Fridge trouble!

My Danfoss fridge compressor with Isotherm controls threw a fault and I found this on the controller and starter 

Bit of a worry!! I'll update once I get to the bottom of why the negative terminal has melted.

Found the problem.  Opened up the red controller to find the negative terminal broken off at the base (top spade terminal in photo...broken nearest to camera).  Shorting out.

Vibration over the years I assume

Update 2/6/21:

As usual I have spent an inordinate amount of time thinking about this problem, and I've become something more than a layman on the topic of marine refrigeration as a result.  I suppose these sort of challenges give me the opportunity to increase my knowledge base.

The solution to the problem begins with an explanation of how a marine fridge works.  The system is made up of at least 3 components:

1. The compressor - this is the metal canister from which the copper tubing carrying the refrigerant is carried to the 'icebox'.  They can be fan cooled or cooled by a water pump or a seawater/ self-pumping system integrated into a through hull (this is what Wireless has).

2. The control module - a device which controllers the compressor pump.  They receive inputs from the thermostat.  They can be DC only or AC/DC (more on this later...it is much larger).  The module is normally attached to the compressor.

3. The thermostat - this device varies from a simple mechanical dial that measures the icebox temperature and signals the control module to turn off and on as required, to a much more complex digital device that adds features like controlling compressor speed, durations, and monitors battery levels/charging.

So with a damaged module and a damaged ASU thermostat unit, I have elected to replace both.  Note the need to install a resistor in the feed from the thermostat to the module to control the compressor speed (the ASU units do this automatically but at a cost).  My self-pumping cooling system likes the compressor running at 3000rpm, so the resistor is selected to achieve this value (692ohm)

I will be installing the Elitech digital thermostat shown above.  This will be installed in the vertical face above and to the left of the hatch under the sink.  It will require a good sized cut-out, so I'll measure twice.  The unit is basic, without the Isotherm ASU battery-monitoring feature, but it's cheap, and has a nice display of fridge temperature (the ASU didn't).

This wiring diagram better describes how to configure a digital thermometer (which requires as 12v supply) into the system.  The lettering corresponds to the lettered terminals on the Danfoss module.

Update 17/6/21

So the module, Elitech thermostat and resistor have arrived.  I quickly realised the much larger AC/DC module could not be directly coupled to the compressor as was the previous unit.   The difference in size is significant,  2 5/8" deep versus 4 3/4".  I consulted with Sean at FridgeTech NZ and he has supplied me with an extension cable that will allow me to mount the module elsewhere under the galley sink.

Extension cable to mount module remotely

I will likely mount the module on the plywood wall that forms the starboard side of the drawer structure.  all of the wiring, both AC/DC and links to the thermostat will lead to this point.

Here are the connectors I use, all waterproof, self-sealing units:

Update:  Installation Day 1

While the DC power is already 'onsite' at the fridge, there is no AC power for the new AC/DC module.  I have a spare circuit breaker on the AC panel, whish was once used for the air conditioner (long ago removed).

I started with a 7m length of 3 core, 20amp cable, feeding it down under the sink floor.  I pulled up the floor, and ran the cable all the way aft to the engine panel adjacent the chart table.  The cable feeds through holes drilled in the ribs.

Once under the engine panel, the cable goes right, through the panel near the diesel filter, through the locker, through the bulkhead under the chart table, up and back into the electrical cabinet.  This is a pretty major undertaking.  I left the wire loose in the cabinet for now.

I also ran the new temperature probe from the sink locker into the fridge box itself, creating a passage through a small opening filled with expanding foam.  I will need to spray some foam back into this cavity once I have the wiring completed.

The digital controller was mounted on the cabinet face below and to the right of the sink.  This required covering an area with masking tape, tracing the outline of the controller with pencil, and cutting out the opening using a drill and a jigsaw.

Once the extension cable to connect the module to the compressor arrives, I will begin wiring up the entire system.  This will include the AC connections at both ends, the DC connection at the module, 2 set of wires from the module to the digital controller, and the temp probe (see diagram above).

If you're going to do a lot of wiring, get good crimpers and a wire stripper, and use the best quality connectors you can find.  I make sure to use a heat gun instead of a torch to seal the connectors.  The digital thermostat has the type of inputs that require the wire ends to be twisted and soldered.

Update:

The extension arrived,  and with a further 3hrs of wiring the fridge is quietly humming away.

It was not without problems as I discovered when I fired the system up for the first time only to be met with an E1 error messag.  This was certainly disappointing given the care I took when installing both the AC and DC wiring. In the end the solution was simple.

While I had been told that the temperature probe, a 2 wire cable, could be wired either way, this is not the case.  It only works if you get the wires the correct way round.  Trouble is you cant tell...  they are both black, and without markings.

If you are considering this setup, with a digital thermostat, make sure you have a jewellers flat head screwdriver...very small screws.

Fridge compressors like these are very quiet, so if you hear a noise or vibration, ensure the cooper tubes aren't touching. 

Exciting, I have a functional fridge again!