Measuring voltage across a closed contactor (line side to load side) under load should be approximately:

Most HVAC electrical problems are “no power,” “open safety,” “bad control signal,” or “failed load.” Use a meter and a simple logic path: Verify line voltage → verify control voltage → verify call signal → follow the circuit through safeties → confirm contactor/relay action → confirm motor/compressor current draw. Key keywords: • Line voltage (120/208/240/480V) vs control voltage (often 24VAC) • Contactor coil, transformer, fuse, breaker, float switch, pressure switches • Open circuit vs short vs ground fault • Voltage drop under load • Continuity (power OFF) for checking open... Show more

Most HVAC electrical problems are “no power,” “open safety,” “bad control signal,” or “failed load.”

Use a meter and a simple logic path: Verify line voltage → verify control voltage → verify call signal → follow the circuit through safeties → confirm contactor/relay action → confirm motor/compressor current draw.

Key keywords:
• Line voltage (120/208/240/480V) vs control voltage (often 24VAC)
• Contactor coil, transformer, fuse, breaker, float switch, pressure switches
• Open circuit vs short vs ground fault
• Voltage drop under load
• Continuity (power OFF) for checking open components

Worked example (common “no cool”):
Thermostat calls for cooling → 24V should energize contactor coil.
If 24V at thermostat but 0V at contactor coil, trace through safeties (float switch, pressure switch) to find the open.

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