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SV436 “SOFT THERMAL (OVC)” means the servo software has detected an over‑current/over‑thermal condition on the n‑th axis; it’s often triggered by high mechanical load, marginal motor/brake, or an amplifier sensing issue, and it usually clears after power cycle but will recur until the root cause is fixed . Below is a field‑proven, step‑by‑step diagnosis and treatment workflow with safe checks first, then isolation test.

What the alarm means

– SV0436 is the servo amplifier alarm for “soft thermal (OVC),” i.e., the digital servo detects a software thermal state rather than a hard thermal trip; common contributors include mechanical overload, tight axis’s motor brake , Axis’s motor power cable damage or an amplifier issue.  

– Community and service notes confirm it often presents on one axis (e.g., X/Y/Z), may briefly clear after a power cycle, and can be related to motor brake.

Safety first

– Power down, lockout/tagout, and allow discharge time before pulling connectors; when testing a vertical axis with motor power disconnected or brakes released, mechanically support the axis to prevent a drop .

Make sure that motor is not Vibrating

If a motor vibrates, the current flowing in it becomes more than required that’s why this alarm generated.

Initial Verification Protocol for External Brake Systems

When addressing servo motor brake circuit anomalies equipped with external electromagnetic brakes, the preliminary diagnostic step involves verification of the 24 V DC supply voltage at the motor brake connector terminals using a calibrated digital multimeter. Measure between the positive and negative terminals of the brake connector to establish baseline voltage values and confirm proper voltage delivery to the brake coil circuit.This 24 v dc voltage will come on motor brake connector when there is no alarm /msg in machine and machine is in ready state & emergency stop button should be released.

in alarm /msg condition 24 v dc supply will not come .

If there is no external brake on the servo motor then you have to check mechanical system .Is there any mechanical support system (counter weight/counter balance) for Vertical axis in many machines spring type support or hydraulic support has been given for vertical axis.

IN such scenario due to problem in mechanical system there will be abnormal load on vertical axis and machine will trigger soft thermal(OVC) alarm.

Supply-Side Fault Diagnosis

In instances where the 24 V DC supply voltage is absent or below nominal levels at the motor brake connector, conduct a systematic inspection of the 24 V DC power supply circuit architecture. Execute the following diagnostic sequence:

Fuse inspection

Verify the integrity of the 24 V DC control circuit fuse (typically located in the machine’s main control panel). A burnt-out or open-circuit fuse indicates an overcurrent condition and requires replacement with an equivalent amperage-rated fuse.

Cable integrity assessment

Perform a comprehensive physical examination of the brake cable harness for visible damage, including insulation degradation, crushed conductors, or pin corrosion at connector interfaces. Damaged brake cable assemblies must be replaced with OEM-specification replacement harnesses to maintain circuit compliance.

Brake Cable Continuity Testing and Substitution

Execute a continuity test on the entire brake cable assembly using the multimeter’s ohms function, measuring between both connector terminals to confirm zero or near-zero resistance.

When continuity is compromised, implement cable substitution methodology

Intermittent Brake Engagement Under Axis Motion Conditions: When brake circuit faults manifest exclusively during machine axis motion, suspect internal conductor fracture within the brake cable jacket—a condition often caused by repetitive flexing or mechanical stress at cable routing points. In such cases, the brake cable must be replaced with an equivalent OEM cable or substituted with a verified functional brake cable from an auxiliary machine to confirm this as the root cause.

Post-Repair Verification and Motor Assessment

When the following conditions are confirmed:24 V DC supply voltage is present and stable at the motor brake connector (motor-side interface)Power delivery circuit continuity is verified across all three motor phase conductors (U, V, W terminals) and equipment grounding conductor. No external brake circuit anomalies are detected .Execute the following corrective action sequence.

Motor Removal and Out-of-Machine Diagnostics

Ensure the machine is de-energized (power OFF state)Implement mechanical support structure beneath the axis to prevent unintended gravitational drop and potential personnel injury or equipment damage.Carefully extract the axis motor assembly following OEM mechanical extraction procedures .Reconnect the motor to external power supply in a controlled laboratory environment external to the machine Re-energize the system and perform isolated motor operation testing Observe motor brake engagement and thermal indicators during trial operation.If soft thermal alarm conditions persist during isolated motor operation (indicating internal motor winding or bearing degradation), submit the servo motor to the nearest authorized FANUC Service Center for comprehensive internal diagnostic assessment, bearing maintenance, and potential windings replacement.

