Dometic RM2551 - Thermister? Other Common Problems??

[MSDeneen]

2011 Flying CLoud 23 FB


Dometic RM2551 shorty refrigerator
=================================


Problems:

1. On AC, the pump or compressor makes a HUGE noise, runs continuously and will never shut off.



2. On GAS, the burner lights and runs, but runs continuously without ever shutting off. This causes the cooling fan at the top to run continuously making one hell of a racket all the time.



I suspect what I see is a temperature "device" (thermistor?) connected to the cooling fins and perhaps it is toasted and doesn't ever reach shutoff?


Also, the fan looks poorly mounted and vibrates the whol trailer.


Any tips, help, partn numbers or advice appreciated. I am handy and can replace that part if that's the issue.



Cheers!


Mark

[ITSNO60]

Hello Mark,

The RM 2551 is an absorption fridge so no compressor or motor. The fan(s) on the back can be improved on by replacing with a new one(s), they are usually just small computer fans. The part on the back coils you refer to is a snap disc which measures the coil temp and tells the rear fan or fans when to run. The thermistor is inside the fridge attached to the cooling fins, it's what cycles the fridge off and on as needed.

Here is a copy of the service manual that includes the 2551:

https://myrvworks.com/wp-content/upl...icana-Plus.pdf

and here's the 2551 parts list:

https://myrvworks.com/wp-content/upl...Parts-List.pdf

Note that they probably won't show the rear fan(s) as those are usually added by the installer to help mitigate poor cooling which can be caused by poor installation and venting restrictions. The fans are not specific, any decent 12 volt PC fan will work.

I used these https://www.amazon.com/dp/B00RUZ059O..._asin_title_11 on my RM8555 and you cannot hear them run.

[MSDeneen]

Originally Posted by ITSNO60

The RM 2551 is an absorption fridge so no compressor or motor. The fan(s) on the back can be improved on by replacing with a new one(s), they are usually just small computer fans. The part on the back coils you refer to is a snap disc which measures the coil temp and tells the rear fan or fans when to run. The thermistor is inside the fridge attached to the cooling fins, it's what cycles the fridge off and on as needed.

Here is a copy of the service manual that includes the 2551:

https://myrvworks.com/wp-content/upl...icana-Plus.pdf

Thank you! I will review the service manual. Since the post, I discoverted the thermistor and have one on order. That "snap disc" I noticed just now is poorly attached where one side is 1/4" awy from the fins. I will try tightening that.



I wonder though why the AC operation sound 10X louder than the gas? I understand you that there is no compressor, but what is the AC doing that is so noisy?


Cheers!
Mark

[ITSNO60]

Originally Posted by MSDeneen

Thank you! I will review the service manual. Since the post, I discoverted the thermistor and have one on order. That "snap disc" I noticed just now is poorly attached where one side is 1/4" awy from the fins. I will try tightening that.



I wonder though why the AC operation sound 10X louder than the gas? I understand you that there is no compressor, but what is the AC doing that is so noisy?


Cheers!
Mark

I don't know, never heard of that before. Note that I edited my first reply and added more info and links.

[ITSNO60]

If you decide to replace the snap disc it just unclips from the coil tube. There should be numbers on it, or a temperature range.

[uncle_bob]

Hi

On an absorption fridge about the only thing that *can* make lots of noise are the fans. If a blade gets damaged or something gets caught in them .... they can really get noisy. Also when they get damaged, they don't ventilate as well. That can make this or that run longer (or maybe even all the time).

I'd check and see if the fans are physically damaged or "full of stuff".

Next on the list would be shot bearings in the fan(s). The noise you get from that is pretty easy to spot once you rule out damaged blades and debris.

Bob

[MSDeneen]

First, thanks to you all for the helpful info!



I've ordered a new fan, a new thermistor for inside the fridge, and I am going to dissassemble the "snap disc" and see if I can replace it.



I am fairly well versed in electronics, and I know a fiar anout about transistors, diodes, tubes, caps, resistors and such, but I have never heard the phrase "snap disc" related to electronics. I'll be curious to see what it actually is.



More after I do some work on it.



Again, thanks!



Cheers to all!


Mark

[ITSNO60]

Originally Posted by MSDeneen

First, thanks to you all for the helpful info!



I've ordered a new fan, a new thermistor for inside the fridge, and I am going to dissassemble the "snap disc" and see if I can replace it.



I am fairly well versed in electronics, and I know a fiar anout about transistors, diodes, tubes, caps, resistors and such, but I have never heard the phrase "snap disc" related to electronics. I'll be curious to see what it actually is.



More after I do some work on it.



Again, thanks!



Cheers to all!


Mark

It's basically just a thin bi-metallic disc inside an enclosure that "snaps" on or off according to temperature. They are fully automatic and can be N/O or N/C. Very common in HVAC. Kind of a heat activated relay

BTW there are also adjustable ones made.

