Outdoor Unit Will Not Start
A structured electrical walk through when the condensing unit stays silent on a call for cooling.
Safety
Symptom
Symptom
Thermostat is calling for cooling, the indoor blower runs, and the outdoor unit does nothing.
Symptom
Or the outdoor fan runs while the compressor only hums and then stops.
Before you start
Work the electrical side safely
Possible causes
- No line voltage at the unit: tripped breaker, blown fuses in the disconnect, or a failed disconnect.
- No 24 volt call arriving at the contactor coil.
- Contactor coil open, or contacts pitted and not passing power.
- Failed run capacitor.
- Open high or low pressure safety, or an open float switch interrupting the call.
- Compressor internal overload open from a previous trip.
- Failed compressor or condenser fan motor.
What to check, in order
Line voltage at the load side of the disconnect
Missing voltage means the fault is in the branch circuit, breaker or fuses, not the unit
24 volts across the contactor coil during a call
Present means the indoor side and safeties are satisfied. Absent means work back through the safety string and the indoor board
Whether the contactor is physically pulled in
Coil energized but not pulled in means a failed coil or a mechanically stuck contactor
Voltage across the closed contactor contacts
Significant voltage across closed contacts means burned contacts that cannot carry load current
Run capacitor value after discharging
Out of tolerance is a very common cause of a humming compressor or a stationary fan
Each pressure safety and the condensate float switch
An open safety is reporting a real condition such as low charge, a dirty condenser or a blocked drain
Compressor and fan motor windings and amp draw
Locked rotor current with no rotation, or open windings, confirms motor failure
Do not jumper a safety to force a start
Next step
Once you identify the failed device, confirm why it failed. A burned contactor often means high current from a struggling compressor or a dirty condenser. A tripped high pressure switch means the system could not reject heat. Correct the underlying condition, then verify operation with pressures, superheat or subcooling, and amp draw before leaving.
Tools involved
- Multimeter
- Clamp ammeter with inrush
- Capacitance meter
- Digital manifold
- Lockout and tagout devices
Symptoms
Practice & Mastery
- Outdoor Unit Will Not Start Troubleshooting Scenarios2
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