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Old 24-04-2014, 03:31   #1
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Isotherm Freezer - Run, Stop, Run, Stop...

I have a small Isotherm freezer on my boat (normally draws about 3A @ 12V). I started it yesterday for the first time in about 9 months and it seemed to run ok until today. Now the compressor keeps stopping.

When I turn it on, the compressor starts, the pipe to the freezer box starts to cool but after about a minute the compressor stops. After a minute of 2, the compressor starts again and this cycle repeats again and again. The thermostat is constantly on so not causing this. I'm guessing that the compressor has some sort of overload protection that trips then resets and restarts the compressor, then trips again.

Could low gas cause this?
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Old 24-04-2014, 04:30   #2
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Re: Isotherm Freezer - Run, Stop, Run, Stop...

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Originally Posted by Cruisin Cat View Post
I have a small Isotherm freezer on my boat (normally draws about 3A @ 12V). I started it yesterday for the first time in about 9 months and it seemed to run ok until today. Now the compressor keeps stopping.

When I turn it on, the compressor starts, the pipe to the freezer box starts to cool but after about a minute the compressor stops. After a minute of 2, the compressor starts again and this cycle repeats again and again. The thermostat is constantly on so not causing this. I'm guessing that the compressor has some sort of overload protection that trips then resets and restarts the compressor, then trips again.

Could low gas cause this?
Have you tried putting a multimeter across the power terminals of the compressor itself? That way you could determine whether the compressor is receiving power all the time (and it's a thermal cut-out issue) or whether the controller board is intermittently providing power to the compressor.

Going further, a leak in the system will cause the compressor to run harder than design. I typical sign of a compressor running with non-optimized differential pressure (or back-leak) is that the compressor will run at maximum rated current draw continually.
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Old 24-04-2014, 05:00   #3
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Re: Isotherm Freezer - Run, Stop, Run, Stop...

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Originally Posted by Cavalier View Post
Have you tried putting a multimeter across the power terminals of the compressor itself? That way you could determine whether the compressor is receiving power all the time (and it's a thermal cut-out issue) or whether the controller board is intermittently providing power to the compressor.
+1

It might be the low-voltage cutoff causing the cycling.

If the voltage at the terminals is low before the compressor is running, from the house batteries being at a lower charge, with the compressor off, the voltage sense circuit could be happy, permitting the compressor to start. Then when the compressor is running, the normal voltage drop makes the sense circuit unhappy, causing the compressor to stop.

Another possible problem could be degraded wiring, causing good voltage w/ no current, but anacceptable voltage once current is flowing, making the sense circuit unhappy once the compressor starts. If the voltage at the terminal drops by more than a volt, then try a temporary direct run of 10 AWG, two-conductor, fused, to a solid supply or the fully-charged battery to check operation.
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Old 24-04-2014, 05:45   #4
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Re: Isotherm Freezer - Run, Stop, Run, Stop...

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Originally Posted by Cavalier View Post
Have you tried putting a multimeter across the power terminals of the compressor itself? That way you could determine whether the compressor is receiving power all the time (and it's a thermal cut-out issue) or whether the controller board is intermittently providing power to the compressor.

Going further, a leak in the system will cause the compressor to run harder than design. I typical sign of a compressor running with non-optimized differential pressure (or back-leak) is that the compressor will run at maximum rated current draw continually.
The electrical connection between the compressor control board and compressor is internal and not available to test. There is power connected to the compressor control board and terminals for the thermostat connection.

If the gas has leaked out of the system, will that cause the compressor to overload and trip?
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Old 24-04-2014, 05:48   #5
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Re: Isotherm Freezer - Run, Stop, Run, Stop...

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Originally Posted by wingless View Post
+1

It might be the low-voltage cutoff causing the cycling.

If the voltage at the terminals is low before the compressor is running, from the house batteries being at a lower charge, with the compressor off, the voltage sense circuit could be happy, permitting the compressor to start. Then when the compressor is running, the normal voltage drop makes the sense circuit unhappy, causing the compressor to stop.

Another possible problem could be degraded wiring, causing good voltage w/ no current, but anacceptable voltage once current is flowing, making the sense circuit unhappy once the compressor starts. If the voltage at the terminal drops by more than a volt, then try a temporary direct run of 10 AWG, two-conductor, fused, to a solid supply or the fully-charged battery to check operation.
The voltage supply at the compressor control board terminal with the compressor running is 12.35V so I don't think that's causing the problem
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Old 24-04-2014, 15:20   #6
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Re: Isotherm Freezer - Run, Stop, Run, Stop...

We also have an Isotherm fridge which is doing the same thing, it has no problem with cooling, the small freezer section need defrosting every couple of weeks (been like that since new) We would be interested if you find a cause to the compressor cycling on and off.
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Old 24-04-2014, 17:53   #7
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Re: Isotherm Freezer - Run, Stop, Run, Stop...

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If the gas has leaked out of the system, will that cause the compressor to overload and trip?
Inadequate refrigerant along with several other conditions can cause an improper differential between the high and low sides of the compressor. This causes the compressor to have to work harder than designed, and it overheats causing the thermal overload switch to kick out until it cools down.

This sounds like why your system is short cycling. Best bet at this point is to get a refrigeration tech onboard to determine what is causing the improper differential pressure. Not always easy to do in the south pacific... but without a set of gauges and the knowledge to use them, you are pretty well stuck.

Wish I could be more encouraging. One of the reasons I don't have refrigeration.
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Old 24-04-2014, 18:08   #8
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Re: Isotherm Freezer - Run, Stop, Run, Stop...

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Inadequate refrigerant along with several other conditions can cause an improper differential between the high and low sides of the compressor. This causes the compressor to have to work harder than designed, and it overheats causing the thermal overload switch to kick out until it cools down.

This sounds like why your system is short cycling. Best bet at this point is to get a refrigeration tech onboard to determine what is causing the improper differential pressure. Not always easy to do in the south pacific... but without a set of gauges and the knowledge to use them, you are pretty well stuck.

Wish I could be more encouraging. One of the reasons I don't have refrigeration.
Thanks, that's what I guessed even though I don't know much about refrigeration.
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Old 24-04-2014, 19:06   #9
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Re: Isotherm Freezer - Run, Stop, Run, Stop...

Hate it when a good guess is an unfortunate situation...

Soldier on..
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