What happens
Damaged contact surfaces introduce resistance into the motor supply.
Typical symptoms
Contactor pulls in but motors hum, start poorly or receive reduced voltage under load.
Checks that distinguish this cause
A qualified technician compares line and load voltage across each pole under the appropriate operating load. Inspect isolated contacts for damage; no-load continuity can miss resistance.
What can look similar
A failing disconnect, supply leg or motor harness can create a similar voltage drop elsewhere.
Repair direction
Replace the approved contactor and evaluate excessive cycling or poor coil pickup that may have damaged it.
Verify the repair
Confirm acceptable load voltage, starts and contact operation.
Equipment-specific limits
Coil voltage and contact arrangement vary. A click proves movement, not that every power pole conducts correctly.
Practice this diagnosis
These simulated calls exercise this component or repair family. The article describes a broader diagnostic process; a call may represent a different cause within that family.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- Fluke: HVAC and industrial electrical testing applicationsElectrical measurement principles, including control-circuit voltage references.
- Fluke: HVAC multimeter troubleshooting guideIntroductory meter applications. Actual equipment ratings take precedence over generic examples.
Editorial review: 2026-09-29. Verify the current manual for the installed model.