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Old 27-05-2019, 10:14   #1
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Danfoss/Isotherm fridge cycling

Hi all,

I have a Danfoss/Isotherm air-cooled icebox conversion system, which i think is pretty common. It has worked fine for the 5 years that i've owned the boat, and for a long time before that as far as I can tell, but recently it has begun acting up.

I have been searching the forums and reading Richard Kollmann's website and posts here, but I don't see anything that matches this exact problem. I am wondering if it is a problem with the thermostat or with the controller perhaps.

Symptoms: quick-cycles when it reaches the set temperature.

IE: everything seems fine, but when it hits the set-temperature, it cycles on-and-off every 30-seconds to 4 minutes or so (seems like random timing) - it will start up and run, then shut off abruptly, then start up immediately again, then shut off, etc. If the thermostat is turned to a higher number, it runs longer, makes the box colder, and then cycles quickly again. Set it to a lower number?
It appears as though whatever tells it to wait a little while once reaching the set temperature is no longer waiting, just trying to always hold it at exactly the right temperature?

problems with the installation
  1. The thermostat is installed _outside_ of the icebox - normally it shoud be inside, correct?
  2. the thermostat sense wire was not mounted directly on the evaporator - it was mounted on the plastic "back" of the evaporator... moving it to be in direct contact with the evaporator has not caused any change
  3. the insulation isn't very thick, particularly in the lid
  4. the batteries are very weak, but i have the same problem on shorepower, I don't see the "low battery" code flash from the Danfoss controller, and it runs fine if I jumper around the thermostat - it's happy running, just having trouble choosing when to run and not run...
  5. the sense tube was touching the evaporator in one place, i imagine it has for years without any problem, but i've re-done the routing so they can't touch anymore, no change.

i have tried:
  1. Install LED on + & D terminals (no errors indicated)
  2. Jumper past the thermostat using C & T terminals (appears to run fine by taking the thermostat out of the loop) - it'll run and run and run without shutting off
  3. disconnecting the fan (no change if the thermostat is connected but the fan isn't - still cycles)
  4. running on shorepower - no change
  5. connecting the sense tube directly to the evaporator instead of to the plastic back of the "o" shaped evaporator - no change
  6. made sure the sense tube can't touch an evaporator line before it touches the evaporator - no change

Thank you all very much for any help
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Old 04-06-2019, 08:41   #2
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Re: Danfoss/Isotherm fridge cycling

The trouble LED will never flash with Thermostat troubles. You have confirmed that your problem is caused by either the thermostat or bad connection on the two electrical wire between module and thermostat with jumper wire across C and T terminals test.

The electrical power between C and T terminals to run compressor is less than power of a small flash light one to five Ma. Isotherm uses four different types of thermostats Mechanical Snap action, Auto Speed Up, Energy control and Digital. If your unit is a standard Snap action switch the best way to rule out thermostat and test its wiring is to disconnect the wires at thermostat and connect them to gather, If compressor runs continuous thermostat is bad. On this type thermostat evaporator temperature is monitored and not box temperature. The amount of thermostat capillary tube in contact with evaporator is how thermostat is calibrated. Most of these thermostats do not have a differential off on adjustment. Buying a new snap action thermostat can be confusing because there are four different temperature ranges and prices for the same unit vary from $12 to $65.
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Old 04-06-2019, 10:54   #3
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Re: Danfoss/Isotherm fridge cycling

Richard,

Nice to have you here since you're the authority on these systems

It is a standard snap-action mechanical thermostat.

Shorting the C and T terminals does cause the compressor to run and it seems to work just fine with no cycling.

Is it ordinary for the thermostat to occasionally give up the ghost without warning? Is this a typical failure mode (IE short-cycling the compressor once up to the set-temp). Any other ways to test?
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Old 04-06-2019, 11:01   #4
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Re: Danfoss/Isotherm fridge cycling

The mechanical thermostat on my refrigerator is "easy" to test using a multimeter.

The leads are nulled to permit accurate measurement of the low-resistance thermostat path to the controller.

In my case the thermostat measured under one Ohm.
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Old 05-06-2019, 09:28   #5
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Re: Danfoss/Isotherm fridge cycling

Yes, they just give up the ghost. Just replaced mine after only two years. It was super easy. Got it from Coastal climate control, Annapolis.
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Old 24-06-2019, 11:43   #6
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Re: Danfoss/Isotherm fridge cycling

thank you all for the great advice.

If i need a new thermostat anyway, should i just get one of the fancy "saves 40%" IC controlled thrermostats? I getalmost all of my power from solar, so having it put more energy into the fridge in the daytime when voltage is high and less in at night when power is scarce is appealing... Thoughts?

https://www.defender.com/product3.js...232&id=4161337

https://www.defender.com/product3.js...232&id=2407016

also can't really tell what the difference between those two models is...


All advice is appreciated!
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Old 24-06-2019, 12:03   #7
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Re: Danfoss/Isotherm fridge cycling

Quote:
Originally Posted by Vengeful View Post
thank you all for the great advice.

If i need a new thermostat anyway, should i just get one of the fancy "saves 40%" IC controlled thrermostats? I getalmost all of my power from solar, so having it put more energy into the fridge in the daytime when voltage is high and less in at night when power is scarce is appealing... Thoughts?

https://www.defender.com/product3.js...232&id=4161337

https://www.defender.com/product3.js...232&id=2407016

also can't really tell what the difference between those two models is...


All advice is appreciated!
The first one, the Isotherm SED00036DA, looks very good for the application.

It says it has this requirement: "The device is compatible only with control units IWM P/N SEG00002DA Secop version 101N0212 and IWM P/N SEG00030GA Secop version 101N0510." Do, make sure your refrigerator has the correct controller before ordering.

It does: "Over Cooling is allowed by the system when a surplus of energy is detected. This can be achieved via the battery being charged (engine on, battery charger on, etc.) or with shore power connected if provided with the AC/DC module control."

My assumption is that will work for your goal. You need to verify and decide.
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