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Old 05-08-2013, 11:41 PM   #71
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Sorry, I think I should have been more clear. By "go out", I meant "fail".
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Old 05-08-2013, 11:57 PM   #72
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I am sure boards do go out. However, I see no indication that the board IS going out because if the cooling unit itself was working, the board would either shutdown altogether, in which case the boiler and absorber pipes would all be cool and the interior would of course thaw out. If the board fails and just leaves the gas valve open then assuming the cooling unit works the fridge would just get colder and colder, freezing the food as my bypass test shows. But even in that case, as shown by my bypass test, the boiler and absorber coils were warm, not too warm - maybe more on the cool side and were both about the same temperature, which is indicative of proper circulation.
Remember, with these fridges, whenever there is a great differential between the boiler and absorber you have an indication that there is either a loss of circulation (absorber coils will be cold with a hot boiler) or a loss of hydrogen charge (boiler will be much cooler than the absorber coils).

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Are refers like the furnaces (boards go out regularly)?
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Old 05-09-2013, 02:59 PM   #73
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Well, well, well

Looks like my 2nd test is already showing some problems before it is ready to start fully. I loaded the fridge with water bottles in the freezer and the fridge to "load it up". Started the fridge up again and I am getting completely different results and a surprise that totally points to a failed cooling unit!
The freezer froze, but the fridge would not come below 40 degrees and the boiler was hot while the absorber coils were still cool. This points to a plugged system. BUT I am going to do a final test after thawing out completely. This will be a bypass test on electric unloaded and loaded.

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Old 05-09-2013, 04:05 PM   #74
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While my frig is not falling short of expectations as much as your I too have never been satisfied with it.

In general it will only cool the frig side to about 40 degrees below ambient. That's fine in cooler weather but not so during the summer. In fact in winter the frig can start to freeze liquids place close to the fins.

I have modified several things in an attempt to improve the efficiency. I pulled the box and added insulation between it and the side wall to remove the ambient load off the sides. I installed a sheet metal false wall behind the unit to provide the 1 in. clearance Dometic requires in the manual. I installed the fan assisted coil mode on the rear. i have a small fan inside the frig to aid circulation. None of these modes had any measurable effect on the operation.

I had come to the conclusion that Dometic just does not make a product that is capable of providing what I was looking for.

As far as the question of level is concerned I would assume, and have always used this practice, that if the coils still have a negative angle, to return the gas, the frig should operate. I have never found this not to be true.
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Old 05-09-2013, 05:38 PM   #75
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By 'bypassing' the thermistor, you are essentially running the cooling unit wide open and should have a continuous flame on LP or constant 120VAC to the heating element. Expect the temps inside the unit to be well below 20ºF in the freezer section (which has no controls anyway) and usually well below 32ºF in the fridge section as there are separate evaporators for each section.

You can also test a thermistor with a glass of ice water, thermometer and an ohm meter to compare the resistance from the thermistor with a table of known values at specific temperatures.

Good luck with your test!
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Old 05-09-2013, 06:43 PM   #76
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Lewster

You mention a separate evaporator for each section. While I have not replaced the cooling unit in this frig my last one I did and it looked like there was only a convection connection, aluminum plate to the fins, between the freezer and the frig side with the thermister in the frig section.

I just assumed the unit cycled against the frig section and it had a 100% duty cycle if the frig was above the set point. I would love to get a cooler frig section in the summer but have run out of ideas.
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Old 05-09-2013, 07:01 PM   #77
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Quote:
Originally Posted by HowieE View Post
Lewster

You mention a separate evaporator for each section. While I have not replaced the cooling unit in this frig my last one I did and it looked like there was only a convection connection, aluminum plate to the fins, between the freezer and the frig side with the thermister in the frig section.

I just assumed the unit cycled against the frig section and it had a 100% duty cycle if the frig was above the set point. I would love to get a cooler frig section in the summer but have run out of ideas.
If you look at a working diagram of a gas absorption cooling unit, you will see 2 sections marked 'low temperature evaporator' and 'high temperature evaporator'. The low temp section is in back of the freezer section and the high temp section is connected to the fins of the refrigerator section. The difference being that the cooling solution (liquid ammonia coating the sidewalls of the evaporator sections which when coming in contact with the hydrogen gas creates a chemical reaction of intense cold) looses some of it's reactive abilities as the ammonia drops in the tubing from high to low temp sections.