[ITSNO60]

Meanwhile, if you're going to be working on it, I like to share this which explains proper venting. https://www.fridge-and-solar.net/fridge_vent.htm

(pay no mind to the mention of 240 volts, that part was written for the other side
of the pond)

When I acquired my 2011 RML 8555 it had a list a mile long of things wrong and improper installations. I worked on it for a solid week and took it from inadequate to more than adequate. I no longer need to run it on MAX all the time either like before.

[MSDeneen]

Originally Posted by ITSNO60

Meanwhile, if you're going to be working on it, I like to share this which explains proper venting. https://www.fridge-and-solar.net/fridge_vent.htm

(pay no mind to the mention of 240 volts, that part was written for the other side
of the pond)

When I acquired my 2011 RML 8555 it had a list a mile long of things wrong and improper installations. I worked on it for a solid week and took it from inadequate to more than adequate. I no longer need to run it on MAX all the time either like before.

Thanks Brian for that link. I liked the info and I have rearranged my cooling of the fridge based on that idea.


I bought two fans and installed them "suking air INTO" the lower chamber. I reversed the top fan to "BLOW OUT". Now, I have fresh air coming into the bottom of the chamber and then the heated air blowing OUT from the top chamber.



That all works fine, but the fridge still doesn't work on AC. I imagine the heating element is blown so I ordered a new one which will arrive tomorrow.



I removed and tested the snap disk. It is normally open and measures open with the VOM. I can heat it a bit over the stove and it becomes about 1 ohm with the VOM, so I think it is fine. It wasn't attached very well to the heat sink.


At the moment I'm not using the snap disc, I am just running the fans whenever the fridge is turned on.


I also installed a new thermister. I didn't imagine an easy way to test it, and they are cheap, so I just replaced it.



Still no joy though. The heating element will be here Friday. We are leaving with the trailer Saturday to go to Spring Training in Peoria, AZ. If no fridge, we will just use a cooler and eat out!


I'm beginning to think we are going to have to get a new fridge. However the next available service appt in Temecula is a month away. So, might as well keep trying to patch this one.


Cheers,
Mark

[ITSNO60]

Originally Posted by MSDeneen

Thanks Brian for that link. I liked the info and I have rearranged my cooling of the fridge based on that idea.


I bought two fans and installed them "suking air INTO" the lower chamber. I reversed the top fan to "BLOW OUT". Now, I have fresh air coming into the bottom of the chamber and then the heated air blowing OUT from the top chamber.



That all works fine, but the fridge still doesn't work on AC. I imagine the heating element is blown so I ordered a new one which will arrive tomorrow.



I removed and tested the snap disk. It is normally open and measures open with the VOM. I can heat it a bit over the stove and it becomes about 1 ohm with the VOM, so I think it is fine. It wasn't attached very well to the heat sink.


At the moment I'm not using the snap disc, I am just running the fans whenever the fridge is turned on.


I also installed a new thermister. I didn't imagine an easy way to test it, and they are cheap, so I just replaced it.



Still no joy though. The heating element will be here Friday. We are leaving with the trailer Saturday to go to Spring Training in Peoria, AZ. If no fridge, we will just use a cooler and eat out!


I'm beginning to think we are going to have to get a new fridge. However the next available service appt in Temecula is a month away. So, might as well keep trying to patch this one.


Cheers,
Mark

It's easy enough to check the AC heating element with an ohm meter. Section 3 page 10 of the manual. Disconnect both leads before checking resistance.

[MSDeneen]

Originally Posted by ITSNO60

It's easy enough to check the AC heating element with an ohm meter. Section 3 page 10 of the manual. Disconnect both leads before checking resistance.

Progress has been made. My AC heater element measured 90 ohms. I bought a replacement yesterday at 4pm on A'zon, which arrived at my door at 7:30 this morning! That alone is cool.



I installed the new heater element, made sure the fuse was good, and we are now testing the fridge on AC. Fingers crossed. The next stop would be a new circuit board for another $120.


I also ordered new LP tank gas lines with gauges (also from Azon) and got them installed. I had broken one of the old plastic nuts that had decayed from heat and age, so good time to replace them with new.


This fridge has been a real learning experience, but I think I finally understand how it all works. And, I have been able to replace parts fairly easily. It's always great when anything is demystified.



Cheers! And many thanks to the Bros and Gals for all the great info!


Mark

[ITSNO60]

90 ohms is really not that far out of the 80 ohm spec, I suspect it will turn out to be something else.

[MSDeneen]

Originally Posted by ITSNO60

90 ohms is really not that far out of the 80 ohm spec, I suspect it will turn out to be something else.

Yup. Still no joy.



Right now, the control board is not activating on AC or Gas. All fuses are ok inside and out.



The new thermistor measures about 1.6VDC when connected to the PCB.


There is this: We are parked in front of the house fully hitched. Which means the trailer nose is about 10 degrees high (as I have shown in previous threads about the hitch).



