Refrigerator Won't Cool Below 40 Degrees

[HarryB1]

A couple of weeks ago my neighbor purchased an older RV with a Dometic RM1350 refrigerator that holds the refrigerator temperature at 40 degrees as verified by a number of different thermometers. I'm wondering if one of the control boards is defective.

The refrigerator is located in a slide, and the air flow channel is well constructed (although it could be slightly improved at the top). The heaters' resistance is within spec, although the wattage they draw is on the low side of spec. And the thermistor's resistance is also within spec when tested in a cup of ice cold water.

The fins occasionally develop some ice, but they are typically wet.

However, the eyebrow display shows 32 degrees when the actual temperature is 40 degrees. And when the thermistor is placed in a cup of ice cold water, the eyebrow display shows 25 degrees.

Even though the interior temperature is 40 degrees, the heaters are cycling on and off. Is there anything we're missing?

[HarryB1]

Instead of replacing the lower control board as I had planned, I decided to gamble $10 by installing a rheostat in parallel with the thermistor. I guessed at an appropriate value for the rheostat's resistance, but it caused the refrigerator to run a little too cold, so my neighbor was able to compensate with the control panel's temp. set button. (I could, of course, have adjusted the rheostat, but this gives him a little more control over the temperature.) The refrigerator's temperature has been holding between 33 and 35 degrees for a couple of weeks now after installing the rheostat.

Reducing the overall resistance of the thermistor's circuit seems to be fooling the control board into thinking the temperature inside the refrigerator is warmer than it is, so it continues to power the heaters until the circuit's output reaches the newly established value.

Of course, I don't know why the control board is looking for a value that is different than the specification---was it shipped this way from the factory and the previous owner lived with the problem, or is something deteriorating in the board itself? Only time will tell.

[HarryB1]

We've been running the refrigerator's heaters wired directly into an AC outlet for a week and a half, and it is cooling very well---it's typically running about 33-34 degrees in the refrigerator and below 5 degrees in the freezer, occasionally below zero. The eyebrow LED typically shows 17 degrees when the refrigerator's actual temperature is about 33 degrees.

The thermistor's resistance is about 8,650 ohms when the inside temperature is about 33 degrees, and as soon as the refrigerator door is opened it begins to drop. And when a warm sponge is held against the thermistor it's resistance drops rapidly to below 3,000 ohms. Because the spec for this thermistor is approximately 8,000 to 10,000 ohms in ice water, does this not rule out the thermistor?

The diagnostic test revealed what I had suspected, namely that the displayed temperature is dramatically lower than the actual temperature.

I did not test the continuity of the upper display board because: a) I couldn't figure out how to access it (there are a couple of small slots on the right side of the trim, but when I put a small screw driver inside one of them I could see the board moving so I didn't want to pry on that), and b) those tests only indicate continuity or the lack thereof. Because the board's display responds to the control panel's buttons, I assume the continuity tests would not provide any additional information.

Unless someone has another suggestion, I plan to recommend my neighbor look at replacing the lower control board.

[dougrainer]

HarryB1 wrote:
After running the refrigerator on LP for 25 hours the inside temperature was about 50 degrees as evidenced by the thermometers inside the refrigerator. But, the eyebrow temperature display showed 33 degrees! The burner was on and the outside fans were running.

I'm not sure what happened during the day, but I suspect the refrigerator door may have been opened many times because my neighbor was cooking up much of the perishable food because of the high temperatures.

Tomorrow morning we're going to wire the heaters directly into the AC outlet again and monitor it throughout the day. And we might let it run that way during the night as well if he feels Ok about it.

I'll report back when we have more information.

Your main problem is, YOU HAVE TOO MANY COOKS TRYING TO WORK ON THE REFER:B You cannot do a functional correct test when you CANNOT control the environment---using the refer, having food in the refer, opening and losing the doors to get items in and out, etc. Also, is it possible you have TOO MANY items in the refer? You MUST have air space between the back bottom wall and the evap fins to allow warm air to flow up and thru the fins to remove heat. Plastic bags with food and shoving things in tightly and packed and the refer will not function correctly. YOU CANNOT DO A FUNCTION TEST WITH ITEMS IN THE REFER AND USING THE REFER. Doug

PS, from Dometic.

The temp sensor in the new generations unit with digital display shows the average temp in the refer.

If it showed the actual temp every time the customer opened the door the temp could go up to 48 to 60 degrees.

If you put the unit in the diagnostic mode it will tell you the actual temp at the thermistor.

[Old-Biscuit]

Contact C.B (posted about replaced his 1350 and has 'parts')
Swap out eyebrow circuit board and see if it allows longer cooling cycles/colder temp in food compartment

[CJW8]

What is an eyebrow temperature display?

[HarryB1]

After running the refrigerator on LP for 25 hours the inside temperature was about 50 degrees as evidenced by the thermometers inside the refrigerator. But, the eyebrow temperature display showed 33 degrees! The burner was on and the outside fans were running.

I'm not sure what happened during the day, but I suspect the refrigerator door may have been opened many times because my neighbor was cooking up much of the perishable food because of the high temperatures.

