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Old 12-09-2013, 01:28 PM   #1
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Thermostat for Heat Pump-switch problem

The thermostat for the heat pump in my 1999 Excella is getting old and ornery. There is a switch on the bottom, just below the words "ON OFF" seen in the attached photo. The switch seems to be loosing contact occasionally. The unit will stop running sometimes, like today when DW closed the fridge door. I wriggle the switch and it comes on most of the time. Sometimes it takes a few right/left on/off switching cycles to get the contact again. It seems to be getting worse, so I need to get to work on it.

I took the cover off and removed the circuit board and looked at the switch. There is no evidence of a reason for a defective switch or cracked solder where it is attached, so I assume the contacts inside the switch are worn or corroded, or both. Is there a way to replace or repair the switch without replacing the whole thermostat?

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Old 12-09-2013, 07:09 PM   #2
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I don't know about replacing the switch, but if you don't find a solution you are happy with, pm me. I have one out of my 1997 which works and I am no longer using since it was less expensive to buy a new one when my original AC died.
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Old 12-09-2013, 07:45 PM   #3
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Thank you Craig, I appreciate this!
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Old 12-09-2013, 08:47 PM   #4
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Your best option is to replace the unit with the same, working t/stat, such as the one Aviator is offering you. There are NONE available new from Dometic. There are some contacts within that fail and a few folks have repaired them, only to have them fail again in 6-12 months.

The only other option is to replace the relay board and t/stat with the newer 5-button model (Comfort Control I), which are still available but have also been superseded by the Comfort Control II, or 10 button unit with it's new, compatible control board.

Let me know if you need any part numbers for option 2 or 3.
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Old 12-09-2013, 10:20 PM   #5
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I appreciate your response.
I did make arrangements to get the one Aviator offered. However, I would appreciate the part numbers for future reference, if it's not to much trouble.
When I receive the replacement from Aviator, I will take the switch apart on the failing thermostat to see if I can repair it.
Hopefully these two will get me through until the 14 year old HP unit fails.

Lewster, when my old HP unit fails, will the newer replacement units use the same patch chord that plugs into the thermostat for the control wiring? Or, does this wiring have to be replaced?

Thanks!
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Old 12-10-2013, 06:35 AM   #6
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Originally Posted by A W Warn View Post
I appreciate your response.
I did make arrangements to get the one Aviator offered. However, I would appreciate the part numbers for future reference, if it's not to much trouble.
When I receive the replacement from Aviator, I will take the switch apart on the failing thermostat to see if I can repair it.
Hopefully these two will get me through until the 14 year old HP unit fails.

Lewster, when my old HP unit fails, will the newer replacement units use the same patch chord that plugs into the thermostat for the control wiring? Or, does this wiring have to be replaced?

Thanks!
Luckily, newer Dometic units use the same communication cable to allow full compatibility on replacement. I'll get the part numbers later today.
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Old 01-15-2014, 10:19 AM   #7
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need advice - heat and AC dead

Woke up to a cold trailer this morning. The furnace stopped running during the night. Fuel is not a problem, with one full tank and one 1/2 tank.

When I got the replacement thermostat from Aviator I plugged it up and the furnace, heat pump, and a/c worked fine (until last night).

This morning the thermostat does not show a display, nor will the light turn on. I removed the thermostat from the wall to check the connection for corrosion, but everything looks good. I snapped the connector in and out a few times, it seems to make good contact.

I pulled the filter covers and junction box cover. I checked the 120v. 120v is good. There are 4 pairs of low voltage wires twisted together with wire nuts. One of them has 12v. So, I assume 12v power is good.

There are also two yellow wires not connected to anything. There is a two wire plug not connected to anything. There is one telephone type connector, not connected to anything. There are two telephone type connectors connected together with a coupler. I do not see any corrosion on any of these wires.

What should I check next?
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Old 01-15-2014, 11:04 AM   #8
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Quote:
Originally Posted by A W Warn View Post
Woke up to a cold trailer this morning. The furnace stopped running during the night. Fuel is not a problem, with one full tank and one 1/2 tank.

When I got the replacement thermostat from Aviator I plugged it up and the furnace, heat pump, and a/c worked fine (until last night).

This morning the thermostat does not show a display, nor will the light turn on. I removed the thermostat from the wall to check the connection for corrosion, but everything looks good. I snapped the connector in and out a few times, it seems to make good contact.

