There are two different series of Danfoss BD 12 volt compressors the older fixed speed and the newer variable speed . If the refrigeration unit is over 10 years old and has a Danfoss BD 2 or BD2.5 or BD3 compressor then it has the older fixed speed discontinued electronic 4 pin module. Troubleshooting Danfoss compressors with 4 pin modules consists of the following steps:

1. All of these compressors have a 4 pin module connector and their modules contain a replaceable fuse. If this fuse is blown there are two reasons why either power wires to module are reversed or module has an internal failure.

2. Check to see that there is actually power at the refrigerator control module.

3. Place jumper wire across thermostat terminals on electronic module, Compressor still does not run go to next step.

4. Disconnect black fan wire from electronic module, Compressor runs, replace fan. Compressor still does not run after fan ground wire is disconnected, go to next step.

5. Run correct size and correct polarity jumper wires direct from a fully charged battery in order to bypass all boat’s wiring. Volt meter readings are of no value when looking for voltage spikes. Compressor still does not run electronic module needs to be removed and tested on another unit. If there are no other units available to test your module on I will test all 12 volt Danfoss control modules free except for BD80 compressor modules. Email me for shipping address and details.

Small 12/24 volt boat refrigeration units using Danfoss BD compressors manufactured after 1996 will have a BD 35 or BD 50 variable speed compressor with a troubleshooting computer chip built into control module. This circuit makes them easier to find troubled area if compressor fails to run. If this unit does not have this $2 LED install one. Without the LED on these new units troubleshooting will be the same as earlier 4 pin Danfoss BD compressors.

Trouble shooting LED will only flash if electronic module sees a compressor problem. In each case problems of compressor’s failures to run are identified by counting number of flashes of LED:
• No LED flashes would indicate either thermostat is open or no power to module.
• One LED flash and a 4 second pause indicates a non Danfoss wiring electrical resistance problem or low batteries. Because of module’s sensitiveity to milliseconds of a voltage spike they cannot be detected by a voltmeter. Solution is to bypass power wiring till resistance problem is located.
• Two LED flashes indicates fan over current cutout. If fan circuit on these variable speed compressors exceeds ˝ amp compressor start up will be aborted. This condition can be confirmed by disconnecting Black fan wire at module, if compressor runs replace fan.
• Three LED flashes indicate excessive torque is required to start compressor. This is commonly caused by turning compressor off and back on too quickly or too much refrigerant or poor condenser cooling. Most people jump to the conclusion that there is a mechanical rotor lock up inside compressor and this is a mistake on Danfoss BD compressors.
• Four LED flashes indicate compressor motor not reaching sustained controlling speed above 1,850 rpm quick enough.

If someone has tampered with refrigerant by connecting gauges to a system letting air in or adding too much refrigerant can cause either a Three or Four LED flashing signal. On water cooled Danfoss condenser systems three and four LED signals are common when seawater gets into refrigerant circuit.