Dometic Duo Therm AC issue

One way you can find out is replace the controller. I ripped out the digital thermostat and "smart" controller from mine and put in a "dumb" relay box that I then hooked up to a Honeywell T6 Pro ZWave thermostat (works without a C-Wire and while not rated for 12V works just fine). With the dumb box in place you could literally use a paperclip to jump the 12V + to the fan low on the AC and make it work (until it iced up).

My thermostat system uses the RJ11 phone style connectors. Not sure how to connect another thermostat that doesn’t use this connector type?

1. Yes, Heat is probably the issue
2. NOTHING on the AC unit will cause this problem
3. The problem is what the dealer stated, a Communication issue
4. The system uses Telephone type 4 wire cable to communicate from the rear to front AC and then down to the wall Tstat. 99% of the time the Front AC is the control point---The rear AC cable goes to the front AC and then the Front AC goes down to the Tstat on the wall.
5. You don't state, but do you have Heat Pumps--specifically Penguin lo profile models. They were prone to the humidity in the evap area where the Phone quick connects were located causing corrosion. Regardless, you need to access both roof top units and verify the
RG11 connectors do not have corrosion. IF they do, (very common) you just cut off and crimp new RG11 connectors. FOLLOW THE COLOR CODE OF THE OLD CONNECTOR. These are NOT phone cables but Comm cables and are wired backwards.
6. Possible but you have 2 Control boards. Each AC has 1 control board. RARE for a Control board to cause this type Com problem
7. Regardless, The FIRST think you do when confronted with your type complaint is check the various RG 11 connectors. Doug

Thanks Doug

My units do have heat pumps.

From what I can see the phone connectors are inside the RV accessible through the air intake grates on the ac units. I cannot see any significant corrosion on the connectors.

I quickly took a look under the AC shrouds and didn’t see any phone connectors near the control boards, but I’ll look again.

Ok further update.

I went up and inspected the RJ11 leads on the back unit (which in my case is the control unit). The wire leads and connection ports on the main board were free of corrosion and/or oxidation.

My RV dealer is now saying that dometic has had struggles with the black AC covers. On a hunch, I put wet white towels over my ac covers. Within a half hour both fired up again. My dealer is suggesting I replace my covers with white ones to see if that helps. He is also going to see if my extended warranty will replace the boards on both units.

Berkyeng wrote:
Ok further update.

I went up and inspected the RJ11 leads on the back unit (which in my case is the control unit). The wire leads and connection ports on the main board were free of corrosion and/or oxidation.

My RV dealer is now saying that dometic has had struggles with the black AC covers. On a hunch, I put wet white towels over my ac covers. Within a half hour both fired up again. My dealer is suggesting I replace my covers with white ones to see if that helps. He is also going to see if my extended warranty will replace the boards on both units.

Sorry------ BS!. Dometic has NEVER had a problem with Black covers. I live and work in Texas and we have extremely HOT weather. WE also have a lot of OEM's with black covers. I have over 23 years of working on and fixing the Dometic CCC system.

Also, On Penguin's you have the 2 modular RG11 connectors ON the circuit board. Those are 12 inch pigtails and those go INSIDE the Evap area where the OEM attaches the OEM run COM cables. There will be 2 cables. 1 is used on the rear AC and BOTH will be used on the front AC. They will have offwhite quick connect splice modules. So, there will be 6 RG 11's at the rear AC and 6 at the front AC. Did you find 6 at each? If not, you missed them. Doug

Yes I inspected the 2 connections on the board. There are 4 connectors in the back evap area, then 3 in the front evap (one of which deadheads into a quick connect as you suggest. All connectors look clean. I also inspected the connector at the back of the thermostat.

Any more suggestions? My unit was up to 42C today.

Thanks again.

It is very possible you have a bad circuit board. Odds are it is the front AC board due to it went blank but the rear worked. But NOT 2 boards with the same identical problem. If it was me and you were at my shop, I would replace ALL the RG 11 connectors, then test. Did you look inside the quick connect off white junction boxes for corrosion?
Also, what YOU can do, is, swap the front and rear AC boards. See what happens. 99% of the time it is the Com Cable/s or connections when you have loss of display or communication. Doug

PS, DISCONNECT the rear to front Com cable at the front AC and see what happens. This removes a possible dirty signal from the rear area ac and cable

Thanks again Doug.

In your PS comment you mention disconnecting the front to back comm line. Is this something I can try independent of the board swap? Not sure I have the know-how to do the board swap.

Update:

I made some sun shades out of white chloroplast and bungees them over each AC. Both units have been running strong since. Maybe white shrouds would do the trick??