Compressor Won't Start — No Response to Start Signal
A field reference for compressor faults — what causes it, how to confirm it on the unit, how to repair it, and what fails next if you leave it. Written for working HVAC & refrigeration technicians.
How to confirm it on site
- Capacitor measurement ±10% off → bad
- Relay contact — click+continuity test
- Winding resistance ±10% or open
- Supply voltage 3-phase balance, ≥90% rated
- Internal thermostat — auto-reset after cooling
- Humming without rotation → insufficient start torque
Root causes
- Start capacitor failure
- Start relay (PTC/current relay) failure
- Run capacitor failure (single-phase)
- Winding open — motor burnout
- Power issue — breaker, phase loss, low voltage
- Internal protection trip not reset
- Insufficient pressure equalization (short-cycling)
If you leave it unrepaired
- No compressor action on start signal → immediate shutdown
- Start cap/relay damaged from repeated attempts
- Breaker/overcurrent trips frequently
- Zero cooling — secondary damage (food spoilage, production stop)
- Replacing compressor without root cause: recurrence
How to fix it
- Sequence: start cap → run cap → relay → winding
- Eliminate power-side issues first
- If internal trip: cool 30 min then retry
- If winding suspect: megger test → <1MΩ replace compressor
- Prevention: voltage monitor + anti-short-cycle timer
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What causes compressor won't start — no response to start signal?
Start capacitor failure Start relay (PTC/current relay) failure Run capacitor failure (single-phase) Winding open — motor burnout Power issue — breaker, phase loss, low voltage Internal protection trip not reset Insufficient pressure equalization (short-cycling)
How do I diagnose compressor won't start — no response to start signal on site?
Capacitor measurement ±10% off → bad Relay contact — click+continuity test Winding resistance ±10% or open Supply voltage 3-phase balance, ≥90% rated Internal thermostat — auto-reset after cooling Humming without rotation → insufficient start torque
How do I fix compressor won't start — no response to start signal?
Sequence: start cap → run cap → relay → winding Eliminate power-side issues first If internal trip: cool 30 min then retry If winding suspect: megger test → <1MΩ replace compressor Prevention: voltage monitor + anti-short-cycle timer
What happens if compressor won't start — no response to start signal is left unrepaired?
No compressor action on start signal → immediate shutdown Start cap/relay damaged from repeated attempts Breaker/overcurrent trips frequently Zero cooling — secondary damage (food spoilage, production stop) Replacing compressor without root cause: recurrence