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Old 10-30-2015, 04:23 PM   #1
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Grunert Mystery

This is a real mystery, and trust me it's not a Halloween gag by any means.

Our Grunert refrigeration system runs on AC current only---generator or shore power, it's not tied to the inverter. The compressor services 2 cold plate refrigerators and one cold plate freezer. As we travel to Florida we anchor most nights. My practice is to start the generator when we anchor, let it warm up and then gradually put a load on it. When I switch on the fridge system (by flipping on the breaker) the compressor and seawater pump kick in immediately. And all is well.

Then we decided to go to a marina one night. We plugged into shore power (50 amp) and fired all the AC things on. Except, the fridge system would not turn on. I checked the 2 fuses, the reset button, jostled the thermostats in each box, checked the sea strainer for water and clogs, all to no avail. Tried it again after dinner with no joy. Went to bed.

During the night I woke up as this was on my mind. Switched on the breaker and the fridge came right on. Weird. For the next couple of nights we anchored and everything worked properly with the generator supplying the AC. Then we arrived at a well known marina with a great service reputation. We planned to spend the night there to visit friends. Plugged into shore power and the fridge refused to turn on. Went to bed. Tried it in the morning and it came right on. I decided to have a refrigeration tech look at it. When I explained the problem he said he would check it over. He checked everything. He worked for an hour with gauges, meters, the works. Diagnosis---nothing wrong with the unit. He was a real gentleman and said there would be no charge.

Off we went. The next several nights we used the generator and all was fine. Then we went to a marina again and plugged in. The fridges refused to come on. The temperature in the fridge boxes rose to 55 degrees and still nothing. We packed them with ice and continued south. Each night since then the fridges have been working fine from the generator.

I have a great refrigeration guy in FL, so we will be anchoring for the next few nights and then have him check it out.

I give up. Anyone want to take a shot at this?

Thanks!
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Old 10-30-2015, 06:23 PM   #2
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Just a wild ass guess, but perhaps a shore power cable issue. Marinco cables don't always play well with Hubble pedestals.


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Old 10-30-2015, 06:31 PM   #3
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That's an interesting guess. The cable we use has not caused this problem in the last 4 years. Our Marina in Florida is the same one we always go to and we get the same slip. Will be interesting to see if it happens there.
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Old 10-30-2015, 07:28 PM   #4
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No marine expert or anything, but I am qualified in general electrical. Anyway, the shore power voltage may have dropped below a threshold where there won't be enough to start up your compressor. Then later at night the marina source has less load from all the boats, the voltage recovers and your unit will start.
The clue is that it works powered by your genset every time. If it isn't blowing a breaker, and it isn't the cable or wiring from the cable outlet in, then maybe it's just an overloaded marina power set-up, and a too-low voltage.
Compressors draw a lot of current the first couple seconds they are starting up.
Especially single phase motors.
You might be sharing the dock with a juice hog, too.

Put a voltmeter on the compressor's terminals as it is started and see what the voltage is doing. If it drops more than 10% below the nominal, this might be the problem.
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Old 10-30-2015, 07:48 PM   #5
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Bluto,
One other fact: when it wouldn't start on shore power, I started the generator and it wouldn't start with that either.
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Old 10-30-2015, 07:49 PM   #6
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After waiting awhile it would then start on the generator.
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Old 10-30-2015, 08:17 PM   #7
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After waiting awhile it would then start on the generator.
Huh.
I suppose there could be a problem in the unit itself, possibly too much liquid refrigerant is getting in the compressor at shut down or after.
It's called slugging, but you would here a definite knock if that happened, unless your unit uses a scroll type compressor which tolerates liquid a little better.

