On a DMC DL 21A with a FANUC 0i-TD control, alarms SV449 and SP1982 remained after the servo amplifier and battery were replaced. The alarms appeared after Reset, the machine did not respond, and the amplifier display showed 8. Cable continuity and the documented supply checks reportedly passed. Installing a known-good amplifier cleared both alarms; the replacement amplifier was therefore returned under warranty.
Confirmed fault isolation
| Observation | Diagnostic meaning |
|---|---|
| SV449 and SP1982 appeared after Reset | The fault persisted through the original service work and prevented machine operation. |
| Amplifier display showed 8 | The evidence associates this indication with DC-line overcurrent on the L axis, but the exact amplifier model was not recorded, so verify the meaning in the matching manual. |
| Wiring continuity and supply checks passed | These checks reduced suspicion of an open connection or incorrect supply, but continuity alone did not exclude an insulation fault or a short under load. |
| A known-good amplifier cleared both alarms | This substitution test isolated the installed replacement amplifier as the confirmed failed component. |
Diagnostic decision path
Do not condemn the motor, encoder, or cabling from SV449, SP1982, or display 8 alone. The evidence identifies three candidate causes for the overcurrent indication: a short in a motor power cable, a short in a motor winding, or an internal SVM fault. The amplifier identification plate is required because the machine reportedly used one unit for the spindle and axes, and the available evidence does not provide its model.
- Record the CNC model, amplifier model, and the specific module displaying 8. Confirm whether it is an axis, spindle, power-supply, or combined unit.
- Inspect each motor power cable through the flexible carrier, especially where chips could crush or cut the outer insulation.
- Disconnect the motor power connectors from the amplifier and test the cables thoroughly. Check the motor windings separately so a cable fault is not confused with a motor fault.
- With the motors disconnected, observe whether the amplifier indication and CNC alarms change after Reset. This separates a downstream motor or cable fault from an amplifier-side fault.
- If the wiring and motors pass these checks, substitute a compatible known-good amplifier. In the documented case, this step removed SV449 and SP1982.
Interpreting the successful substitution
The known-good-unit test provides stronger evidence than continuity measurements alone: both CNC alarms disappeared without a reported motor, encoder, or cable repair. The practical conclusion is that the newly installed amplifier was defective or otherwise unsuitable. The evidence supports warranty return of that unit, but it does not identify its internal failed circuit.
Do not treat the battery replacement as the demonstrated cause. The alarms existed before and after that work, while changing the amplifier resolved them. Also avoid assuming an encoder fault unless a separate test or alarm points to encoder feedback.
Verification before returning the machine to service
After installing the known-good amplifier, confirm that SV449 and SP1982 no longer return after Reset, that display 8 is absent, and that the control responds. Preserve the amplifier identification and test results with the warranty claim. If either alarm returns, resume isolation at the motor power cables and windings rather than assuming a second amplifier failure.
FAQ
What caused FANUC SV449 and SP1982 after servo amplifier replacement?
In this case, the installed replacement amplifier was the failed component. A compatible known-good amplifier cleared both alarms, and the suspect unit was returned under warranty.
What does 8 on the FANUC servo amplifier display mean?
The available evidence associates 8 with DC-line overcurrent on the L axis. Because the amplifier model was not recorded, confirm that interpretation in the manual for the exact unit before replacing hardware.
How do I separate an amplifier fault from a motor cable short?
Inspect the power cables, disconnect the motor power connectors, and test the cables and motor windings separately. Then observe whether the indication changes with the motors disconnected; if those checks pass, substitute a compatible known-good amplifier.