Troubleshooting Vapor-Compression (Mechanical)
Refrigeration & Air-Conditioning Systems

From Air Conditioning & Refrigeration Toolbox Manual, David J. Tenenbaum, 2nd edition, Jan. 2006, ISBN 0-7689-2235-6, Thomson Peterson's, $14.99 Sample pages
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Start at primary problem:
1: Unit fails to start
3: Unit runs, inadequate cooling
2: Unit runs erratically 4: Compressor loses oil
1: Unit fails to start





No hum

No power at motor or relay

Open switch or blown fuse

Test across line side of starter to ground with volt meter

Find out why protector is open and close it, replace fuse


Motor, starter, or relay getting power, but contacts in starter or relay are open

Open motor control–no signal to coil

Test holding coil with voltmeter (relay is normally open), if coil is not powered, test through control circuit with continuity tester or voltmeter

Isolate problem and correct



Open relay with energized holding coil

Test through holding coil with continuity tester or voltmeter

Replace holding coil


Contacts closed

Burned starter contacts

Check load side of starter with voltmeter, check contacts with ohmmeter for low resistance

Replace faulty starter if resistance is high



Open windings in motor

Check motor continuity with voltmeter

Replace faulty motor

Motor hums

Any motor

Shorts or grounds in winding

Check compressor windings with ohmmeter

Replace motor



Open winding

Test with voltmeter

Replace compressor and/or motor


Three-phase motor

Compressor is stuck

Check oil level in crankcase with sight glass or dipstick, try to rock compressor manually

If good, reverse any 2 power wires to reverse rotation and/or rebuild compressor


Single-phase, capacitor start and run.

Compressor is stuck or start capacitor is bad

Check oil level in crankcase with sight glass or dipstick, try to rock compressor manually, check start and run capacitors

Replace capacitors if needed, try a hard-start kit, rebuild compressor


Single-phase motor, permanent split capacitor

Compressor is stuck

Inspect starting relay visually or test with voltmeter

Replace starting relay


 Single-phase, capacitor start and run, or permanent split capacitor

Bad capacitor

Test with capacitor checker

Replace capacitor

Compressor is stuck, motor tests OK

Refrigerant pressure not balancing in off-cycle

Check with gauge manifold

Low-pressure cut-out control set too close together

Expand differential; check metering device; replace plugged capillary tube or valve to balance pressure