Troubleshooting 12/24 volt Danfoss BD compressors.
If compressor runs continuously with less than desirable or no cooling
there is a refrigerant flow problem or loss of refrigerant or poor
condenser cooling. If at least 90% of evaporator’s surface area is
covered with a layer of frost no matter how thin refrigerant quality
and volume are correct. To confirm compressor runs continuously remove
thermostat wires from control module and place jumper wire across C
and T control module terminals.
If Danfoss compressor fails to run continuously this is caused 98% of
the time by an electrical problem.
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 ½ 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.