Sub-Zero Refrigerator Loud Bang When Compressor Stops:... | appliancefaults.com

Technical Background:

The MCU (Microcontroller Unit) controls the compressor via relays. When the setpoint temperature is reached, the MCU de-energizes the compressor relay, cutting power to the compressor. A sudden stop can cause mechanical stress in the compressor and surrounding components. Voltage paths are 120V AC from the breaker to the control board and then to the compressor relay.

Symptoms:

The homeowner reports a loud 'bang' or 'thump' noise emanating from the Sub-Zero refrigerator shortly after the compressor shuts off. The refrigerator otherwise seems to be cooling normally.

Possible Causes:

Faulty compressor start relay or capacitor
Worn compressor mounting grommets or springs
Refrigerant floodback or liquid slugging
Valve plate issue inside the compressor
Defective defrost termination thermostat allowing excessive ice buildup
Loose refrigerant lines

Solution:

1

Disconnect power to the Sub-Zero at the breaker panel (LOTO).
2

Access the compressor compartment, typically at the bottom rear of the unit. You may need a 1/4-inch nut driver to remove access panels.
3

Inspect the compressor start relay and capacitor. Use a multimeter to check the capacitor for proper capacitance (in microfarads). Replace if out of spec.
4

Check the compressor mounting grommets for wear or damage. Replace as needed.
5

If accessible, use a manifold gauge set to check refrigerant pressures (high and low side) at startup and shutdown. Note if pressures are excessively high or fluctuate wildly.
6

Use a non-contact infrared thermometer to check the compressor shell temperature during operation. Overheating could indicate internal compressor issues.
7

If refrigerant floodback is suspected, consult a certified refrigeration technician for proper system evacuation and recharge.

Safety Warning:

Always disconnect power to the appliance at the breaker panel before performing any diagnostics or repairs. Lock Out / Tag Out (LOTO) procedures must be followed. Refrigerant systems contain high pressure and should only be serviced by qualified technicians. High voltage is present even with the unit off; capacitors can hold a charge.

Prevention:

Schedule regular maintenance checks, including cleaning the condenser coils (located at the bottom or back of the unit) every 6 months. Ensure proper ventilation around the refrigerator. Replace the start relay/capacitor preventatively every 5 years.

Expert Tip:

Some Sub-Zero models have a built-in service mode. To enter service mode, simultaneously press and hold the 'LIGHT' and 'FREEZER DOWN' buttons for 5 seconds. This allows you to manually cycle components and check for proper operation. This is model dependent; consult the service manual for your specific model.

Frequently Asked Questions

•What is the proper capacitance range for the compressor start capacitor on my Sub-Zero model?

The capacitor value is typically printed on the capacitor itself. Consult the Sub-Zero service manual for your specific model number for the exact specifications.

•How do I properly discharge a capacitor before handling it?

Use a resistor (approximately 10,000 ohms, 2 watts) with insulated leads. Connect the resistor leads to the capacitor terminals for several seconds to safely discharge the capacitor.

•What are the typical refrigerant pressures I should expect to see on a Sub-Zero refrigerator?

Refrigerant pressures vary depending on the refrigerant type and ambient temperature. Consult the unit's service manual or the information sticker located on the unit for the correct pressure ranges. R134a systems typically run lower pressures than older R12 systems.

Sub-Zero Refrigerator Loud Bang When Compressor Stops: Troubleshooting Guide

Technical Background:

Symptoms:

Possible Causes:

Solution:

Safety Warning:

Prevention:

Expert Tip:

Frequently Asked Questions

•What is the proper capacitance range for the compressor start capacitor on my Sub-Zero model?

•How do I properly discharge a capacitor before handling it?

•What are the typical refrigerant pressures I should expect to see on a Sub-Zero refrigerator?

Related questions

Technical Background:

Symptoms:

Possible Causes:

Solution:

Safety Warning:

Prevention:

Expert Tip: