HVAC Zone 3 and Dometic Thermostat Issue

[KenNJ]

I am currently experiencing a problem with zone 3 in my motorhome. Two days ago I noticed a code E1 (loss of communication) with my zone 3. I reset the system and all was well again for a couple days. Yesterday I noticed that I now only show two zones on the thermostat. I went through the reset process again but no luck. I no longer even get the E1 error just my zone 3 will not show up on the dometic thermostat, PERIOD.

I contacted Newmar and Dometic to ensure I was completing the trouble shooting steps in the correct sequence and confirmed I was doing that portion right.

I went up on the roof and removed each units cover to check the circuit boards and connections for each device and found nothing that looked wrong. I was checking for loose connections paying particular attention to the communication cables for each zone but again, no problems. I even unplugged the phone type connection and the plugged in back in for each zone directly on the circuit boards. I also cycled the dip switches returning everything to the original position. I noticed no corrosion of any kind on the connections or circuit boards. Next I removed the stat from the wall, un-plugged the stat and checked visually using a magnifying glass looking for burn marks etc, again nothing. I plugged the stat back in and attempted the start up procedure but still only two zones 1 and 2 show up not all three. Worth noting, zones 1 & 2 work perfectly.

This same thing happened to me back in February of this year and corrected itself in a day or so but not this time around. I have ordered a replacement thermostat (got the part number from Newmar) but honestly I am not sure that is really the problem. Neither Dometic, Newmar or a local Newmar dealer could provide me any further steps so now what ? Well I should add the local Newmar rep did state he replaces these Dometic thermostats all the time. Frankly this is the only reason I ordered a replacement thermostat. The service manager sounded confident with his recommendation even though I did not purchase the thermostat from him.

I don't mind paying for someone to come out but what exactly would a mobile tech do at this point ? The only thing I can think of is to have him check the control board on the third AC unit but if anyone has any other ideas I am open to it.

[MtnTrek]

Sorry to hear you're having difficulties.
Sure sounds like you are going about the troubleshooting process well.
Often the shop service techs do have the luxury of being able to swap out parts until they isolate the problem(s). After which they can return items to stock or rely on the vendor to resupply. Having been a commercial controls tech for decades, yes we did swap parts frequently. Field component level troubleshooting is seldom billable.
Unfortunately as consumers we don't have that luxury.
The other vulnerable link is the communication cable which may have been compromised? With a spare female socket (RJ-11, RJ-45 ?) you might use some jumpers etc. on the one end and tone it from the other, at least eliminating the cable?

I'm dealing with a faulty communication cable on a different type of subsystem and pondering on the most palatable solution considering it's inaccessibility. I'd almost welcome replacing a circuit card.

Best of luck.

[David D.]

Exactly what happened with mine, I lost zone 2. I even took t-stat to the roof and plugged directly into unit 2 and all it would reconize was zone 1. Circuit board was bad.

[KenNJ]

David D cannot thank you enough for that info and reply. It's always nice to hear when you ideas are confirmed. Tomorrow I am changing delivery of the new thermostat to overnight so I am hoping to report back on Wednesday or Thursday this week.

MtnTrek as a FYI Dometic does not provide a spare jack but that was a good idea if they did.

[David D.]

Ken,
Sorry if I wasn't clear but it was not the t-stat, it was the control board in the roof topA/C that was bad. The board the cables plug into.The only way to eliminate a bad cable from 1 A/C to the other is to remove the t-stat from the wall, take it to the roof and plug it directly into unit 3 with a small jumper cable. You can use a telephone cable but you need to cut the wire into and reverse the wires.

[KenNJ]

There are a couple of RV techs in my area that I am going to call in the am to see if anyone has this in stock. From Dometic this looks to be about $143.00 bucks and looking at the darn thing it certainly does not look like is should cost that much and for that much it certainly should never fail like it has. O well what can you do about electronic parts.

Well I am glad you pointed this out because it makes sense.

[MtnTrek]

Originally Posted by KenNJ

MtnTrek as a FYI Dometic does not provide a spare jack but that was a good idea if they did.

Yeah, wouldn't expect they did. It would be a small inexpensive part you'd purchase at a supply store, to aid you to troubleshoot by your lonesome. I worked alone so many yrs. I think that way. As David noted they use a crossover VS a std. patch configuration.
No worries, sounds like you have it by the horns.
Please give us an update.

[bruceisla]

Your A/C top units are in a daisy chain. Thermostat goes to unit 1 (front), unit 1 then connects to unit 2 (mid). If you have a hydronic unit (HydroHot, etc) it is probably unit 3, and unit 4 is the rear unit. Could be slightly different on your coach. In short, you need to check the other end of the communication cable at the unit preceding the rear unit.

It sounds like you have a bad connection between units. You may have lost 12vdc at the rear unit. 12vdc is typically carried in the communication line from the mid to the rear unit.

It is not likely the thermostat is bad if the other top units work correctly.
Check that your hydronic works correctly. Usually the thermostat "thinks" it is unit 3.

[KenNJ]

Okay as suggested in another post it was the Control Board directly on the Zone 3 unit. I was able to cancel the Thermostat order from Amazon and then I struck gold because my local dealer (Berryland) had the replacement board in stock. I replaced what is called the CCC2 Relay Replacement Board part # 3312022.000. This was a direct replacement for the original. There are many places you can purchase the board from, just use that exact part # and you will find it. Also, this bad boy was $145.92 at my dealer, Dometic wanted $8 dollars more.

I am back in business and thrilled. I have 3 15k units (upgraded during my build) and was fine with 2 units but being down in Louisiana, yeah I am glad I have 3 of these.

Hope this info is helpful should someone else run into the same problem.

Cheers

[MtnTrek]

Thanks for the update, beat that Creole heat.