Dometic Single Zone LCD Thermostat/AC/Furnace Problem

[CEB]

I have a 35' trailer with a Dometic AirCon & Furnace system which uses a Dometic Single Zone LCD thermostat. The furnace and fan system seem to be functioning properly. I can select "fan" mode on the thermostat and run the fan at "hi" or "low" speed. I can also select "furnace" mode and the furnace starts. But, when I try to select "cool" mode on the thermostat the "cool" mode light on the thermostat does not illuminate. The circulation fan does turn on but the air compressor does not. The thermostat is getting the 12VDC supply voltage. Does anyone out there have any experience with Dometic RV systems. I'm trying to figure out if it is a thermostat issue or if the problem is in the system control box. I have managed to download a Dometic Diagnostic Service manual (Form No. 3311071.017 01/12) but it doesn't address whether the lack of the indicator light is indicative of a thermostat malfunction. Would the thermostat "cool" mode indicator light illuminate even if the control box was malfunctioning or is the lack of the indicator light illuminating indicative of the problem with the thermostat not supplying the required signal to the control box to engage the AC?

[ncmurf00]

Is the thermostat temp set low enough for it to kick on?

[CEB]

When the problem first appeared it was earlier in the year when temps were well in the 90s during the afternoon. This was during our last outing for the year and we haven't taken the trailer out since but now that we are approaching spring in Arizona it's time to get the problem resolved.

When the system was functioning normally during warm weather the indicator light on the thermostat would always illuminate when "cool" was selected. Granted the air conditioning compressor would shut down when the thermostat set temperature was reached but the "cool" light did not turn off. It remained on as the compressor cycled.

[kenandterry]

There’s so many different RV thermostats out there, even within Dometic.

Any chance you can post a photo of yours?

[ncmurf00]

Thanks for the additional background.
If the cool indicator doesn’t come on, I’m leaning towards the thermostat itself. Maybe the contacts on the switch are corroded. I’ve never seen one wait for feedback from the unit to illuminate that indicator.
Cheapest one to try, if you wanted to replace parts. There is a way to jump it off to force a test but I wouldn’t recommend it, in case something accidentally shorts in the process.

[CEB]

The thermostat is a Dometic Single Zone LCD model 3313193.xxx as shown on the attached. It's the one with "fan, cool and furnace" indicator lights only. No "heat" indicator.

[Bob K4TAX]

We have a model that looks like that.

I suggest setting the FAN to AUTO and then selecting Cool. I find unless we specifically want the FAN to run on Hi or Lo, then AUTO is the best as it allows the Furnace fan to come on before the gas is turned on and allows the furnace fan to continue running for a few minutes after the gas is shut off.

As to the AC, again with the FAN set to AUTO, it provides both HI speed fan and Lo speed fan as needed for cooling.

As to the AC LED not illuminating, this may indicate a thermostat issue. Check wiring on both ends, the thermostat, and in the AC, to be assured something hasn't come loose.

Bob

[kenandterry]

Originally Posted by Bob K4TAX

We have a model that looks like that.

I suggest setting the FAN to AUTO and then selecting Cool. I find unless we specifically want the FAN to run on Hi or Lo, then AUTO is the best as it allows the Furnace fan to come on before the gas is turned on and allows the furnace fan to continue running for a few minutes after the gas is shut off.

As to the AC, again with the FAN set to AUTO, it provides both HI speed fan and Lo speed fan as needed for cooling.

As to the AC LED not illuminating, this may indicate a thermostat issue. Check wiring on both ends, the thermostat, and in the AC, to be assured something hasn't come loose.

Bob

Bob K4TAX described the operation of our thermostat too.
Here’s the photo of our model just for comparison, and you’ll notice the photo shows the fan in “AUTO” mode as described.

[bubbles]

Originally Posted by CEB

The thermostat is a Dometic Single Zone LCD model 3313193.xxx as shown on the attached. It's the one with "fan, cool and furnace" indicator lights only. No "heat" indicator.

The mode light should illuminate regardless fan speed setting. Might run the initialization sequence again:

• Make sure the Single Zone LCD thermostat is in the Off condition. See page 4, “Quick Reference To Control Buttons”.
• Press the “+” button and, while holding it, also press and hold
the On/Off Mode button for three seconds. LCD will show
― ― . Press the On/Off Mode button again to turn system off.
This completes the initialization.

If you have more than one furnace swap the stats, it's just 3 wires at the stat.

[CEB]

I have tried running the initialization sequence several times to no affect. Selecting "auto", "hi" or "Lo" fan speed makes no difference. Still the "cool" mode light does not illuminate.

I'm going to take a look into the control box which is mounted in the ceiling to see if there is any obvious loose connection.

I am inclined to agree that the fault is most likely in the thermostat itself but without access to a circuit diagram or "principles of operation" description I am still not 100% sure there isn't a problem in the control box.

Thanks for all the replies.

[bubbles]

This is all I have. I'm leaning towards the board in the control box. If it was a home unit I would think start capacitor but since the TS cool light isn't coming on don't really know.