CFC Compressor & Component Diagnostics 2 — Questions and Answers
Question 1: A scroll compressor exhibits abnormal noise during startup but runs quietly at full speed. What is the most likely cause?
- Worn scroll tip seals
- Loose mounting bolts causing resonance at startup RPM
- Refrigerant slugging due to liquid floodback (Correct answer)
- Overcharged system reducing head pressure
Correct answer: Refrigerant slugging due to liquid floodback
Liquid refrigerant entering a scroll compressor during startup causes slugging noise as the incompressible fluid damages scroll wraps.
Question 2: When diagnosing a reciprocating compressor with low suction pressure and low discharge pressure, which condition is most likely?
- Refrigerant overcharge
- Worn or broken valve reeds (Correct answer)
- Plugged liquid line filter-drier
- High ambient temperature
Correct answer: Worn or broken valve reeds
Broken or worn valve reeds cause low compression efficiency, resulting in both low suction and low discharge pressure simultaneously.
Question 3: A technician measures a compressor's amp draw at 85% of the rated full-load amps with normal pressures. This indicates:
- The compressor is undercharged with refrigerant
- Normal operation within acceptable tolerance (Correct answer)
- The compressor motor windings are partially grounded
- A failing capacitor on a single-phase motor
Correct answer: Normal operation within acceptable tolerance
Amp draw at 85% of rated FLA with normal pressures indicates the compressor is operating normally within standard tolerance bands.
Question 4: What does a high superheat reading at the compressor suction combined with normal subcooling at the condenser outlet most likely indicate?
- Refrigerant overcharge
- TXV or metering device starving the evaporator (Correct answer)
- Condenser fan motor failure
- Compressor valve failure
Correct answer: TXV or metering device starving the evaporator
High superheat with normal subcooling indicates insufficient refrigerant flow through the metering device, starving the evaporator of refrigerant.
Question 5: A compressor trips on its internal overload repeatedly on hot days but resets overnight. The most probable diagnosis is:
- Refrigerant undercharge causing low suction pressure
- High ambient temperature causing high head pressure overloading the motor (Correct answer)
- A faulty crankcase heater allowing refrigerant migration
- Low voltage to the unit causing high amp draw
Correct answer: High ambient temperature causing high head pressure overloading the motor
High ambient temperatures elevate condensing pressure, increasing compressor motor load and amp draw until the overload trips.
Question 6: When performing a megohm test on a compressor motor, a reading of 0.5 megohms indicates:
- Excellent motor insulation
- Acceptable insulation at normal operating temperature
- Marginal insulation that may indicate moisture or contamination (Correct answer)
- A definitive open winding requiring compressor replacement
Correct answer: Marginal insulation that may indicate moisture or contamination
A megohm reading of 0.5 MΩ indicates marginal insulation resistance, suggesting moisture contamination or degrading motor insulation.
Question 7: A technician finds oil logged in the evaporator of a CFC system. Which compressor condition most commonly causes this problem?
- Compressor running at correct speed but low oil viscosity
- Excessive refrigerant velocity carrying oil out of the compressor
- Worn compressor rings allowing oil to bypass into the refrigerant circuit (Correct answer)
- Oil separator failure allowing oil into the discharge line
Correct answer: Worn compressor rings allowing oil to bypass into the refrigerant circuit
Worn piston rings in a reciprocating compressor allow excessive oil to pass into the refrigerant stream, resulting in oil accumulation in the evaporator.
CFC Compressor & Component Diagnostics 2
A scroll compressor exhibits abnormal noise during startup but runs quietly at full speed.
What is the most likely cause?