Fanuc spindle alarm 43 means the position coder signal of the MASTER spindle used in differential spindle speed control is disconnected. This alarm only exists on machines using differential speed control — typically multi-spindle lathes and machines where a sub-spindle must track the main spindle's rotation (synchronized part hand-off, sub-spindle pickup while turning). The sub-spindle's drive needs the master's position coder signal to compute the differential — and it stopped arriving.
The diagnosis is AL-27's — pointed at the master
Fanuc's manual says it directly: troubleshoot as for alarm code 27. The full procedure is in that guide; the differential-specific point is which hardware to apply it to:
- Work on the MASTER spindle's coder chain, not the sub-spindle's — the alarm may display against the sub/differential spindle, but the missing signal is the master's. Check the master's position coder, its cable, and the routing of that signal to the amplifier that raised the alarm.
- Sensor parameters: verify the differential-mode and sensor settings per B-65280EN — on multi-spindle configurations the master-coder routing is parameter-defined, and a restore or amplifier swap can silently break it.
- The inter-amplifier signal path: in differential setups the master's coder signal often passes between amplifiers — an extra cable and two extra connectors that single-spindle machines don't have, and therefore the most common failure point. Reseat and inspect that link specifically.
- Cable and coder checks: then the standard AL-27 sequence — wiggle-test, continuity, coder mechanical drive, amplifier input last.
Tip: A quick sanity check that saves an hour: does the MASTER spindle itself still orient and thread normally? If the master also misbehaves → its coder chain is genuinely down; fix it and AL-43 clears. If the master is perfectly fine → the coder is healthy and the fault is in the branch feeding its signal to the differential spindle — the inter-amplifier cable, its connectors, or the parameter routing.
Diagnosis checklist (in order)
- Confirm the machine actually uses differential control (sub-spindle synchronization) — and note which operation trips the alarm.
- Test the master spindle's own orientation/threading: master faulty → fix its coder chain per AL-27.
- Master healthy → inspect the inter-amplifier signal cable and connectors; verify differential-mode parameters.
- Persists → the receiving amplifier's coder input: board repair or exchange.
Related guides
- Fanuc spindle alarm 27 (position coder disconnected) — the full procedure
- Fanuc spindle alarm 42 (Z signal not generated)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained
AL-43 on your multi-spindle lathe? JJ Automation diagnoses differential-spindle feedback chains on site in NCR, stocks position coders and cables, and repairs amplifiers at component level. WhatsApp us or call +91 88022 12394.