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Old 10-22-2019, 07:05 PM   #1
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Grunert Refrigeration - Controls Problem

I have an old Grunert Refrigeration system still running on my '84 Grand Banks. Refrigerant was recharged 18 mos ago and it still cools well but the problem is the control system. It runs 24/7 and the temp controllers do not shut off the compressor at temperature set point. I made a temporary fix by putting the power supply on a timer and I planned to buy new Isotherm systems for my Refrig and Freezer boxes but am now thinking I may be able to extend the life of my Grunert by fixing the temperature control system. I have one compressor/condenser system in my engine room that services both the Refrig Box and the Freezer Box. The temp controller on the Freezer is new but the one for the Refrig is an old Ranco temp controller. My first thought is the Ranco controller may be bad and to replace it with a new Johnson Controls SPST temp controller like the one on the freezer. There is no control schematic available from Grunert because they say this one was a custom made unit. So, I traced the tubing and cables and devices and came up with the attached schematic. If anyone on this forum reading this has expertise on controls systems for refrigerators and can look at this schematic and tell me if they think the most likely culprit is the temperature controller, let me know. I realize the controls are way-old technology using relays for turning the compressor on and off, one from freezer control and other from refrig control. Also, I marked as a "solenoid ?" is a bit confusing to me. It has the small refrigerant line from the refrig going thru it has the refrig control cable going thru it. If anyone with refrig controls expertise can tell me what purpose it serves, pls do.
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Old 10-23-2019, 11:28 AM   #2
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It is a pretty simple system. The solenoid valves are in the refrigeration lines going to the freezer and fridge. I have three of them (2 refrigerators, one freezer). The thermostats would open/close the solenoid valves based on the desired temps. Once both units have reached the target temps, the compressor and seawater pump or fan motor in your case would shut off. The only other "feature" would be a delay to restart the compressor to avoid high pressure damage.

First, make sure you have sufficient refrigerant in the system. The system will run continuously if you don't. I'd bet this is your issue. There should be a sight glass close to the compressor/accumulator. You should see just a few bubbles in the refrigerant. If your holding plates are only partially covered in frost after running for an hour you are low on refrigerant.

Once you have made sure you have sufficient refrigerant, and your plates are being pulled down within an hour or two, your compressor and holding plates are working and the system should shut off.

If it does not shut off, your problem could be bad relays or thermostat(s) not working. For the relays, check continuity on the coils and contacts when coils are activated. If they are standard automotive relays, just replace them. The high-amp contactors for the fan motor and compressor seem to be working. The thermostats are still available for a reasonable price ($90).

Note, the thermostats measure the cold plate temperature about 2/3rds down not the air temp in the units. The sensing coils of the thermostats need to touch the holding plate. The refrigeration holding plate should be around 27 deg F, the freezer holding plate around 0 deg F.

Good luck!
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Old 10-23-2019, 06:18 PM   #3
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Things to remember here: The compressor is cycled on and off in response to the pressure in the system. For each separately cooled space, a thermostat controls a solenoid, which controls the flow of refrigerant. Once all the solenoids are closed, the pressure difference builds and turns off the compressor. This would be the Ranco and its usually a dual device, one switch on the high pressure side turns off in response to excessive pressure there and another plumbed to the low pressure side turns on in response to increasing difference in pressure there. An open solenoid bleeds pressure, requiring the compressor to run to maintain the supply of pressurized refrigerant. There has to be enough refrigerant in the system. Get pressure gauges on it to see what's going on.
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Old 10-24-2019, 09:25 AM   #4
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re: Grunert Refrig Controls problem

My control system only has one solenoid and it's for the refrigerator refrigerant control. The freezer has no solenoid (that I can find).
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Old 10-24-2019, 11:29 AM   #5
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I can't get the larger image or download your drawing for some reason. The freezer tstat should be controlling the compressor directly, and the fridge tstat and solenoid piggyback - it has pressure available when the freezer calls for compressor run. If not solved, PM me an attachment.
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Old 10-24-2019, 03:13 PM   #6
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Grunert Refrigeration - Controls Problem

You are correct according to my read of the schematic. see attached smaller file. Without putting pressure gages on the system (I don't have refrig tech equipment), it appears the refrigerant level is good based on time it takes to get cold and that I have to manually shut off compressor or else refrig will freeze. So, would you think the problem may be with a faulty refrigerator thermostat? The refrig thermostat is an old Ranco; the freezer thermostat is a much newer Johnson Controls.
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Old 10-24-2019, 06:13 PM   #7
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When we refurbed the Grunert system (new compressor, new seals on the fridge doors, regas etc) on our GB, we also replaced the old mechanical thermostats with new digital ones. I will try and post a photo when I am on the boat this coming weekend.

Good luck with it! Our experience has been that once you get the system running again, it is very effective.

Hamish.
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