Electrical4 min
Contactor and voltage drop
Pitted contacts look closed and still starve the compressor. Measure under load.
24V side
- Call for cool. Coil should be 20–28 VAC. Chatter is low voltage, bad common, or a failing coil.
- If the thermostat is calling and the coil is dark, chase R, Y, and C — not the compressor yet.
Line side under load
- Measure L1 to T1 and L2 to T2 with the compressor running. More than about 1 VAC drop across a pole is a bad face.
- Line voltage at the disconnect vs at the compressor under load tells you lug, whip, and contactor losses.
- Single-phase brownout: many compressors want 197 VAC minimum under load — check the nameplate.
Replace notes
- Match FLA / inductive rating, not just ‘30 A’. A 1-pole vs 2-pole swap can leave a leg hot — don’t.
- Dress the new lugs. A loose T2 is a callback fire.