Dometic Duo-Therm expert needed - compressor DOA

[saxmandan]

I decided to try to upgrade the analog Duo-Therm thermostat, which has inaccurate temp setting, with a digital unit. I tried two different Honeywell units, and was never able to get it working as it should. I was also going to add the toggle switch option for hi-lo fan speed. I decided to put back the original and just order a new Duo-Therm replacement, and now I find the AC compressor isn't kicking in. The fan comes on, and the furnace works as it should, but no compressor. It's very possible I caused damage trying different wire combinations with the Honeywell thermostat. Any ideas on what part is damaged. Maybe a control board?

[Xmcdog]

Are you sure you have the stat retired correctly?
What are your colour codes?

[Ivylog]

Start with replacing the start capacitor... less than $20.

[saxmandan]

I took a photo of the thermostat before I disconnected anything, so I know the wiring is correct. Left to right , red, yellow, white, blue, orange and brown.

I checked the 3A fuse in the control box, which is on the roof, and it was OK.

Changing the capacitor sounds like a good idea. Could this have been damaged mis-wiring a thermostat? I'll order one and keep my fingers crossed!

[Old-Biscuit]

Duo-therm uses DC + (RED) for relays
RED-----7.5V (RH/RC)
Yellow---Cool (A/C Compressor)
White---Furnace
Blue----Hi Fan
Orange---Fan
Brown (Green) ---DC Grd





Honeywell uses DC - (Green) for relays
Red...Not used (cut/tape off)
Yellow----Cool (A/C Comressor)
White----Furnace
Blue-----Hi Fan
Orange...Fan
Brown (Grd)....RH/RC

[Dom]

If you are able to use a voltmeter you can check voltage at compressor. If you take a pic of terminals, we can tell you where to place meter

[jacwjames]

Did you try to do a reset to the thermostat?

[saxmandan]

Well, with the cover off today I checked and the compressor seems to be working, or at least it's vibrating when the thermostat is set to cool, and the temp is turned down. Unlike my last RV, a Winnebago Vista, I can't hear the compressor kick in from inside the coach. So, at this point I won't do anything until a warm day where I can test to see if it's putting out cold air. If not, I guess I have other issues. I used it in Key West over the winter, and it was working fine, so I'll stop checking till I get a warm day.

Thanks for all the suggestions.

BTW, as well as the Dometic analog thermostat I have ordered, I have an Coleman Airxcel coming also:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
It uses 12v, whereas the Dometic is 7.5v, but some on Amazon say it works, and it uses 6 leads, which is the same as the Dometic.

[Stajones]

Originally Posted by Old-Biscuit

Duo-therm uses DC + (RED) for relays
RED-----7.5V (RH/RC)
Yellow---Cool (A/C Compressor)
White---Furnace
Blue----Hi Fan
Orange---Fan
Brown (Green) ---DC Grd





Honeywell uses DC - (Green) for relays
Red...Not used (cut/tape off)
Yellow----Cool (A/C Comressor)
White----Furnace
Blue-----Hi Fan
Orange...Fan
Brown (Grd)....RH/RC

Hi,
I’m having a couple of issues as well and you seem to have a pretty good handle on the Duo Therm control wiring.

Is it possible to bypass the thermostat and “jumper control wiring” to get a unit up and running temporarily? I have done this before on a residential unit (without heat pump).
My coach units are the Penguin A/C-Heat Pump models.

I’ve been through a bit of troubleshooting thinking I had a bad start cap on the front unit...long story...and front unit is back working but now my rear unit doesn’t show up as Zone 2 on the thermostat...thus no run!

I’ve cleaned all the “straight wired” phone cable comm connections - swapped control boards from front to rear - reset DIP switches (Furnace on front, Z2 on rear) - reset Thermostat (was getting “EE” before swapping control boards).

So...at this point I just need to get the rear unit running to help out the front unit for a few hours on these West TX 100 degree afternoons, until I can get the control issue solved.
Any suggestions?
Thanks in advance!

[Old-Biscuit]

Originally Posted by Stajones

Hi,
I’m having a couple of issues as well and you seem to have a pretty good handle on the Duo Therm control wiring.

Is it possible to bypass the thermostat and “jumper control wiring” to get a unit up and running temporarily? I have done this before on a residential unit (without heat pump).
My coach units are the Penguin A/C-Heat Pump models.

I’ve been through a bit of troubleshooting thinking I had a bad start cap on the front unit...long story...and front unit is back working but now my rear unit doesn’t show up as Zone 2 on the thermostat...thus no run!

I’ve cleaned all the “straight wired” phone cable comm connections - swapped control boards from front to rear - reset DIP switches (Furnace on front, Z2 on rear) - reset Thermostat (was getting “EE” before swapping control boards).

So...at this point I just need to get the rear unit running to help out the front unit for a few hours on these West TX 100 degree afternoons, until I can get the control issue solved.
Any suggestions?
Thanks in advance!

Sorry Stan
Not up to snuff on those Dual Zone T-stat wiring



T-stat goes to Front A/C and then cable goes to Rear A/C Unit
Bad connections?
Use swap and some alcohol to clean up

[Stajones]

Originally Posted by Old-Biscuit

Sorry Stan
Not up to snuff on those Dual Zone T-stat wiring



T-stat goes to Front A/C and then cable goes to Rear A/C Unit
Bad connections?
Use swap and some alcohol to clean up

Thanks for response!
Yes - reset thermostat several times, and soft “filed”/alcohol cleaned all comm connections. I’ve ordered a 50FT straight wired cable to troubleshoot with - will be here tomorrow - but I might pull the rear board this morning while it’s cool outside and bring it down to see if the thermostat will recognize it at that front to rear connector.
Thanks again - I figure there’s a way to jumper at the unit but I’m afraid I’d end with a “smoke test” -;(

[cavie]

[QUOTE=saxmandan;5247887]I took a photo of the thermostat before I disconnected anything, so I know the wiring is correct. Left to right , red, yellow, white, blue, orange and brown.

I checked the 3A fuse in the control box, which is on the roof, and it was OK.

Changing the capacitor sounds like a good idea. Could this have been damaged mis-wiring a thermostat? I'll order one and keep my fingers crossed![/QUOTE]

Very unlikely that you damaged anything.

[saxmandan]

Originally Posted by cavie

Very unlikely that you damaged anything.

Yes, I called a local RV facility that's also a Dometic installer and they agreed that it wasn't likely I did any damage. They said the compressor is probably shot. It's a 15 year old unit, so I suppose it's on borrowed time anyway. They want me to stop by the facility so they can give me a quote on a new system installed, completed with a digital 3-wire thermostat. Depending on the price they come up with, I might do the job myself. To keep the cost down I would probably just replace the roof unit and keep everything else in place (with a new thermostat).
I'm still checking the capacitor today, since although the compressor is vibrating, I'm wondering if it's trying to start or actually running. I can say it does get very hot, to the point that you can't touch it.

[saxmandan]

Well, after doing some research and finding that a new AC system is well north of $1000.00 I decided to go out and buy a multimeter that has a capacitance setting. Turns out the 50+15 capacitor is likely bad. The 15 side of it was ok, but the 50 side was nowhere near 50uf. I ordered that capacitor, as well as the black start capacitor that was also on my unit. Came out to over $60.00, but it's a lot cheaper than $1500.00. The problem is I have a low-profile 15k unit, and they seem to be more expensive than the standard height units. So, I'll keep my fingers crossed. BTW, the capacitors have a money back guarantee, so if I do need a new AC unit I'll definitely return them.