Tomorrow morning we're going to wire the heaters directly into the AC outlet again and monitor it throughout the day. And we might let it run that way during the night as well if he feels Ok about it.

I'll report back when we have more information.

[wa8yxm]

Even with a fan on the coils (inside and out) I have noticed with the outside Temps go into the "I'm melting" range (say 80 or higher) the units do not cool as well.. Just so you know. Now I have fans both inside and out so the effect is to self-defrost but still.. I'd like it cooler inside.

(The Freezer still freezes though. I did have to defrost it last week... Man did I get more room).

[dougrainer]

You can run the refer 24 hours longer with the 120 elements hot wired. You have verified it is not a cooling unit failure/leak. You can have a defective Thermister. Defective Thermister's will test(ohm) out correctly. The ONLY way to verify a bad Thermister is to replace it. The defective Thermister will indeed cause short cycling, but will also shut the LP on and OFF also. Doug

[YC_1]

Don't overlook the drain tubes for cracks or improper installation

[HarryB1]

I am still convinced the main problem is that the heaters are cycling on and off at too high of a temperature. Permit me to provide some additional information:

A couple of days ago I had my neighbor run the refrigerator on LP for about six hours. (He was low on LP so didn't want to run out during the night and have the refer heat up and spoil his food.) After the six hours the fins had some ice on them, which soon disappeared after switching to AC.

The next day I wired the heaters directly into the 120 AC outlet and ran it that way with the refrigerator turned off, so all of the controls and wiring of the refer were bypassed. (Consequently, the external fans did not run at all.) I did not feel comfortable doing this overnight (especially because it's not my RV), so we only ran it during the day when we could monitor it. After about 7 or 8 hours we checked the fins and they had some ice on them.

I plugged the refrigerator into my Kill-A-Watt meter and that is when I discovered that the heaters are cycling on and off because sometimes the refrigerator is drawing 380-400 watts, and at other times it's not drawing anything.

Something is telling the heaters to cycle on and off, but we haven't sat there all day watching the Kill-A-Watt meter to see if this happens at random intervals or on a regular cycle.

In summary:

1) The heaters are cycling on and off and maintain the refrigerator at a near constant 40 degrees whether during the day or at night

2) The eyebrow temperature display shows 32 degrees when in fact the actual temperature is 40 degrees

3) The thermistor's resistance is within spec

4) When the thermistor is immersed in ice cold water the eyebrow display shows 25 degrees

5) If the door has been open for a while, the eyebrow display shows a much higher temperature

All of this leads me to conclude that the refrigerator's controls falsely "think" the inside temperature is 32 degrees when in fact it is 40 degrees, and therefor the controls cycle the heaters on and off to maintain that temperature.

[HarryB1]

Thank you Old-Biscuit for providing the three corrections.

In reply to your comments:

a) The two outside fans seem to be working properly because they cycle on and off as the temperature of the condenser fins changes in response to the heaters turning on and off

b) My neighbor had borrowed a number of thermometers from other neighbors---at one time he had 5 different thermometers in there! I had him fill a small plastic bottle with water and poke a hole in the cap through which he inserted the shaft of a small stem thermometer, and this we placed on the top shelf of the refrigerator under the thermistor. Prior to doing this we checked the accuracy of that thermometer by placing its stem in a beaker full of ice cold water, ensuring it read close to 32 degrees.

c) Regarding the "automatic cooling unit cycling system": I had read about that in the manual and concluded it shouldn't be a factor at this point in time because we've turned the refrigerator off a number of times in the last couple of days to test various components, and that cycle isn't initiated until 60 hours after the refer is turned on

d) This afternoon I asked him to switch the refer to LP and run it that way for 24 hours. I'll check back with him tomorrow afternoon and report back what I find.

e) Regarding air leaks: One of the freezer doors isn't sealing properly as evidenced by the dollar bill test, but at this point I don't think it is the main culprit. I did not look closely at the drip tray drain tube---I'll try to remember to do that tomorrow---but I do recall seeing the check valve.

f) I did not perform the upper display panel tests, but I'll take a look tomorrow to see how long the refrigerator doors would need to be open to do so---we're trying to keep the doors closed as much as possible because he needs to keep the food as cold as possible.

[C_B_]

After chasing all the supposed fixes for 2 years to make my RM1350 operate properly I cured the problem!!!

It's called Samsung RT21M6213SG/AA







I'll admit this cure is not cheap.

BUT------ Freezer keeps ice cream hard.

Refrigerator has not gone above 35 degrees.


Went from 13.5 cu. ft. to 21.2 cu. ft.


If anyone is interested the RM1350 with all the fans and duct work are out in the shed.



C.B.

[YC_1]

Great advice:

Fans must be working. Remove the upper outside panel and look inside. If you can't peek down then use a camera to see if there are some fans. There should be a snap thermistor that controls them. You might be able to trace the wires down to find it. A simple alligator clip will bypass it for testing.

The other thing is the vent tubes. If they are cracked they let in warm air. If you need to replace them you might be able to slide the refer forward just a bit after finding all the hidden screws holding it in place. Front and back of course.

[midnightsadie]

listen to old biscuit, he knows his stuff. one other thing ,it takes about 12hrs or more for these frigs to get cold.