Q: How does the fridge know it is tilted? Is there an electronic mercury switch? I don't see one. When we camped last weekend, the GAS fridge worked fine. Now, no-go.



How much tilt makes it a no-go?


I hestitated to unhitch because A) I am not supposed to park on the street, so I kept it hitched to move it around. B) hitching up on the slopped street is a bit of a PITA. And C) we are headed out tomorrow AM for Peoria.



If I don't get it working by noon Saturday, we will live out of the cooler, and tackle it again when we come home.


Cheers,


Mark

[ITSNO60]

There is no switch to prevent operation when un-level on any absorption fridge that I know of. I do know that they will operate off level but doing so can result in permanent damage to the cooling unit.

One thing I didn't mention, on mine, all the connections were badly oxidized and I would get intermittent failures. Clean up all the spade connectors that go to the gas valve, igniter and burner areas. Pull any plugs that go to boards and modules and spray contact points with electronics cleaner. Clean accessible pins with a clean pencil eraser. Sand off corroded male spades with a small file or fingernail board. Replace any shady looking female spade connectors. Check to make sure the ground wire(s) are firmly attached and clean. This haunted me for a good while till I finally did them all. The ground (spade) connector on the burner was the worst, deteriorated from heat and no more spring pressure to hold it in place. I put on some gold plated connectors and covered everything with heat shield sleeves.

I'm sure you are aware that the fridge needs 12 volts regardless of gas or AC operation to operate the controls so your battery switch must be on. Also in many cases there is a small fuse about 3 amps that runs the control for the fridge either in your trailer fuse panel or some have found it elsewhere, but it sounds like you already checked all that. There is likely a fuse on the board as well.

If it was working on gas before and now it isn't the connections on the back are a good place to start. Cleaning the burner orifice should also be on your list but first we need to see if it's getting spark and gas. When you switch to gas do you hear a rapid "tic tic tic" from the rear of the unit as the igniter fires?

If you recently ran out of propane there could be air and the fridge will only attempt to light a few times before locking out. Air can be bled from the system by using your stove burners.

So far it has been my experience that failures in these units are almost always something fixable without replacing expensive boards and such.

[MSDeneen]

Originally Posted by ITSNO60

There is no switch to prevent operation when un-level on any absorption fridge that I know of. pressure to hold it in place. I put on some gold plated connectors and covered everything with heat shield sleeves.

I'm sure you are aware that the fridge needs 12 volts regardless of gas or AC operation to operate the controls so your battery switch must be on. . When you switch to gas do you hear a rapid "tic tic tic" from the rear of the unit as the igniter fires?

If you recently ran out of propane there could be air and the fridge will only attempt to light a few times before locking out. Air can be bled from the system by using your stove burners.

1. Ok - no switch that senses tilt. Glad to hear.
2. Yes, we had the battery ON.

3. No - there was no tic tic tic of the ignition trying to light.

4. The gas tank is full, and has been used frequently of late for hot water and cooktop. So, no "air in the lines."


The logic seemed to be: The fridge is not "ASKING FOR COOLING" so the board doesn't connect the 120V AC heater, nor will it initiate a spark sequence. I'm suspicious of the new thermistor we installed. However, I measured the old one at 1.4V when connected to the PCB, and the new one measures about 1.45V. So it seems ok.


I will no more sanitization of the connections. That's always a good thing to do with old assemblies like this.



I agree - - -it's usually some small thing that is overlooked. I will redouble my observations and cleaning of the contacts.



CHeers and Thanks!


Mark

[Wazbro]

The problem with the fridge being off level is that the ammonia (coolant) will flow to points where it will block the flow of the evaporating coolant, sometimes causing popping/banging sounds as some gets through.

[MSDeneen]

Originally Posted by Wazbro

The problem with the fridge being off level is that the ammonia (coolant) will flow to points where it will block the flow of the evaporating coolant, sometimes causing popping/banging sounds as some gets through.

Last week, when we tried using AC intense banging is what we heard. I shut it all off after about 30-minutes of that racket! Mystery solved on that.



Cheers,
Mark

[ITSNO60]

Yikes, didn't realize you were trying to operate it off level. That's a big no-no and can cause permanent blockages in the cooling unit.

There is a device on the market called Fridge Defend which will protect against this by cutting power to the fridge. It does this by monitoring boiler tube temperature which goes high when operating off level. So it doesn't actually sense when you're off level but it senses the resulting over-heat condition and shuts you down before damage or fire can occur.

[uncle_bob]

Originally Posted by MSDeneen

Last week, when we tried using AC intense banging is what we heard. I shut it all off after about 30-minutes of that racket! Mystery solved on that.



Cheers,
Mark

Hi

The obvious issue there is that you can't repair the damage to the sealed cooling system. Once it's compromised it's time for a whole new fridge. In some cases it fails totally. In others it just limps along with worse performance. How much tilt for how long equals damage? That's not at all clear.

Bob