-chain module’s fiber has come loose — in a serial topology one loose link loses every axis behind it. If all three are clean, run the FSSB automatic setup (No.1902#0 FMD = 1) to re-recognize. If it happens repeatedly with no physical fiber damage, replace the fiber.

I can’t find the motor ID (No.2020) — what now?

The motor ID is not something the amplifier learns at power-up: it must be looked up from the motor nameplate type (A06B-XXXX format) in the motor parameter manual. Three field moves: ① copy it from a same-model motor’s nameplate; ② take it from the machine’s archived parameter table — this is exactly why the pre-swap record matters; ③ use the model-to-code table in the motor manual. If you cannot confirm it, stop and verify rather than filling in a close value from experience — a wrong motor ID gives mismatched characteristics at best and a power-up alarm at worst.

Does servo gain need re-tuning after an amplifier swap?

Not necessarily. First complete the three verification checks in section 7: direction, no-load load meter, test cut. Only if the test cut shows larger following error or light chatter should you consider fine-tuning — the reason is that a new module’s current-loop characteristics may differ from the old one. Follow the standard flow: confirm whether the inertia ratio needs re-identification (see the inertia ratio article), then adjust gain and FFG in the order speed loop first, position loop second, one change at a time. “A bit different after the swap” and “not fixed” are two different claims — separate them with data first.

Last updated: 2026-10-25