I pulled the filter covers and junction box cover. I checked the 120v. 120v is good. There are 4 pairs of low voltage wires twisted together with wire nuts. One of them has 12v. So, I assume 12v power is good.

There are also two yellow wires not connected to anything. There is a two wire plug not connected to anything. There is one telephone type connector, not connected to anything. There are two telephone type connectors connected together with a coupler. I do not see any corrosion on any of these wires.

What should I check next?
Alan, check the ground wire for the thermostat. A very common issue with them is a bad ground. Usually contacting the ground wire to ground through a test light, then turning on the thermostat, will verify this. A bad ground will light the test light.
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Old 01-15-2014, 12:05 PM   #9
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Terry, I appreciate your response!

I'm not sure which wire is ground on the back of the t-stat, since it is a modular plug.

Where should I test it? Is this connection the ground inside the junction box at unit on top? While I wait for your response, I will go on top and loosen/retighten that ground.

I removed the cover and tested the fuse with an ohm meter. The 3 amp fuse is good.

add edit: In the JB inside on the roof I found two ground wires, one green and one sheathed telephone type wire. I cleaned corrosion with sandpaper to bare metal and reconnected. Still no display or light on the t-stat.
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Old 01-15-2014, 01:00 PM   #10
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need advice - heat and AC dead

I have confirmed that the t-stat is not receiving 12v to the on/off switch, using a multi-meter. When I switch the mult-meter to Ohm mode I can confirm a ground at that point.

There is 12v where the light blue wire connects to the dark blue wire where they hang down above the filter.

I also swapped out the newer t-stat I got from Aviator with the older one that was working when I removed it. There is no display or light with either of them. That makes me think the problem is not within the t-stat.

I wiggled all of the blade connectors on the circuit board inside the junction box on the roof to see if anything was loose. There is no visible corrosion on the circuit board, so I think those connections are good. Shouldn't this wiggling movement have made contact if there was any corrosion that I could not see?

What should I check next?
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Old 01-15-2014, 01:38 PM   #11
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advice needed!

Quote:
Originally Posted by A W Warn View Post
I have confirmed that the t-stat is not receiving 12v to the on/off switch, using a multi-meter. When I switch the mult-meter to Ohm mode I can confirm a ground at that point.
edit to above: I unplugged the modular jack from back side of the t-stat. I checked each conductor in the jack, all four conductors are showing continuity to ground. Should all four wires be grounded?
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Old 01-15-2014, 03:23 PM   #12
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Check on the A/C circuit board for a 3 amp fuse. This protects the 12VDC wiring and might be the bream in the DC DC supply
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Old 01-15-2014, 05:28 PM   #13
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Still have questions, though I finally figured out what happened. Heat Pump, AC, and furnace are operating normally now.

The low voltage wire above the interior filter inside the lower portion of the unit had power when I check it several different times, where the light blue and dark blue wires are connected. The low voltage wire supplying the thermostat did not have power. I did not realize there would be more than one source of low voltage.

When I turned on the interior light near the rooftop unit it did not come on. I found the breaker for that circuit was tripped and would not reset. After searching for a cause, I discovered the insulation had worn down to copper on the wire supplying power to the light fixture. I repaired the worn spot then reset the breaker. All is back to normal.

My questions: Is there a transformer somewhere in the unit that converts 120v to low voltage? Or, is the unit connected to two 12v circuits? I am wondering if my unit is wired correctly, or has it been modified since it left the factory?
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Old 01-15-2014, 08:32 PM   #14
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Still have questions, though I finally figured out what happened. Heat Pump, AC, and furnace are operating normally now.

The low voltage wire above the interior filter inside the lower portion of the unit had power when I check it several different times, where the light blue and dark blue wires are connected. The low voltage wire supplying the thermostat did not have power. I did not realize there would be more than one source of low voltage.

When I turned on the interior light near the rooftop unit it did not come on. I found the breaker for that circuit was tripped and would not reset. After searching for a cause, I discovered the insulation had worn down to copper on the wire supplying power to the light fixture. I repaired the worn spot then reset the breaker. All is back to normal.

My questions: Is there a transformer somewhere in the unit that converts 120v to low voltage? Or, is the unit connected to two 12v circuits? I am wondering if my unit is wired correctly, or has it been modified since it left the factory?
Most A/C installations that I have seen are connected to only one 12VDC power source, generally fused at the DC distribution block. I have no idea why someone would have 2. I don't see the value in it.
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