Dometic RM1350 cooling

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Dometic RM1350 cooling

May 06, 2020, 02:57:42 pm

The Dometic RM1350 refrigerator has performed flawlessly until the last trip. In 85 degree weather, box temperature in the morning would be 34 degrees but gradually warmed up to 50 degrees during the day. The condenser cooling fans would not come on. Without removing the refrigerator it was determined that power was going to the limit switch.

Now that I am home with lots of time on my hands, I removed the refrigerator. Bench tested fans and they worked OK. Connected 110v and 12v to test. Box temperature overnight was 32 degrees. Using a heat gun on the limit switch, the fans came on at about 130 degrees.

I am stumped.

Any ideas?

Re: Dometic RM1350 cooling

Reply #1 – May 11, 2020, 12:00:04 pm

Hi Don and Sue,

Check the burner and chimney and make sure they're burning and drafting correctly.

I happen to have a used RM1350 from my fifth wheel sitting in my garage that I'm diagnosing and repairing in preparation to sell it. It likes to hang out too close to 40-44 deg for my happiness.

You may find this service guide helpful: https://bryantrv.com/docs2/docs/Service%20Manual,%20RM1350.pdf

I'm starting by dividing the problem into the cooling unit and control side as recommended here: https://www.youtube.com/watch?v=k2bBSGuwU_o

I'm going to guess that your control side is fine since you're getting to 32 in the morning.

Since I'm already down to 35 after directly wiring the heater six hours ago and have a new cooling unit, I'm suspecting a bad thermistor or something else on the control side.

Interestingly, you adjust temperature by location of the thermistor. The 1-5 switch only changes how long the heater stays on after the thermistor exceeds a certain temperature according to this guy: https://www.youtube.com/watch?v=bY7cy4Kf3VQ&t=

In my original installation, the RM1350 was located in the slide, so the exhaust air had to make a u-turn to get out instead of directly drafting out the roof. Even with the new cooling unit, it never handled outside temperatures in the 90s which is a lot of my camping, so we ended pulling the fridge and installing a residential unit.

Good luck. Don't forget to tell us what the solution finally was.

Re: Dometic RM1350 cooling

Reply #2 – May 11, 2020, 03:12:00 pm

What was the outside temps like during this time?

Re: Dometic RM1350 cooling

Reply #3 – May 11, 2020, 03:15:10 pm

Check your door gaskets for tightness using a dollar bill. It may be the control board or a bad thermistor. Dinosaur is good (or used to be) at troubleshooting electronic boards over the phone.

Re: Dometic RM1350 cooling

Reply #4 – May 11, 2020, 03:19:59 pm

Quote from: Dakota Slim – May 11, 2020, 03:15:10 pm

Check your door gaskets for tightness using a dollar bill.

No, being a high end coach you need to use a ten dollar bill.

Were it a Prevost, you would need to go to the bank to get a hundred dollar bill to test it properly.

Make sense???

Re: Dometic RM1350 cooling

Reply #5 – May 11, 2020, 04:06:34 pm

I think that's the proper gasket gap procedure.

Re: Dometic RM1350 cooling

Reply #6 – May 11, 2020, 04:40:53 pm

Quote from: Dub – May 11, 2020, 04:06:34 pm

I think that's the proper gasket gap procedure.

Yes proper procedure.

And whether you use a one, ten or hundred dollar bill depends on your coach and whether you have already gotten your allowance for the week.

Re: Dometic RM1350 cooling

Reply #7 – May 11, 2020, 05:18:57 pm

Thanks for your responses.

Refrigerator had been working properly the previous 4 weeks in ambient temperatures up to 85 degrees. Not a chimney or door gasket problem. The problem is the condenser cooling fans quit.

Additional thoughts?

Re: Dometic RM1350 cooling

Reply #8 – May 11, 2020, 05:23:34 pm

Most condenser fans are just good quality 12 VDC muffin fans. IF on a thermostat, that also could be your failure point.

Re: Dometic RM1350 cooling

Reply #9 – May 11, 2020, 05:31:50 pm

Hi Brett,

I understand that it could be the fans but I bench tested them and they worked fine.

The fans are controlled by a limit switch (thermostat?) on the condenser fin. I used a heat gun on the limit switch and it worked properly.

Re: Dometic RM1350 cooling

Reply #10 – May 11, 2020, 05:35:31 pm

Have you checked for 12 VDC positive to the thermostat? Likely there is an in-line fuse (at least that is how I would set it up from the 12 VDC coming from coach to back of refrigerator/the PC board.

Re: Dometic RM1350 cooling

Reply #11 – May 11, 2020, 05:56:19 pm

I don't think the thermostat is relevant.

I believe that there should be continuous power to the limit switch regardless of what the thermostat is doing. Heat activates the limit switch which turns the fans on.

Does that make sense?

Re: Dometic RM1350 cooling

Reply #12 – May 11, 2020, 06:53:31 pm

Quote from: djpauls – May 11, 2020, 05:56:19 pm

I don't think the thermostat is relevant.

