Dometic 5 button thermostat screen is blank

[johnfr80504]

On a recent trip our rear furnace stopped working in our 2007 Monaco Cayman PDQ. The 5 button thermostat screen was blank indicating it was getting now power. I checked for 12V power at the furnace and all was well. I then used a jumper to activate the call for heat from the thermostat and the furnace came on and ran OK.

I knew that the thermostat got it's power from the AC unit and that the call for heat signal was simply passed through the AC control board. I checked all the fuses I could find that might be feeding 12V to the AC unit. Today I finally figured out that the AC unit gets it's 12V power via the CO2 detector supply line. I hunted down the fuse for the CO2 detector and sure enough it was blown. I replace the fuse and I can now measure 12V at the input of the AC units circuit board.

But the thermostat still isn't getting power. I checked the fuse on the circuit board in the AC unit and it was good. It has a Dometic #3311557 controller board. (NOTE: several years ago I revamped the system such that I added a separate thermostat for the rear furnace & AC.) I ran a new telephone type cable to the second thermostat. I was attempting to measure the voltage at the telephone connector and inadvertently shorted out 2 of the pins and blew the 3A fuse on the board.

If I swap the 2 thermostats, the one that isn't working on the rear AC/furnace works on the front AC/furnace. And the front thermostat doesn't work when plugged into the rear AC.

At this point I am not sure if I have a bad board or a bad cable. I made up a test cable this evening to bypass the cable in the ceiling. I will pick up a box of 3A fuses in the morning.

Just curious if anyone has ran into this issue and if so what they found was the solution?

[johnfr80504]

OK I found the issue. A mouse or rat had chewed the telephone cable clean in two. Probably popped the fuse for the CO2 detector in the process.

This morning I picked up a box of 3A fuses at the parts store. I replaced the 3A fuse on the circuit board and connected the thermostat using the test cable I made up. It came alive and I was able to start the furnace. At this point I knew I had a bad cable. the connectors looked good so I began to investigate what it was going to take to pull a new wire. As I tugged slightly on the AC end of the cable I found lots of slack. Pulled a bit more and it was showing no resistance. Soon the bitten off end fell in my face. Must have been a big rat! When the problem showed up we were camped in the Fishing Bridge RV Park in Yellowstone. Guessing a local forest packrat decided to pay us a visit and found his way into the ceiling.

Now I just have to fish the cable back through the ceiling and splice the ends together.