Topic: Dometic CT Thermostat error code (Read 3752 times) previous topic - next topic

We have a 2018 Wonder FTB that we purchased new. I have never been happy with the location of the thermostat. It is, in my opinion, too close to the furnace resulting in a long delay in the furnace coming back on after it has reached it's temperature. So I thought that I would move it closer to the living area in hope that we could get the temperature a bit more stable.

It has 3 wires, a 12v+, a 12v- and a coms wire that runs up to the control board in the AC. Before I drilled any holes I thought that I would test the thermostat with the longer wires to see if wire length or gauge would effect operation. When i removed from the bulkhead and added the needed length of wires the thermostat displayed an E1 error code (communications to control board).

I thought that the length of the coms wire was critical, so I put everything back the way it was, but now I cannot get rid of the E1 error code. I have done a reset by holding in the Mode and + button until it displays the -- reset code, but to no avail. I have tried every combination of reset with the AC powered on and off, furnace fuse in and out with and without a power down in between.

Next I guess I will go up into the AC and make sure that the coms wire has continuity. I don't know why there would be a problem in the factory wiring harness as the furnace was working just before I removed the thermostat.

Is there a reset sequence that I am not following? Any help greatly appreciated .

Garnet

Are you able to find the error code in the manual? That is my best advise. Please let us know when you figure it out.

The E1 code is loss of communication between the thermostat and the control board is the AC.

I have since confirmed that I have continuity between the Coms terminal on the stat and the plug on the control board. Next we hooked up an oscilloscope to the Coms wire and can see that there is a signal that changes when I touch the Mode button. So the stat and the control panel are talking, there just not communicating.

Another update:

My friend with the oscilloscope discovered that he had a suspect probe when we were checking yesterday so ran the tests again. It turns out that we cannot detect any activity on the Coms terminal of the thermostat.

After much chatting we have come to the conclusion that I must have touched the 12v+ to the Coms wire at some point and that there was some current that may have been stored in a capacitor in the control board of the AC, and that may have damaged it. I have found the board and the thermostat at an RV dealer in WA for a decent price so I will order them on Monday. Once they are here we can do a postmortem on the board and hopefully find the problem.

That's too bad. I also thought of moving mine - IMO it is too close to the entry doorway. The door is the least insulated place so likely the warmest place in the house during the summer.
I'd also like to upgrade the T-stat to the Bluetooth version. I hope the control unit and wiring are the same as the non-BT version. But regardless I'll need to be extra careful protecting the wires.

I believe the Bluetooth and regular CT stats are interchangeable, but you should check first.

My mistake, I think, is that I should have unplugged the thermostat harness from the control board at the AC before I started (very easy to do). It is really difficult to get the heavy gauge wires from the LTV harness back into the tiny screw terminals in the thermostat. Inevitably one of the wires will touch the wrong terminal or each other. I thought that just having the battery switch off was good enough.

Problem repaired! 

I replaced the control board in the AC , $56, and the thermostat, $45, and everything works as it should. I did not have the nerve to try just one and then the other to see which had failed. As I said earlier I suspect it was the control board that I damaged, but I did not want the old thermo to transfer problems to the new board if I was wrong.

Anyway, a big thanks to Panther RV just outside of Vancouver WA. They where the only ones I found on the interweb who's website could match my Dometic AC numbers to the correct replacement parts. They replied to my email promptly, confirming the parts where correct and  international shipping was just $20. Big thumbs up.

Garnet