I believe that there should be continuous power to the limit switch regardless of what the thermostat is doing. Heat activates the limit switch which turns the fans on.

Does that make sense?

NO. Think of a limit switch as a FAIL-SAFE. The current should run through both the thermostat and limit switch. But unless the thermostat fails and temps exceed the setting of the limit switch, the limit switch should just function as a "pass through" device.

Re: Dometic RM1350 cooling

Reply #13 – May 11, 2020, 08:21:17 pm

You're both right.

I think y'all are talking about two different things. OP (Don & Sue) are talking about the thermostat on the high fins that control the fans and Brett is talking about the limit switch associated with the heating unit.

Re: Dometic RM1350 cooling

Reply #14 – May 11, 2020, 09:40:58 pm

OK, anyone have a schematic or can trace the wires to the Foretravel-installed condenser fans on the 2009 Nimbus? Can you look up in the outside refrigerator access area and physically see the wires to the fans?

That will tell us where the fans get their 12 VDC and how they are controlled.

Re: Dometic RM1350 cooling

Reply #15 – May 12, 2020, 05:53:58 pm

Power to limit switch is on all the time (power with refrigerator on and no power when refrigerator is off).

Limit switch closes and supplies power to fans as designed at about 140 degrees and opens at about 120 degrees.

The problem must be intermittent but seems odd that it didn't work for about a week straight and then works as designed after I pulled the refrigerator.

I have checked for loose connections and all seems tight.

Tried calling Dometic for tech help. I keep getting the message that their phones are down. Called an RV dealer friend who told me that he has been getting the same message from Dometic for at least the last 2 weeks.

Hopefully the attachments help.

Thanks

Re: Dometic RM1350 cooling

Reply #16 – May 13, 2020, 10:06:44 am

Quote from: djpauls – May 12, 2020, 05:53:58 pm

The problem must be intermittent but seems odd that it didn't work for about a week straight and then works as designed after I pulled the refrigerator.

Maybe cutting 12v power to the control board caused it to reset.
Or maybe there was a blockage in the tubes that freed itself with the motion.

Re: Dometic RM1350 cooling

Reply #17 – May 13, 2020, 11:30:03 am

One thing you might consider since you've seen a change in behavior is this weirdness -- the RM1350 is sensitive to when it's turned on. Maybe you turned it on at a different time? See this excerpt from p.12 of the Service Manual I linked in my first reply.

The refrigerator has been designed with an automatic cooling unit cycling system that helps reduce frost build up in the fresh food compartment. The first automatic frost reduction cooling unit cycle begins 60 hours after turning "on" the refrigerator (for best operational results the refrigerator should be turned on anytime between 4 and 10 PM), and will last for approximately 120 minutes. Thereafter, the cycle will automatically repeat every 48 hours for as long as the refrigerator continues to run.

If it's running the frost reduction cycle during the heat of the day, that could explain your reduced cooling. If it's not running, or reduced running, it's not heating up the top coils that will trigger the fans to come on.

Re: Dometic RM1350 cooling

Reply #18 – May 13, 2020, 01:55:49 pm

Quote from: trailertrashrick – May 13, 2020, 11:30:03 am

One thing you might consider since you've seen a change in behavior is this weirdness -- the RM1350 is sensitive to when it's turned on. Maybe you turned it on at a different time? See this excerpt from p.12 of the Service Manual I linked in my first reply.

The refrigerator has been designed with an automatic cooling unit cycling system that helps reduce frost build up in the fresh food compartment. The first automatic frost reduction cooling unit cycle begins 60 hours after turning "on" the refrigerator (for best operational results the refrigerator should be turned on anytime between 4 and 10 PM), and will last for approximately 120 minutes. Thereafter, the cycle will automatically repeat every 48 hours for as long as the refrigerator continues to run.

If it's running the frost reduction cycle during the heat of the day, that could explain your reduced cooling. If it's not running, or reduced running, it's not heating up the top coils that will trigger the fans to come on.

My Norcold does that about every 43 hours and in hot weather the inside temp can go well over 40 -- especially if I open the door for some reason. I have an indoor/outdoor thermometer sitting on my desk that shows the inside reefer temp and when I see the reefer temp go over 40 I will turn the reefer off for 30 seconds and then restart it. I'm pretty sure that restarts the clock. Frost build up is not much of a problem for me because I am usually in low humidity areas. If I see frost built up I will let the cycle go.

*Before I knew about this defrost cycle I called tech support at Norcold. They told me to bang on the coils on the back of the refrigerator with a rubber mallet.

Re: Dometic RM1350 cooling

Reply #19 – July 22, 2020, 11:43:18 am

Update.

Even though I suspect the limit switch is the problem, I couldn't get it to act up.
Before reinstalling the refrigerator I installed an ARP Fridge Defend with an additional blower added to the side vent. Wired the OEM cooling fans to activate with the ARP sensor eliminating the OEM limit switch.
Works better than ever.

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