In a nutshell (and to finally answer your question ), they are 2 separate sections but are joined together with many tubing sections which would not allow a separate, higher capacity high temp evaporator to be installed.:

I DO have a fix for anemic fridge temps though. I have been using 12VDC computer fans in gangs of 2-4 situated under the roof vent to pull ventilation air across the condenser fins to keep it within it's normal operating temperatures. It's this condenser that gets overheated and throws a wrench into the absorptive cooling process. Keep the condenser 'cool' and you will have lower fridge temps....especially in high ambient temps.
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Old 05-09-2013, 07:22 PM   #78
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Lewster

OK you have defined my next project. I think I will mount 4 or 5 on a piece of sheet metal and connect them to one or two adjustable thermostat mounted behind the unit. Then hunt for the point to turn the fans on. I would like to have the thermostat set about 10 degrees apart and each having about a 10 degree difference between on and off.

During cooler weather convection should still work even with the reduced opening. As the ambient goes up the fans would come on 2 and then 5

What do you think?

And could ventilation be Tim's problem while on the road. A possible down draft forcing heat retention behind the frig rather than exhausting it.
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Old 05-10-2013, 06:36 AM   #79
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Howie,

I think you are over complicating it. K.I.S.S. if you know what I mean. I use 4 on large motor homes with the 12 cu. ft. Norcolds and they work great. Connect to the 12VDC block at the back of the fridge for power, run thru a fuse and use an SPST switch (mounted inside the trailer if you like) and that's all you need. The fans are so quiet that they can actually run continuously if necessary in very high ambient temp situations.

And yes, Tim might be getting a downdraft when traveling. I'm sure that the fans would help his situation also.

A while back, there was a kit available from Stu Snyder that tried to accomplish increased air flow, but it also used a plenum and single very small fan. It still pushed the air over the condenser instead of pulling it. I installed many of these and while they did help a bit, they were never the final solution. Multiple pulling fans just might be the answer.
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Old 05-10-2013, 07:05 AM   #80
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I've been reading this thread with interest because my 2008 Classic 25fb we just bought about a month ago experienced this on our first extended trip. Our fridge is the Dometic 3762 2 way. I'm having issues with my converter not charging the batteries at 13 v while on shore power but I used a charger to keep the batteries topped up on this trip.

I had the fridge on Gas during the time at camp to test out the LP system. The freezer compartment seems to be level with the floor adjacent to the fridge. At setting 4 (1 through 5) the temperature displayed on the fridge was 38F. The weather had been cool in Missouri but on Sunday the weather had cleared and the temperatures got into the high 70s low 80s. The fridge seemed to maintain temperatures or maybe it got up to 40 deg. The trailer was in the full sun. We even ran the AC inside to learn and test it out. (Unfortunately we have that dreaded problem with water leaking inside the interior from the AC, another story for other threads)
We broke camp on Monday around 11am to start the 350 mile drive home from Missouri to Dallas, TX. The wind was blowing strong out of the south so most of the time it was a headwind. For a period of about an hour on I40 its a crosswind with the street side of the trailer getting the wind and that is where the fridge is located. We stopped for lunch at a rest stop and my wife made lunch. I didn't notice the temperature reading because I didn't know there was going to be a problem. The wife knows not to keep the door open long when getting food out the fridge.

When we arrived home and the trailer was in storage I checked the fridge and it was at 54F. It was about 80 degrees. I checked the LP flame and it was hot and the flue part was too hot to touch.

Earlier you mentioned touching parts A and B and they should feel about the same hot to touch. I'd like to try that too. Can you post a link to the Dometic document that shows this troubleshooting test. My fridge documents down show that procedure.

When I had the Dometic recall performed this week, I talked to the RV tech about this issue and he said traveling shouldn't be a problem with the movement. He did mention that air coming across the top vent can cause flame outs and exhaust issues and my AS has a fence between the AC and the fridge vent on the roof. Here is the interesting thing he said about RV fridges. He says that a certain percentage of them have an issue with cooling while traveling. He couldn't explain the cause.

I'm admire your tenacity with this problem but by now I would've bit the bullet and bought a new fridge. I may be forced to go this route and if the if it still doesn't work while traveling I'd be forced to sell my AS with this disclaimer and take a big loss.

