Fanuc spindle alarm 73 (AL-73 / SP9073 ‘MOTOR SENSOR DISCONNECTED’) means the spindle amplifier is not receiving a valid signal from the spindle motor's speed/position sensor — the MZi, BZi or CZi sensor on αi systems, or the pulse generator on older motors. The amplifier shows 73 on its 7-segment display and refuses to run the motor: without rotor feedback it cannot commutate safely. Unlike AL-31 (feedback disagrees with command), AL-73 means the signal is absent or invalid at the connector — which makes the diagnosis pleasingly physical: sensor, cable, connector, or the amplifier input that reads them.
Where the signal path runs
Motor sensor → sensor cable (often through the drag chain or along the spindle motor loom) → JYA2 connector on the spindle amplifier. AL-73 lives somewhere on that line — in our repair intake, roughly: cable 50%, sensor 25%, connector seating/corrosion 15%, amplifier receiver 10%.
Step 1 — The five-minute checks
- Reseat both ends of the feedback cable (power OFF): JYA2 at the amplifier and the connector at the motor. Look for bent pins, coolant inside the shells, and green corrosion. A surprising number of AL-73 cases end here.
- Inspect the cable run: follow it end to end — oil-swollen or cracked insulation near the motor, crush points in the drag chain, and rub-through where it crosses cabinet edges. Wiggle-test while a helper watches the display: if the alarm state flickers, you have found the spot.
- Recent work? If the alarm appeared right after motor replacement, bearing work, or belt service, the sensor gap or connector is disturbed — go straight to Step 3.
Step 2 — Meter the cable
- Disconnect at both ends and buzz each conductor for continuity, and each against its neighbours and the shield for shorts. Flex the cable at suspect points while measuring — broken strands inside intact insulation pass a static test and fail in motion.
- Verify the shield is intact and terminated — a floating shield lets VFD/servo noise corrupt the signal, which can present as intermittent AL-73 (or companion alarms 81/82/83) under load rather than a clean disconnection.
Step 3 — The sensor itself
- Gap-type sensors (MZi/BZi): check the air gap between sensor head and the sensor ring/gear against spec — typically around 0.15–0.3 mm (the exact value is on the sensor's data sheet/label). A gap knocked wide during bearing work reads as no signal; too tight risks a strike.
- Inspect the sensor ring/gear: swarf stuck to teeth, a dented ring, or a ring that shifted on the shaft all corrupt the signal.
- Check the sensor head: cracked body, oil ingress at the cable gland, damaged mini-connector. Sensor heads are replaceable separately on most αi motors — a fraction of a motor's cost — but the gap must be set precisely on installation.
Step 4 — The amplifier and parameters
- Amplifier receiver circuit: known-good sensor and cable, alarm persists → the JYA2 input stage on the SPM/αi SP control board has failed — a routine board-level repair for us.
- Parameter mismatch: after an amplifier swap or motor change, wrong sensor-type settings (spindle parameter 4000-range, e.g. sensor selection bits) make the drive look for a sensor that is not there — AL-73 from the first power-up. Verify motor code 4133 and sensor parameters against the actual motor before condemning hardware.
- Swap logic: if a spare amplifier is available, moving only the amplifier tells you within minutes whether the fault travels with it.
Can you keep running?
The motor will not run with AL-73 active, so the question answers itself — but resist the temptation to ‘fix’ an intermittent version by taping the cable and hoping. A sensor signal that drops at 8,000 RPM mid-cut is a scrapped part and a possible tool strike. Replace the damaged section properly or fit a new sensor cable.
Related guides
- Fanuc spindle alarm 31 (motor lock / speed detection)
- Fanuc spindle alarm 12 (DC link overcurrent)
- Identifying and replacing a faulty Fanuc encoder