Why is my boat fridge not cooling, and what should I troubleshoot first?
My 12V boat fridge has stopped getting properly cold, although the interior light still works. It sometimes hums briefly and stops, and seems better on shore power. What should I check before calling a refrigeration engineer?
Answer
First, measure the voltage at the fridge's power terminals while the compressor tries to start; a working interior light does not prove the supply can run the compressor. Then check condenser ventilation, thermostat settings, ice build-up and door seals before suspecting a refrigerant leak.
1. Check voltage under load
Most permanently installed yacht fridges use a 12V or 24V compressor, commonly a Danfoss/Secop unit fitted to brands such as Isotherm, Dometic and Frigoboat. Read the identification plate first. Portable thermoelectric coolboxes and gas absorption fridges need different checks.
Isolate DC and shore power before cleaning terminals, changing wiring or removing electrical covers. Leave mains-voltage testing to someone competent.
- Check the fridge breaker, fuse and battery isolator. Replace a failed fuse only with the specified rating; if it blows again, investigate the fault.
- Turn the thermostat towards colder and measure DC voltage across the compressor controller's positive and negative supply terminals during an attempted start.
- Compare that reading with voltage measured directly across the battery posts under the same conditions.
If battery voltage stays healthy but controller voltage drops sharply, inspect both positive and negative cables, fuse holders, switches, plugs and crimps. Green corrosion, loose connections and undersized cable are common causes. A difference approaching 0.5V on a 12V installation deserves investigation.
Many Secop BD35F/BD50F controllers have a standard 12V battery cut-out around 10.4V and restart around 11.7V, though external resistors and manufacturer settings can change these figures. These are protection thresholds, not desirable operating voltages. A fridge that works on shore charging but fails at anchor strongly suggests a battery or supply problem.
2. Listen to the compressor
Listen at the compressor compartment while the thermostat is calling for cooling. A brief hum followed by silence tells a different story from a compressor running continuously.
- Completely silent: check supply, thermostat demand and controller indications. The interior lamp may have a separate circuit.
- Repeated start attempts: check voltage during each attempt first, then inspect the fan and controller fault indication.
- Runs continuously but barely cools: inspect heat rejection, frost and door sealing before considering a refrigeration fault.
- Stops while the cabinet remains warm: check thermostat operation and whether its sensor has become displaced.
Some Secop installations have a diagnostic LED that flashes a repeating code. On common BD controllers, one flash indicates battery protection, two indicate fan overcurrent and three indicate a motor-start error. Confirm the codes in the exact controller manual. A three-flash code alone does not prove the compressor needs replacing.