Critical Safety Considerations Hazard Warning

Control cabinet interiors and motor brake circuits operate at lethal voltage potentials. Execute live voltage diagnostics exclusively when operationally necessary and only when qualifications meet OEM technical certification requirements. Adhere strictly to applicable FANUC technical procedures and facility lockout/tagout (LOTO) protocols during all diagnostic procedures

– Check for recent heavy cuts or program changes increasing load; if recurring on Z, consider counterbalance health (hydraulic or mechanical) that can elevate constant load and heat .

Operator reset sanity 

– After clearing any protective devices, power back on and see if SV0436 returns immediately or only with motion; this timing distinction guides isolation.

Load and axis feel

– In jog, watch axis load meters; a constant ~100% standing load or spiking with slow moves suggests mechanical drag (ways, ballscrew thrust, contamination) or brake partially engaged on a holding axis . 

– If the alarm clears with power cycle but reappears at enable or first move, suspect brake drag or motor temperature sensing path; a stuck brake especially on Z can mimic overload/overcurrent .

Isolation test: drive vs motor/cables

– With CNC off, disconnect the motor power leads for the suspect axis at the amplifier; then power up and enable carefully (support vertical axes), observing if the alarm 436 (or any high‑current 8/9/A) appears with the motor disconnected .

– If the alarm persists with the motor unplugged, the amplifier is likely faulty; if it does not, focus on the motor, brake, and power cable set for shorts/grounds or mechanical binding .

Motor and cable checks

– Using a DMM, check phase ‑to‑phase and phase‑to‑ground at the motor plug: phase‑to‑phase should be low and consistent; phase‑to‑ground should be open; inconsistency or phase‑to‑ground continuity indicates a shorted motor or cable . 

– Use a megger at 1000 V to test each phase to ground; expect very high resistance (e.g., 500 MΩ or infinity); low values indicate insulation breakdown that can drive soft thermal/overcurrent .

Feedback and temp sensor path

– Intermittent soft thermal can also stem from temp sensor/feedback harness anomalies; community reports note that if an alarm behavior changes when feedback is disconnected, suspect the feedback cable or pulse coder; a defective internal motor thermal sensor line can also misreport heat .  

– If disconnecting feedback with CNC main power on changes which alarms appear (per abnormal current 8/9/A rules), it helps identify whether the amplifier or feedback chain is culpable before swapping parts .

Amplifier health

– If isolation points to the drive, replace or exchange the amplifier; reputable repair houses document SV0436 as soft thermal. 

– After a replacement, follow any model‑specific parameter or side‑card compatibility steps if other drive alarms (e.g., 368/369/466) appear on install; clear PWE changes properly and power cycle to confirm normal status .

Mechanical causes to rule out

– Verify axis moves smoothly by hand when decoupled (as allowed by the machine builder) and check ballscrew, linear guides, and thrust bearings for binding; persistent high idle load points to mechanical friction leading to thermal/OVC .  

– On Z of VMCs, inspect counterweight/hydraulic counterbalance state; low pressure or counterbalance faults add constant motor torque demand and heat that can trigger SV0436 .

When it’s intermittent only under heavy cutting

– If the alarm only occurs during aggressive cuts, reduce feed/accel limits and check servo tuning/load inertia; a process‑induced overload can be the root cause rather than an electrical failure .  

– Ensure spindle and axis programs aren’t imposing abrupt direction changes that spike axis current; smoothing or ramp adjustments can mitigate transient overcurrent that leads to soft thermal .

Practical “treatment” sequence (checklist) 

– Electrical: confirm no tripped MCBs; power‑cycle correctly; re‑test; then perform motor‑disconnected alarm test to split drive vs field side .

– Metrology: DMM and megger on motor and cable; repair/replace if shorted or grounded; verify connectors and shielding integrity . 

– Mechanical: support vertical axes, verify brake releases fully, listen for brake drag, inspect axis for binding and counterbalance health; correct friction sources .  

– Parts: if alarm persists with motor disconnected, swap amplifier; if it shifts with feedback cable changes, inspect/replace feedback cable/encoder; document parameter compatibility if a new module is installed .

Notes from the field

– “Soft thermal (OVC)” is a software‑detected condition; it often precedes hard trips and is worth addressing early to avoid drive damage or nuisance downtime .

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These blogs should not be considered as official repair or service manuals. For detailed instructions, critical repairs, or advanced troubleshooting, it is always necessary to contact and work under the guidance of the respective *machine manufacturer* or *CNC controller support team*.  

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