Dometic 5 button thermostat screen is blank

[johnfr]

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?

[MyronTruex]

I made and then bought a breakout box for the phone cable so I could actually measure the 12 volts on the phone cable.

When you did your modification to split the AC units via a second thermostat, you may have still been picking up 12 volts from the other AC phone type cable.

I would get fresh/new inline couplers in any case. They live in the direct path of a hurricane filled with moisture at times. Recently taking mine apart after chasing an intermittent for more years than I am proud to admit, I found most of the inline couplers very suspect looking. After replacing them I have not had any more problems.

As for making your own cables, do you have a tester for the cables? 

I decided to re-crimp my plugs and why it did not occur to me that the crimper would short all the wires together is baffling. I am getting a bit old these days and makes some silly mistrakes.  This led to a blown fuse on the circuit board. Fortunately no more damage was done except my ego is still sore. 

When using the break out box, mine keeps the thermostat inline so there is a load on the line. That is, the voltage measurement is while under load so there is little change of getting fooled.

Finding a decent ground for the meter can be a bit tricky too but at least there is plenty of metal on the ac.

I bought a factory made phone cable, tested it with my Cat 5 cable tester, and have it labeled for testing in the future. It is long enough to stretch from the front AC to the Rear AC.

I think you are on to the problem. Just having confidence in what and how you are testing is important and think you are right there. There has been a couple of very long and thorough explanations/trouble shooting threads. A search should pop up a good days readings. 

Getting the thermostat to "light up" is the goal of course. Keep in mind after that, how long the delays are for the  AC units to come alive and that you need to do a Reset when moving a thermostat around. You can't do a reset till you can get  it to come alive of course.

[jacwjames]

Word of caution on making your own connectors.  I bought a good quality crimper  that came with a couple styles of ends, but not the ones that I needed.  I had to order the type I needed before I actually got some I could use. 

I needed a cable for a Progressive HW50C display, the original wire failed, I still had the display outside but the inside display quit working.  I took the display outside and used a short cable and it worked so had to replace the longer display.  They specified the cable ends, which I ordered but the first two didn't work, either too loose or oversized to fit in the crimper.  Too loose could cause a problem as Myron suggested.  Third order got some that were very snug in the crimper but they worked. 

[johnfr]

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. By removing the light in the ceiling of the bathroom I found the other end of the cable. I am going to slice them because the end that goes to the thermostat is embedded in spray foam insulation that I sprayed into the channel it passes through to keep mice out of the area behind the control panel. I worked for 40 years as an electronics engineer so I know how to solder etc.. 

 

[Tom Cherry]

6 hours ago, johnfr said:

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. By removing the light in the ceiling of the bathroom I found the other end of the cable. I am going to slice them because the end that goes to the thermostat is embedded in spray foam insulation that I sprayed into the channel it passes through to keep mice out of the area behind the control panel. I worked for 40 years as an electronics engineer so I know how to solder etc.. 

 

Hard to read the BLACK cut and paste of a “picture”.  We prefer that the text is typed or copied and pasted.  Please conform to the usual posting rules in the future.

Seems like you have it under control.  But some members think that the “phone line” is a PHONE.  It is NOT. It is a Data Cable.  There is a schematic in the Dometic install manual.  You have to use a cable that is terminated properly….otherwise NO JOY.  Next up.  The connectors in the plenum that connect the thermostat to the Danglers or the ones hanging down are SPECIAL.  If you toss them and put in a RJ11 replacement….again NO JOY.

The Monaco connector is like this.  Lay it on your bench.  Then put a VOM on the pin furthest from you on one side….then on the mating one on the other side.  The wires RUN STRAIGHT through.  But the “STRAIGHT” ones, advertised, do NOT work.  Trust me on that.  I’m trying to get a PN.  If you order one from Amazon….the top left pin is connected to lower right.  That’s right…they are crossed…NOW WHY…no idea…but myself and others have ordered all different brands.  They ALL come in REVERSED.  So…google a DATA cable diagram.  Then cut off ONE end of your “phone” cable.  Reverse them so they match the Dometic….that is what you need.  If you find the mate to what Monaco used….let us know…I have calls into several large suppiers and “THEY DONT KNOW”.  All they sell is for PHONES.  We have DATA cables.

You obviously know the reset protocol and will have to do that.

Good Luck and keep us posted….

[johnfr]

Sorry about the white text on black background. I have no idea how or why that happened. I didn't notice it when I made the post.

As a follow up, I fished a new wire I found in one of my junk drawers. I cut off one end and spliced it onto the the wire coming from the thermostat per the schematic in the Dometic install manual. When I plugged the thermostat in it still didn't work. I ohmed out the cable and discovered that one pin on the AC end was not connected. I went to Home Depot and purchased a new one and fished it through to the AC unit. That time before soldering everything up I checked for continuity end to end and all ohmed out OK. I then plugged in the thermostat and it came on.  I soldered up the splice and insulated each wire with heat shrink tubing. I retested it and all was well.