Dometic absorbtion fridge

[Hofty]

Folks,

I have a dometic RM2551 fridge. 2 way. LP/AC. The control board is a dinosaur P-711 and currently the temperature setting is on 4/5.

On LP the refridgerator maintains -17.8 [Freezer] and 3 [Fridge].

On 110V AC the Refridgerator fluctuates from getting cold to warming up then randomly will cool down again.

Not sure why this happens but wondering which component I'd start with swapping out. Not wanting to replace entire main control board.

Any help would he appreciated.

 

[Hofty]

I'm a noob. The Thermistor was not in between the fins. First RV I have owned and I placed the thermistor randomly in the fridge as I figured it would measure the temperature that way.

Let's see how it works out now. I'm going to let the wireless thermometer meter the performance overnight

 

[Gary RV_Wizard]

That should do it!

 

[Kirk]

The thermistor is typically clipped to a fin, usually near the right side of the cooling coils and moving it to the left will lower the operating temperature.

 

[Hofty]

So I have changed over to propane and the freezer gets down to -20 centigrade and fridge down to 3 degrees but now the issue is that the freezer warms up to +1.5 degrees. I'm sort of at a loss now.

Fridge is at temperature but freezer has warmed up.

Top temperature = Freezer
Middle temperature = Fridge

 

[Gary RV_Wizard]

So I have changed over to propane and the freezer gets down to -20 centigrade and fridge down to 3 degrees but now the issue is that the freezer warms up to +1.5 degrees. I'm sort of at a loss now.

Fridge is at temperature but freezer has warmed up.

Top temperature = Freezer
Middle temperature = Fridge

I'm confused. Did the freezer first drop to -20 C and then later warm up to 1.5C? And I don't know what you mean by top and middle temperatures.

The freezer on that model of fridge should be able to maintain the freezer somewhere between -12 C and -18 C. 3 C is the expected temperature for the fridge area.

 

[Hofty]

Yes,

The freezer got down to -20 degrees then warmed up to +1.5 degrees. Now the fridge is 2-4 degrees. I'm baffled why this is happening. I ordered a new Thermister so will replace that and see how it goes. When I disconnected the thermistor yesterday immediately the electric heater turned on so I'm hoping that fixes the issue

 

[Gary RV_Wizard]

There is only one thermistor for temperature feedback. It gives fridge temperature and the freezer will be some arbitrary amount less. The cooling unit is a two stage thing, first chilling the freezer and using leftover chill to cool the fridge. There is no way to control the temperature difference between fridge & freezer. Since the fridge temperature is as expected, the thermistor is not the problem.

When the circuit board loses contact with the thermistor to goes into a constant cool mode, what Dometic calls Limp Mode. See Page 10 of the RM2551 manual.

Electric heat operation is automatically selected whenever 120v is available. The exception is if Gas Mode is manually chosen.

 

[Kirk]

I was unable to locate a copy of the service manual for that specific model but I did find a copy of the installation and operation manual and it has some pretty good drawings and schematics. (I see that Gary just posted a link to the same document.)

Not sure why this happens but wondering which component I'd start with swapping out. Not wanting to replace entire main control board.

I think that you could spend a lot of money by just replacing parts based on a guess about what is happening. If you disconnect the thermistor that will put it into a constant cool mode and should stabilize the temperatures but the chill box will probably be at or near freezing as well. 0°C At this point I can only guess but it sounds like it may be having problems with the cooling unit. Have you noticed any ammonia odors or possibly yellow gunk in the rear of the unit?

 

[JudyJB]

My fins are similar to the picture posted, but I have a white plastic tube that I am supposed to move up and down to control the temperature. Frankly, it does not seem like a very good system, but it does seem to work. Is it possible that moving the thermistor thing up or down on a fin would affect the temp??

 

[CharlesinGA]

Dometic thermistor with the white plastic clips that hold it to the fins.

Oddly, mine does not cool worth a flip if the thermistor is clipped to the fins, so it is in the plastic clip, hanging from the RH end of the top shelf grate. The thermistor works perfectly, I used a temp sensing probe on my multimeter clipped to the thermistor and on the outside I unplugged the thermistor and connected it to another multi-meter to measure the thermistor ohms. My numbers were almost exactly what the Dometic chart and the same chart on the Fridge Defend website call for. (note that the thermistor ohms at +37F in the chart is incorrect, both in the Dometic manual and the Fridge Defend website.)

