Skip to main content ↑ ↓
Topic: Duo Therm help needed (Read 762 times) previous topic - next topic

Duo Therm help needed

I have a friend with a 96  U295 with the 4 button  duo therm thermostat.
Here is the problem:  It appears the thermostat is getting no power.  Nothing happens when you turn it on.
Here is what has been looked at:  He took a look at the breakers behind the white panel, none tripped.  Actually he has found nothing tripped.  I have a 99 U270 and none of my prints show the thermostat, so I am not sure where it gets power from.
Does anyone know where the power to the thermostat comes from?  Also, any trouble shooting advice would be appreciated.
Jerry
Jerry & Nanci
1999 U270 34'WTFI
2011 Malibu
A smart man knows what to say, a wise man knows when to say it.

Re: Duo Therm help needed

Reply #1 –
clean contacts with pencil eraser
Tim Fiedler    2025 LTV Unity MBL on Order
2000 Chevy Tracker 2 Door Convertible 4WD Now lifted 4.5"
1985 Citroen 2CV6 Charleston
Murphy Rebel on wheels with 175HP Titan
Cessna P337
1980 48' Westport MY (!/4 Share)

Re: Duo Therm help needed

Reply #2 –
If you have a friend who has a 4 button thermostat, you first might try his on your coach. If your thermostat fails that test usually it can be rebuilt . Or sometimes the switch has broken free from the circuit board and needs to be re-soldered. Or maybe the switch is simply dirty - you might have to un-solder and disassemble the thermostat's power switch in order to clean the contacts. Be careful not to lose the spring and contact bar inside the black plastic slide. Call me for more more details 308-440-5154.

The system's wiring, either from a furnace or aqua hot, sends a 12 volt signal up to the front air conditioner board. There it is converted to a digital signal and sent on via the telephone phone type wire to the thermostat. The door master switch needs to be on.

I would try to decide where the 12 volt signal has been lost. This can be from the sources noted above or anywhere along the line to the thermostat. So first check the 12 volt fuses on the furnace board, the or the air conditioner board, the circuit breaker for the A/C and the 12 volt fuse on the A/C board.

Continuing, check the thermostat itself (with the phone plug inserted) for the presence of 12 volts on the solder side of the thermostat ( between the power switch contracts and the ground plane of the biggest chip). If voltage is absent here, next look for 12 volts on the thermostat's phone cable plug itself (disconnected) by using safety pins held to the volt meter's probes to the red and black wire contacts of the plug.

Next look for unplugged or corroded phone plugs and their jacks at the front air conditioner and on the back of the thermostat. Your vintage A/C system should have the air filter (inside the coach) directly below the A/C unit. If so take down the filter holder to see the phone cables. The four brass colored contact springs need to be clean, aligned and separated perfectly in their black plastic comb of the jack. Tough to see them, so use a flashlight and maybe a mirror.

I've heard of situations whereby the plugs on the phone cables are at fault or the cable itself is damaged somewhere. Replacement of the plugs is easy if you find a Radio Shack store (hopefully still open for business near you) to buy the plugs and the crimping tool and phone wire.

Hopefully you won't have to do all these tests to find the problem. The other common problem with these 4 button thermostats....the four button switches start to fail to make contact, particularly when trying to change zones. That is very frustrating.

Jim
Jim Frerichs
2002 U320 42'

Re: Duo Therm help needed

Reply #3 –
Jim,
Thanks for the steps for diagnosis.  We would swap the thermostats, but mine has the 5 button, and the bigger problem is he is in sunny FLA and I am in GA.
Jim, do you rebuild the thermostats?

Thanks,
Jerry
Jerry & Nanci
1999 U270 34'WTFI
2011 Malibu
A smart man knows what to say, a wise man knows when to say it.

Re: Duo Therm help needed

Reply #4 –
Yes, I do rebuild the 4 button thermostats; fortunately the 5 button thermostats very seldom have a problem with their button switches. Rebuilding a 4 button thermostat is a whale of a lot cheaper than upgrading to the 5 button system (new thermostat plus new A/C boards).

Jim
Jim Frerichs
2002 U320 42'

Re: Duo Therm help needed

Reply #5 –
Thanks Jim for the info.  I am passing it on.
Jerry & Nanci
1999 U270 34'WTFI
2011 Malibu
A smart man knows what to say, a wise man knows when to say it.

Re: Duo Therm help needed

Reply #6 –
I think you will find all wire connections in the large space above the front roof air filter.  12 volt positive & ground, "telephone" type RJ-11 to thermostat, 120-volt, any temp sensor.  Some connections also daisy chain to rear roof air.

I looked at wiring diagrams and only find breakers identified as furnace or Aqua Hot.  Since furnace is also connected to system so wall thermostat can also control the furnace, furnace wires are also up in front air conditioner.

Since over time many of the 4-wire thermostats have issues, you may find that the thermostat is the only problem, and that 12-volt power supply is ok.  There is good info on converting to much better 5-button thermostat on BeamAlarm.com & Foreforums.

See if these diagrams help.

Re: Duo Therm help needed

Reply #7 –
Barry,
Thanks for the info.
Jerry
Jerry & Nanci
1999 U270 34'WTFI
2011 Malibu
A smart man knows what to say, a wise man knows when to say it.

Re: Duo Therm help needed

Reply #8 –
Thanks all for the help.  Turns out a mice enjoyed biting thru the Wiring to thermostat.  Repaired wiring and unit is back in operation.
Jerry & Nanci
1999 U270 34'WTFI
2011 Malibu
A smart man knows what to say, a wise man knows when to say it.

Re: Duo Therm help needed

Reply #9 –
Jerry, D...... mice, Too bad there is not some use for them, I Know tastes like chicken.
Glad to hear that Larry found the problem.
Gary B