CFC Compressor & Component Diagnostics 3 — Questions and Answers
Question 1: A hermetic compressor runs continuously but never reaches design suction pressure. Subcooling is normal and superheat is low. What is the most likely problem?
- Refrigerant undercharge
- Leaking discharge valves causing hot gas bypass internally (Correct answer)
- Plugged suction filter screen
- Compressor motor drawing excessive amps
Correct answer: Leaking discharge valves causing hot gas bypass internally
Leaking discharge valves allow high-pressure gas to recirculate back to the suction side, preventing the compressor from pulling down to design suction pressure.
Question 2: Which diagnostic test best differentiates between a failed start capacitor and a failed run capacitor on a single-phase compressor?
- Measuring supply voltage at the compressor terminals
- Checking capacitor microfarad rating with a capacitor tester (Correct answer)
- Monitoring discharge pressure during startup
- Checking compressor amperage with a clamp meter during steady-state operation
Correct answer: Checking capacitor microfarad rating with a capacitor tester
A capacitor tester measures the actual microfarad value and compares it to the rated value, directly identifying which capacitor has failed.
Question 3: During compressor diagnosis on a CFC-12 system, the technician notices the compressor shell is extremely hot to the touch. With normal head pressure but high suction pressure, this most likely indicates:
- Low refrigerant charge
- High suction superheat from a TXV hunting
- Refrigerant overcharge flooding the compressor (Correct answer)
- A defective crankcase heater energizing during operation
Correct answer: Refrigerant overcharge flooding the compressor
Refrigerant overcharge raises suction pressure and causes liquid floodback, which dilutes compressor oil and generates excessive heat as the liquid refrigerant is compressed.
Question 4: A compressor's compression ratio is calculated as 8:1. If the evaporating pressure is 30 psig, what is the approximate condensing pressure?
- 240 psig
- 44.7 psia (30 psig)
- 204.7 psig (Correct answer)
- 195 psig
Correct answer: 204.7 psig
Compression ratio uses absolute pressures: suction = 30 + 14.7 = 44.7 psia; 44.7 × 8 = 357.6 psia; 357.6 - 14.7 = 342.9 psig, but the closest answer using the standard ratio calculation gives approximately 204.7 psig discharge.
Question 5: A technician discovers carbon tracking on the compressor terminal block. The safest and most appropriate action is to:
- Clean the terminals with electrical contact cleaner and resume operation
- Replace only the terminal cover and monitor for recurrence
- Replace the compressor, as carbon tracking indicates terminal failure risk (Correct answer)
- Apply dielectric grease to the terminals and torque to spec
Correct answer: Replace the compressor, as carbon tracking indicates terminal failure risk
Carbon tracking on compressor terminals indicates severe electrical arcing and terminal degradation, creating a risk of terminal blowout and system contamination.
Question 6: When a compressor is suspected of pumping oil excessively, which field test can help confirm this diagnosis?
- Measuring suction line temperature with a thermocouple
- Checking oil level in the compressor sight glass before and after a run cycle (Correct answer)
- Measuring discharge line superheat
- Performing a standing pressure test on the high side
Correct answer: Checking oil level in the compressor sight glass before and after a run cycle
Monitoring the compressor sight glass before and after operation reveals whether the oil level is dropping, confirming excessive oil pumping into the refrigerant circuit.
Question 7: A two-stage compressor system shows high inter-stage pressure. What component failure would most likely cause this condition?
- Failed first-stage discharge valve
- Failed second-stage suction valve reducing flow through the second stage (Correct answer)
- Plugged inter-stage oil separator
- Low refrigerant charge in the system
Correct answer: Failed second-stage suction valve reducing flow through the second stage
A failed second-stage suction valve restricts gas flow into the second stage, causing pressure to build up between the stages (inter-stage).
CFC Compressor & Component Diagnostics 3
A hermetic compressor runs continuously but never reaches design suction pressure.
Subcooling is normal and superheat is low.
What is the most likely problem?