Dometic DuoTherm AC Problems (No Compressor)

[Woodtroll]

Hello folks,

I have the 2003 version of the Dometic Duotherm dual-control thermostat, like this one:

Dometic Thermostat

and a model 57915.531 Duotherm rooftop AC unit.

On this weekend's camping trip, when I cut the control to "Cool" with the fan on "Auto", the fan would come on but the compressor did not come on. I checked all the fuses, breakers, etc. including the 3A fuse inside the control box just above the intake filter panel, and all appear to be good.

At home, I figured I'd start with the obvious, and check the thermostat. The lug labeled "Cool" (which I assume is for the AC) has no power on it with the switch in the "Cool" position and the temp. slide all the way down, but has 12.8-12.9 volts on it in the "Off" and "Heat" positions. This lug has the yellow wire on it. I would have thought that voltage would be the "signal" to turn the compressor on, but this one seems to be backwards. ???

How do I check this thermostat to make sure it is working correctly, and does anyone have any other suggestions as to what to check to get my AC compressor running again?

Thanks very much!
Regan

[Woodtroll]

Well, using the manual, the wiring diagrams, and a lot of patience, I figured out that the problem is the relay (in the circuit board on the AC unit itself) that turns the signal from the yellow thermostat wire into a coil control for 120VAC to pass through the NO relay to the compressor. The relay is mounted to a circuit board labeled Etratech 50-001401-A. I cannot find this board available anywhere for sale (the camper is now 13 years old, after all), but the relay itself is still available in identical form for $12 or so shipped. I ordered one thinking I might be able to solder the new one in if I can't find the board. I have good wire soldering skills, but have never tried to work on a circuit board before. Luckily, it looks like this relay has sizeable lugs that are not cramped up against other solder points. If I can get the old one loose, I should be able to get the new one in.

Anyone know a source for the whole board, or an updated part number, just in case my replacement work does not turn out well?

Thanks very much!
Regan

[Woodtroll]

Chris, I figured out the freeze sensor. Mine is an NO type, and was not the problem.

I found two Dometic RV AC manuals online that were pretty helpful, in case someone else has need of them:

http://bryantrv.com/docs2/docs/acservice.pdf

http://bryantrv.com/docs2/docs/olderac.pdf

The good news is that it does not seem to be my compressor. Based on the manual and the simple multimeter I have, it seems the start capacitor may be the issue. I will have to either risk $15 or so or find someone who can test it for me.

I'll let you know how this works out; thanks for everyone's help!
Regan

[Woodtroll]

Thanks to both of you for the replies, I appreciate your time!

Chris, when you have time, could you give me a little more info on locating and shorting the freeze sensor, please? I do understand the part about grounding the yellow at the unit, and I have a pretty good basic understanding of AC theory as it applies to autos, but I have never gotten gut-deep into one of these rooftop units before.

If there is a good troubleshooting site or link to a similar issue that someone else has had with these systems, I have been unable to find it. I can follow directions, but can't come up with any on this particular problem. If y'all know of one, please let me know and I won't have to keep bothering you!

Thanks again,
Regan

[Chris_Bryant]

You got the correct readings- Do NOT jump the yellow to red or you might burn up the thermostat. DuoTherm switches the grounds on that system, not hots.
Just ground out the yellow up at the roof unit- that will bypass the thermostat and wiring. If it still doesn't come on, it could be a bad freeze sensor. That you can usually short out to test.

[Toddupton]

Usually jump yellow to red and ac should come on. If it does bad stat if not then most likely not the stat.