Kelvin
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Old 05-10-2013, 07:20 AM   #81
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Quote:
Originally Posted by KJRitchie View Post
I've been reading this thread with interest because my 2008 Classic 25fb we just bought about a month ago experienced this on our first extended trip. Our fridge is the Dometic 3762 2 way. I'm having issues with my converter not charging the batteries at 13 v while on shore power but I used a charger to keep the batteries topped up on this trip.

I had the fridge on Gas during the time at camp to test out the LP system. The freezer compartment seems to be level with the floor adjacent to the fridge. At setting 4 (1 through 5) the temperature displayed on the fridge was 38F. The weather had been cool in Missouri but on Sunday the weather had cleared and the temperatures got into the high 70s low 80s. The fridge seemed to maintain temperatures or maybe it got up to 40 deg. The trailer was in the full sun. We even ran the AC inside to learn and test it out. (Unfortunately we have that dreaded problem with water leaking inside the interior from the AC, another story for other threads)
We broke camp on Monday around 11am to start the 350 mile drive home from Missouri to Dallas, TX. The wind was blowing strong out of the south so most of the time it was a headwind. For a period of about an hour on I40 its a crosswind with the street side of the trailer getting the wind and that is where the fridge is located. We stopped for lunch at a rest stop and my wife made lunch. I didn't notice the temperature reading because I didn't know there was going to be a problem. The wife knows not to keep the door open long when getting food out the fridge.

When we arrived home and the trailer was in storage I checked the fridge and it was at 54F. It was about 80 degrees. I checked the LP flame and it was hot and the flue part was too hot to touch.

Earlier you mentioned touching parts A and B and they should feel about the same hot to touch. I'd like to try that too. Can you post a link to the Dometic document that shows this troubleshooting test. My fridge documents down show that procedure.

When I had the Dometic recall performed this week, I talked to the RV tech about this issue and he said traveling shouldn't be a problem with the movement. He did mention that air coming across the top vent can cause flame outs and exhaust issues and my AS has a fence between the AC and the fridge vent on the roof. Here is the interesting thing he said about RV fridges. He says that a certain percentage of them have an issue with cooling while traveling. He couldn't explain the cause.

I'm admire your tenacity with this problem but by now I would've bit the bullet and bought a new fridge. I may be forced to go this route and if the if it still doesn't work while traveling I'd be forced to sell my AS with this disclaimer and take a big loss.

Kelvin
Kelvin,

I would try adding several fans under your roof vent for the fridge before doing anything so drastic as selling your trailer. There are also other work-arounds for your problem to keep your fridge cold and you and yours happy.
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Old 05-10-2013, 09:52 AM   #82
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Yes a bit of over kill but as one that suffers from "terminal tinkeritis" I tend to over engineer my projects.

I installed a Synder some time ago with no noticeable improvement. My problem with it was the fan had no means of spreading the air flow over the covered area of the coils.

I like the idea of evacuation of the heat, per your suggestion, rather than movement behind the frig.
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Old 05-10-2013, 09:55 AM   #83
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Lewster

Yes a bit of over kill but as one that suffers from "terminal tinkeritis" I tend to over engineer my projects.

I installed a Synder some time ago with no noticeable improvement. My problem with it was the fan had no means of spreading the air flow over the covered area of the coils.

I like the idea of evacuation of the heat, per your suggestion, rather than movement behind the frig.
GO FOR IT!

Let us know how it turns out............
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Old 05-10-2013, 06:42 PM   #84
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I ran a bypass test (thermistor disconnected from the control board) last night on electric and damned if the fridge didn't cool down to something like 7 degrees in less time than it took to get it to 15 degrees on Gas. Electric works better than gas on this trailer and to me at least shows that in a bypass situation, works. None of this explains why it STOPS working once we get on the road.
One of the diagnostic manuals I found is HERE. It is somewhat dated, but still valuable information as the cooling units themselves have not changed much at all in the way that they work. You put heat into them and away you go. If the absorber is about the same temperature as the boiler, you have good circulation.
BTW, this morning, my boiler was warm and the absorber coils were actually cool. I suspect that is because the fridge temp was so low that the ammonia was simply not evaporating much all the way through. Perhaps it was staying liquid all the way down the absorber.
I'll post pictures of my unloaded and loaded electric bypass tests later.

One thing I am going to do is install a removable fan with a motor speed controller directly on the lower compartment door to force air up and across the absorber (not needed) and cooling fins (necessary for hot weather) and I will show how I did that later. I just ordered the parts, should arrive in a week or two.
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