That's about all I have.
Have the unit checked out by a competent refrigeration man and go from there.
Good luck!
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Old 10-30-2015, 08:18 PM   #8
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Well thanks. I will post the outcome when the issue is resolved.
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Old 10-30-2015, 08:34 PM   #9
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Sorry couldn't be more help. It could be something as simple as a loose or corroded lead on a terminal or as bad as a compressor going out.
You might just want to take a look at those power connections with the breaker off and taped down.
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Old 10-30-2015, 08:38 PM   #10
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Your input is appreciated. I'm betting on a voltage issue. Compressor is fine, runs like a champ on the generator.
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Old 10-30-2015, 09:54 PM   #11
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If you turn it on while connected to shore power, and it does not start, you can walk through the circuit with a voltmeter and find out where volts are going in and not coming out. Not a refrigeration issue, but an electrical issue. Contactors, thermal breakers are first things to check.

But have to catch it in the act.
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Old 10-31-2015, 06:35 AM   #12
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What other electrical devices share that power leg (assuming your boat is 240 Volt)? Maybe something like a water heater is pulling the voltage down. Would definitely check the breaker with a volt meter next time it doesn't come on.

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Old 10-31-2015, 06:46 AM   #13
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When we sailed we had a engine driven holding plate refrigerator that we charged every twelve hours. Maybe the thermostats are telling the units not to operate because they are at the desired settings. What are the box temps at ?
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Old 10-31-2015, 07:15 AM   #14
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On my air conditioning compressors, when you switch them off in a compression cycle and try to turn them back on...no joy for a bit...not sure how long that bit is...usually in minutes not hours depending on how long the pressure needs to bleed off from what I understand.

Are you running the compressors on the Genset and shutting them off then trying them on shore power pretty quick? As in minutes?

Try them on the Genset, turn them off and in a minute or so see if they come back on when still using the Genset. If so...sounds like a pressure/sensor issue like I have seen on AirCon compressors....which seem to be similar in design on some of the boats I am used to.
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Old 10-31-2015, 08:00 AM   #15
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MY guess would be a problem with the actual switch that shifts from shore power to noisemaker.
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Old 10-31-2015, 08:13 AM   #16
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I agree with FF. Begin at the switch and work your way out to the inlet. You may want to do this sooner than later. It sounds like one of the hot legs is the problem but if the neutral becomes involved and drops out things will fry. You may be able to spot the problem visually by discolored cabling from overheating or by lightly jiggling connections power off of course to see if any are loose. I don't recall you mentioning trying different cords but might be worth trying that too if you have one available.

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MY guess would be a problem with the actual switch that shifts from shore power to noisemaker.
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Old 10-31-2015, 08:29 AM   #17
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We've been enjoying an intermittent problem with on of our AC/reverse cycle systems. The symptom was that the compressor would not start -- sometimes. But then other times, it would. We could hear it attempting to start, reversing valve moving, etc... but sometimes... no workee.


After mucho puzzling, it may have turned out to be corrosion (or other gremlins) at the wire connections to the compressor. Took those off and cleaned up the terminals a bit, put everything back together... and the system has been back to working as normal since then. No actual corrosion was visibly evident, but...


But "since then" only means about 4 days worth of activity... so I'm not declaring complete victory just yet. Still... seems promising, so far.


That kind of fault was not addressed in our AC manual's troubleshooting guide, but the local AC guys were very helpful on the phone, and that's partly how I stumbled here from there.


FWIW, the symptom psneeld describes is addressed in our manual; says not to switch off and then back on right away, and gives a time limit... forget what it is, though...


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Old 10-31-2015, 09:10 AM   #18
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Genset neutral is usually connected to ground. Shore power is not.
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Old 10-31-2015, 11:51 AM   #19
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Don't give up on the Gunnert you will be shamed from your marina and loose status among the boating community. Back in the day of ice boxes when only the uber rich had refrigeration aboard their yachts, a Gunnert freezer on your boat was the ultimate status symbol. Much the same as having redundant 72mi radar systems with dual 6 foot open array antenna's on your 8ft Caribbe tender would be today.

Having a Gunnert on a "Grand Banks", now that is simply over the top and arrogantly ostentatious.

While this problem may cause others to think less of you or question your skills as a mariner, us serious boaters here on TF with Gunnerts know it is impossible that there could ever be anything wrong with it so it has to be something else at fault.
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Old 10-31-2015, 03:23 PM   #20
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Are you 100% sure it's just not the thermostats working normally?
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