⚠️ Disclaimer: These tasks are guidelines only. They do not include lockout/tagout (LOTO), energy isolation, or other safety requirements. Review and verify suitability for your specific equipment and application. Add all required safety procedures per your company's policies and regulatory requirements before use. You are responsible for the safe and appropriate execution of all maintenance activities.
Speed sensors don't seize. They don't make noise. They just start lying — sending a clean signal that no longer reflects what the shaft is actually doing. By the time your drive faults or your control system reacts, the sensor has already been wrong for a while.
This checklist covers the mechanical and electrical checks that keep speed sensors accurate and reliable. It's for maintenance technicians executing the PM and maintenance managers building or auditing the program.
For a broader look at how sensor PMs fit into your overall instrumentation strategy, see your sensor and instrumentation PM program
How to Use This Checklist
Record findings with specificity — not checkboxes dressed up as findings. "Sensor gap within spec" tells you nothing in six months. "Gap measured 0.038 in., within spec of 0.020–0.060 in." gives you a trend. A bad finding sounds like: "cable jacket abraded at conduit entry, approximately 2 in. of insulation exposed, routed for replacement." A good one does the same: it says what you found, where you found it, and what you did or flagged. Trend your air gap measurements and output signal deviation over time — those two numbers will tell you when a sensor is drifting before the control system does.
Field Checklist — Critical Tasks
Visual Inspection
| Task | Freq | Type |
|---|---|---|
| Inspect speed sensor body and mounting bracket for physical damage, cracking, or looseness. Tighten mounting hardware if needed. | Every PM | MEC |
| Inspect sensor cable and connector for cuts, abrasion, pinching, or moisture intrusion. Verify connector is fully seated and locked. | Every PM | ELE |
| Clean the sensor face and target wheel of grease, dirt, and debris. Use a clean rag — avoid solvents on plastic sensor bodies unless approved. | Every PM | MEC |
| Inspect target wheel (toothed wheel, encoder disc, or magnet ring) for missing teeth, cracks, or debris buildup that could cause signal dropout. | Quarterly | MEC |
| Check all conduit fittings and cable management hardware securing the sensor wiring. Verify no movement or vibration-induced wear on cable routing. | Quarterly | ELE |
Mechanical Inspection
| Task | Freq | Type |
|---|---|---|
| Check sensor-to-target gap (air gap) using a feeler gauge or dial indicator. Verify gap is within manufacturer specification — typically 0.020"–0.060" for magnetic sensors. | Every PM | MEC |
Electrical Inspection
| Task | Freq | Type |
|---|---|---|
| Verify speed sensor output signal at the controller or drive display matches actual equipment speed. Confirm reading is stable — no erratic or zero readings at operating speed. | Every PM | ELE |
| Test speed sensor with a handheld tachometer or frequency meter and compare output to sensor signal. Flag any deviation greater than ±2%. | Semi-Annually | ELE |
Reference Checklist — Full Task Library
Visual Inspection
| Task | Freq | Type |
|---|---|---|
| Inspect speed sensor body and mounting bracket for physical damage, cracking, corrosion, or looseness. Tighten mounting hardware to manufacturer torque spec if accessible. | Every PM | MEC |
| Inspect sensor cable from sensor to junction box or panel for cuts, abrasion, pinching, UV degradation, or moisture entry. Replace damaged cable sections before next PM. | Every PM | ELE |
| Clean sensor face and target wheel of grease, metallic debris, dirt, and buildup. Metallic debris on an inductive sensor face can cause false triggering — ensure face is clear. | Every PM | MEC |
| Inspect target wheel (toothed wheel, encoder disc, or magnet ring) for missing or damaged teeth, cracks, runout, or eccentric wear. Replace if damage would cause signal errors. | Quarterly | MEC |
| Inspect all conduit, cable trays, and flexible conduit sections protecting sensor wiring. Verify no movement, vibration wear, or conduit looseness. Resecure any loose fittings. | Quarterly | ELE |
| Inspect and clean junction box interior associated with speed sensor wiring. Check for moisture, condensation, insects, or damaged terminal blocks. Verify all terminations are tight. | Annually | ELE |
Mechanical Inspection
| Task | Freq | Type |
|---|---|---|
| Check and record sensor-to-target air gap using a feeler gauge or dial indicator. Compare to manufacturer specification (typically 0.020"–0.060" for inductive/magnetic sensors). Adjust if outside spec. | Every PM | MEC |
| Verify spare speed sensor(s) of correct model and specification are stocked. Confirm spare sensor has been tested or is from a verified supplier. Update spare parts inventory record. | Annually | MEC |
Electrical Inspection
| Task | Freq | Type |
|---|---|---|
| Check connector seating, locking ring or latch, and sealing boot at the sensor connector. Clean contacts with electrical contact cleaner if corrosion is present. | Every PM | ELE |
| Verify speed sensor reading at the PLC, controller, or drive matches actual equipment speed measured with a calibrated handheld tachometer. Record both values. Flag deviation >±2%. | Every PM | ELE |
| Check sensor output signal waveform using a multimeter or oscilloscope at operating speed. Verify clean, consistent signal with no dropout, noise, or erratic pulses. Inductive sensors should show clean AC sine wave; Hall effect and proximity sensors should show clean square wave. | Quarterly | ELE |
| Confirm sensor cable shielding is terminated correctly at one end only (per installation design) to prevent ground loops. Verify shield drain wire connection at the panel or junction box. | Semi-Annually | ELE |
| Perform sensor output voltage or current check at full operating speed. Record measured output vs. expected output per sensor spec sheet. Flag values outside acceptable range. | Semi-Annually | ELE |
| Review speed sensor alarm and fault history in the controller or SCADA system. Identify any recurring fault codes, signal dropouts, or speed deviation faults since last PM. | Semi-Annually | ELE |
| Perform full end-to-end calibration verification — compare sensor output to a calibrated tachometer across three speed points (low, mid, full speed). Document results and update calibration record. | Annually | ELE |
Failure Modes This Checklist Targets
Air Gap Drift The gap between sensor and target changes over time from vibration, thermal cycling, or mounting hardware that loosens incrementally — eventually producing erratic or absent signal.
Cable and Connector Degradation Continuous vibration, heat, and exposure abrade cable jackets and loosen connectors. An intermittent connection produces exactly the kind of phantom signal dropout that takes hours to diagnose.
Target Wheel Damage A missing tooth, crack, or debris buildup on the target wheel creates missing pulses in the output signal — which the drive or PLC may interpret as underspeed, overspeed, or total signal loss.
Inductive Sensor Face Contamination Metallic debris attracted to an inductive sensor face generates false pulses. The sensor reads shaft movement that isn't happening. The control system responds to data that isn't real.
Shield Ground Loop Incorrectly terminated cable shielding creates ground loops that inject noise into the sensor signal. Noise at the input level produces speed readings that oscillate, drift, or spike — none of which reflects actual shaft speed.
Calibration Drift Sensors that have never been verified against a known reference can be wrong from day one — or drift wrong gradually. Either way, the control system makes decisions based on a number nobody has confirmed.