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
3: Unit runs, inadequate cooling
Minor Symptom Problem Test Solution
Room or product warm

Lack of refrigerant

Check for bubbles in sight glass

Repair leak and recharge


High suction pressure and low head pressure

Leaking valves in compressor head, or blown head gasket

Test pressures with gauge manifold; pump down, remove head and check gasket and valves

Replace faulty parts, reassemble and recharge

High-pressure cutout locks out, does not automatically reset

Air- or water-cooled condener

Low air flow, low water flow

Check fan belt and motor; check water pump

Repair or replace faulty parts; clean tubes with acid



Non-condensables (air) in refrigerant tubing

Check for air

Purge air from system


". Refrigerant leaving condenser too hot

Overcharge of refrigerant

Check level of liquid refrigerant in condenser with back of hand (area with liquid refrigerant will be cool, area with vapor will be warm). High level indicates excess refrigerant

Remove excess refrigerant


Water leaving condenser too cool

Fouled or limed condenser tubes; or water pump inoperative

Compare incoming and outgoing water temperatures; check for poor contact between water and refrigerant due to fouling or liming; check pump and valve operation

Clean tubes with brush or acid; repair or replace pump or valve

Low head pressure

Leaving condenser water is cold

Water valve stuck open

Check leaving condenser water flow and temperture

Repair water valve

Bubbles in sight glass

Low charge

Check sight glass

Locate leak, repair, recharge

Suction pressure rises fast on shut-down

Discharge valves leaking back.

Check valves with gauge manifold; pump down, pull head, inspect valves and head gasket

Fix problem found

High suction pressure while running (high side to low side)

Internal relief open or blown; head gasket

Install gauge manifold and try to pump down, if unable, pull head to inspect

Fix problem found

High suction pressure

Compressor runs continuously

Large load on evaporator

Check if load is abnormally large; system may be undersized

May be a normal reaction to large cooling load; if it ain't broke…

Suction line too cold

Thermostatic expansion valve (TXV) overfeeding or oversize

Check superheat and sensing bulb placement; check valve rating

Adjust superheat; repair or replace valve with proper size; reattach bulb securely

Compressor noisy

Broken valves

Pump down, pull head, check valves. Check for floodback, check temperature of multiple heads; cooler heads have the broken valves

Repair or replace as indicated; check metering device and superheat to correct floodback before restarting

Inadequate cooling

Compressor does not pump because unloader is stuck

Check oil pump discharge pressure, unloader oil pressure, assembly solenoid, unloader valve filter, compare full-load amps (from nameplate) to actual amp draw.

Fix problem found

Low suction pressure

Bubbles in sight glass

Lack of refrigerant

Check sight glass

Find leak, repair, recharge


Unloaders not working due to dirty screen or wrong setting

Check screen and setting

Correct or consult manufacturer's information

Temperature drop at filter-drier

Plugged filter-drier

Compare temperature on each side of filter-drier

Pump down system, clean and recharge

Temperature drop at solenoid valve

Valve not fully open

Compare temperature on each side of valve

Pump down system, dismantle, and clean valve, recharge

Low flow through TXV

Power element dead; inlet screen partly plugged

Check power element; check superheat; check screen

Pump down, replace power element; clean screen in TXV