Odd problem with a/c thermostat

Narrowed it down Doug. I took the sensor off then connected it with a separate pair of wires directly to the connectors coming out of the thermostat thus bypassing the original connection which is snaked through the ceiling. Just connecting with alligator clips and hanging down on the kitchen counter. The thermostat worked correctly so the problem is in the wiring from behind the Tstat to the remote sensor. Just need to run another set of wires. Easier said than done in this ducted ceiling coach. I got the sensor from coleman. Its identical to the original. Thanks Doug

OK, odds are you have the WRONG remote sensor. Your sensor for the wall tstat should be RVP P/N 6537-3191. IF you purchased the upper control board remote that part number is 8330-5191. They are NOT compatible. Where did you get your replacement parts? Tiffin? Doug

Thank you Doug. I can't be sure there was no work done, but I am the second owner and the original owner hardly used the coach and told me he'd had no maintenance issues with the ac. Previous owner only had 9000 miles on it. I just came back from a 5 week excursion of about 5000 miles during which time the AC worked perfectly.

I will check the gasket issue, but even though my coach is under cover, we are having a heat wave here in Miami and I can't work on it until it cools down. And, unfortunately, RV maintenance in Miami is hard to come by. (Not really an RV town) The closest dealers are in FT Myers on the Gulf coast. Still, that's my next step. Always prided myself on being a DYI guy. Oh well.

Have you connected the OLD remote sensor direct to the wall tstat 2 wires? If you are the 2nd/3rd owner HOW do you know nothing was done on the front AC????? I feel the separation gasket on the roof mount is bad or not installed. That would be a cause of cold air in the duct but NOT the 99 degree problem. Doug

Thank you Doug. I am not the original owner, but the AC is original and has had no modifications. I did replace the rear air, but this is working fine. My model is the RV Comfort HP and does have the green wires coming out of the thermostat. As long as the remote sensor is connected to the thermostat it reads 99 degrees. To replace the remote sensor I cut and spliced the leads near the AC intake. When cut, about 4 inches from the sensor the display read "--" and after splicing it went back to 99. Thus, if the problem was a short upstream from the sensor I would not have expected a change. This is what led me to think the Tstat was bad. So I replaced it. I had trouble finding the right replacement, but I'm confident I have it now.

I am concerned the the cold air circulating inside the duct near the intake duct next to the sensor might be an issue? Don't see why, but feels odd since I don't find this to be the case in the other intake ducts. Seems like there might be a hole in the duct work nearby, but I can't see it.

1. On Tiffin the remote sensor is next to a RETURN vent. There should be NO cold air in that area. The COLD exhaust is on the Passenger side of the ceiling.
2. As you found out, replacing parts is not the best way to tackle a problem. The remote sensor goes to the front AC control board, You need to go on top and remove the top AC cover and the Evaporator cover to access the control board and look at the wiring connections on the control board. The connections will be labeled.
3. Are you stating that regardless of the air temp or condition, the tstat shows 99 all the time? What I would do, is take the original Sensor and go up to the front AC control board and connect its 2 wires to the control board and see what the Wall Tstat shows. IF it shows a correct different temp then the wire from the ceiling to the roof AC control board is pinched somewhere. If so, you just run a new 2 wire harness down the return ductwork to the remote sensor. Doug

Are you the original owner? If so, has ANY work been done on the front AC? Replaced?

You did not state what model tstat you have. RVP has about a dozen models. The RV Comfort HP model does have the remote sensor connected to the wall tstat. They are the 2 Green/white wires coming out of the wall tstat. All others, the sensors go to each AC control board.

Update on my problem, not solved, unfortunately. I was convinced my remote sensor was bad so I changed it out. I noticed that the temperature reading on the thermostat stayed at 99 degrees even though it was much cooler. So after that I decided that the problem is at the thermostat, so today I received a new one. The problem remains. The item on the new stat is 99 degrees. If I disconnect the remote it just goes blank. I wonder if anyone knows if a short in the sensor line might cause it to read the max temp, 99 degrees? From what jyrostng said it might be that with no resistance in the line it might either read the max or the min?

Thanks folks. I've learn a lot. I'm replacing the sensor. BTW, the sensor is mounted just adjacent to an ac return register. When I remove that register I can access the wiring fortunately so it won't be difficult to switch out. I noticed however that with the AC running there is a lot of cool air blowing on my hand as if there is a hole in the duct work. This doesn't occur in any of the other intake ports. This one however is the closest one to the ac unit. Curious.

I believe, like Dometic, if the remote sensor is not connected to the upper control board and the Board cannot sense the OHMs from the remote sensor the temp sensing function reverts back to the wall tstat. Doug

Are you sure you're on the correct sensor. You said, "when you disconnect the sensor there is no display"

There are foam pads you can mount behind the thermostat to insulate it from the wall.

Seal up any wire holes so no outside air affects.

2 wire sensors are thermocouples that are dis-similar metals joined at the tip that generate a tiny voltage that varies at different temperatures, make sure the connector at your thermostat has a good connection, A bad connection can make the reading vary.