 

[CharlesinGA]

The ground wire connection just above the circuit board in the first pic (post #1) is questionable and you should verify all electrical connections by removing and replacing them, thus cleaning the contacts. If the varying temps are on gas only, you may have erratic or weak gas pressure.

My travel buddy had problems on one of our trips, with his Dometic fridge on gas, we hooked up the generator and it worked perfectly. The issue turned out to be mud wasps or dirt daubers blocking the chimney. Once he cleaned it out (which required removing the fridge from the cabinet) it worked great.

Charles

 

[John From Detroit]

You need to understand the meaning of "Freezer" and "Deep freeze"
Freezer is above zero F but below Zero C (0-32 F) In practice around 10-20 f
Deep freeze is -F anything below zero.
So 1-2 F is well within Freezer range.

 

[Ray-IN]

Dometic RM2551 diagnostic service manual

 

[Ray-IN]

My fins are similar to the picture posted, but I have a white plastic tube that I am supposed to move up and down to control the temperature. Frankly, it does not seem like a very good system, but it does seem to work. Is it possible that moving the thermistor thing up or down on a fin would affect the temp??

For Norcold, yes. Norcold says to move the thermister down on the fin to fine-tune tempeature. For coarse adjustment move the thermister one fin left every 24 hrs. until fridge temperature is satisfactory, then fine tune. moving the thermister down increases temp, up decreases temp., because warm air rises.

 

[Hofty]

The issue is that on either LP or 115v the freezer gets down to -12 to -20 degrees. The fridge gets down to 3-5 degrees. Then it's almost like the system forgets to monitor the thermistor [Just replaced that component today, so that rules out the thermostor being the issue?].

The system will work again if I turn the refrigerator off and then on again. It will cool everything to optimal temperature and then warm up again. It does this whether it's it's cool or warm outside. I'm in Canada and it's 3-5 degrees at night and 12-18 degrees during the day.

I'm at a loss what to do.

 

[Hofty]

Might be worth mentioning I have a Micro P-711 Main Control Board. The issue persists each time whether on LP or 110V.

2:00am

-16.5 degrees [Freezer]
0.50 degrees [Main Fridge]

7:45am

- 1.40 degrees [Freezer]
2.30 degrees [Main Fridge]

So it seems like the system fails to maintain temperature. I have replaced the thermistor.

 

[Gary RV_Wizard]

The thermistor simply reports a temperature reading to the circuit board. It doesn't control anything itself. The control board reacts by turning on the heat source (propane or electric) to activate cooling whenever the reported temperature is too high. The temperature differential between fridge & freeze is fixed by the cooling unit design, so it cannot be altered by the control board. The control board has no knowledge of the freezer temperature - just what the thermistor reports about the fridge.

Your fridge section maintains temperature OK (as closely as these systems work, anyway), but the freezer does not. My best guess is that your cooling unit is in the early stages of failure and unable to continuously deliver enough chill to hold the freezer temp where it should be. It will probably get worse over time.

 

[Hofty]

So if I understand this correctly the entire system might be failing i.e. the ability for absorbtion to cool the fridge/freezer?

My thoughts are that the control board is not triggering the heating element and/or LP burners correctly regardless of the thermistors ohm readings. Might be a good time to replace the Board? Or are you referring to the coils not providing the rights cooling ability?

 

[Kirk]

My best guess is that your cooling unit is in the early stages of failure and unable to continuously deliver enough chill to hold the freezer temp where it should be. It will probably get worse over time.

So if I understand this correctly the entire system might be failing i.e. the ability for absorbtion to cool the fridge/freezer?

That is exactly what Gary is trying to tell you and as one with a very similar background to Gary's, I completely agree but I would also point out that Gary did say "best guess" as since he isn't there to do any tests, he can only guess. Gary has many years of experience with this type of refrigerator and like myself, he has repaired more than one but only after troubleshooting it first.

Might be a good time to replace the Board?

Since it is your refrigerator and it is your money to spend, you are free to make your own decision. You need to do so knowing that if you try a new circuit board the seller will not take it back since he has no way to know if you might have damaged it. Have you looked at step 5 in the service manual from the link supplied by Dave? Have you cleaned up the ground connection that was pointed out by Charles? I would at least do those things first.