No 12v at thermostat for ac unit???

RV t-stats work differently from home-type units, and it sounds like the previous owner of your unit may have replaced the t-stat with a digital unit, then removed it prior to selling the unit.

I just changed my factory t-stat to a digital unit last night so the wiring is fresh in my mind. The factory unit was a Dometic Duo-Therm that controlled both the a/c and the furnace:



I find that digital t-stats just work better and also display the ambient temp as well as giving you a better read of the set temp.

Mine had 6 wires:

Red 12v+
White furnace
Yellow a/c
Blue hi-fan
Brown lo-fan
Green ground.

You colors may vary but you will have these functions.

The digital home-type t-stat only doesn't have a provision for a 2-speed fan, so a separate SPDT switch is needed to enable both hi and lo fan.

I found a non-programmable heat and a/c t-stat on Amazon, along with an RV-style SPDT switch to control the fan. Here's the install:



For the wiring, you don't use the red 12v+ wire at all, it gets terminated unused. The W furnace and Y a/c wires get connect as normal to those terminals on the new stat. The ground wire goes to the RC and RH terminals....this is not a typo. This stat uses a pair of AAA batteries, so no 12v+ is needed.

The blue hi fan and brown lo-fan wires go to the outside terminals on the SPDT switch and a new wire from the center terminal on the switch goes to the fan terminal on the t-stat.

Hope this helps.

dougrainer wrote:
If you have a wall tstat, you will have the standard multiple wires coming from the roof top unit down to the wall tstat. YES, on Jayco's they use the one HOT(12 volt pos) blue wire from the furnace to power the 12 volt side of the Roof AC system. There are NO 120 tstat wires. The ONLY 120 is the roof top 120 voltage to run the unit. ALL controls to operate the roof top unit are 12 volt. This link will show you the wiring for a RVP/Coleman. Just remember, there are 2 blue wires from the furnace. ONE has 12 volts. THAT wire will be connected to the RED wire up at the AC unit. Doug

***Link Removed***


***Link Removed***

Thanks bud for your reply. This is where I’m at today. I ohmed the blue+ wire that is supposed to go to the red positive, there must be a cut or torn wire in the chase of the wall. My guess that all the wires run from the top down is they are looped so the wires do not pull down. My guess is someone with a screw ruined the red 12v power wire to thermostat. I ended up running belding cable and ran it right to the furnace. Now I’m getting the correct voltage at thermostat. I have a new thermostat coming in tomorrow. Previous owner cut all the wires short. I guess I will update later.

dougrainer wrote:
If you have a wall tstat, you will have the standard multiple wires coming from the roof top unit down to the wall tstat. YES, on Jayco's they use the one HOT(12 volt pos) blue wire from the furnace to power the 12 volt side of the Roof AC system. There are NO 120 tstat wires. The ONLY 120 is the roof top 120 voltage to run the unit. ALL controls to operate the roof top unit are 12 volt. This link will show you the wiring for a RVP/Coleman. Just remember, there are 2 blue wires from the furnace. ONE has 12 volts. THAT wire will be connected to the RED wire up at the AC unit. Doug

http://www.cleggind.com/postal_tech/pdfs/AC2/airxcel_1976376%5B1%5D.pdf


https://www.google.com/search?q=atwood+furnace+wiring+for+a+Jayco+trailer+AC+unit&sxsrf=APq-WBubrUaj6NaaivrCRqv2LDcvs4a2Ig:1648479813151&tbm=isch&source=iu&ictx=1&vet=1&fir=EWQc7uQNBQDthM%252CUPMtp-kEXf7IrM%252C_%253BDzyeAKkqFWwseM%252C5jT6CdYXkBNbOM%252C_%253BH_j0_geGqGFF4M%252CtJH03Tz1_g-DtM%252C_%253BPwCC5EuvTuW5rM%252CEaockhFXBsAbrM%252C_%253BoDpcU-yZPx8u7M%252CMiFqCfELwZBpsM%252C_%253BJ_SwdDtu8PFdHM%252Cwzwil3ijHfmF8M%252C_%253BMQYZflyRCKZ9TM%252CDXDSzuNmIZYcTM%252C_%253BYFbFinVyzZMcPM%252CtJH03Tz1_g-DtM%252C_%253BYRbu1X-23wsufM%252ClrokvToSln8faM%252C_%253BEwNSF8coq8os9M%252CWytUIwpciEyMGM%252C_&usg=AI4_-kSo6zE9sXEVemVuK_DT0gl1TgkTHA&sa=X&ved=2ahUKEwibobrtien2AhWQmmoFHfknAkoQ9QF6BAhSEAE#imgrc=EWQc7uQNBQDthM

Not my issue, but I really appreciate the kinda in depth knowledge that member Doug brings to this forum - Many thanks to Doug for his continued tutelage :) !!

3 tons

If you have a wall tstat, you will have the standard multiple wires coming from the roof top unit down to the wall tstat. YES, on Jayco's they use the one HOT(12 volt pos) blue wire from the furnace to power the 12 volt side of the Roof AC system. There are NO 120 tstat wires. The ONLY 120 is the roof top 120 voltage to run the unit. ALL controls to operate the roof top unit are 12 volt. This link will show you the wiring for a RVP/Coleman. Just remember, there are 2 blue wires from the furnace. ONE has 12 volts. THAT wire will be connected to the RED wire up at the AC unit. Doug

http://www.cleggind.com/postal_tech/pdfs/AC2/airxcel_1976376%5B1%5D.pdf


https://www.google.com/search?q=atwood+furnace+wiring+for+a+Jayco+trailer+AC+unit&sxsrf=APq-WBubrUaj6NaaivrCRqv2LDcvs4a2Ig:1648479813151&tbm=isch&source=iu&ictx=1&vet=1&fir=EWQc7uQNBQDthM%252CUPMtp-kEXf7IrM%252C_%253BDzyeAKkqFWwseM%252C5jT6CdYXkBNbOM%252C_%253BH_j0_geGqGFF4M%252CtJH03Tz1_g-DtM%252C_%253BPwCC5EuvTuW5rM%252CEaockhFXBsAbrM%252C_%253BoDpcU-yZPx8u7M%252CMiFqCfELwZBpsM%252C_%253BJ_SwdDtu8PFdHM%252Cwzwil3ijHfmF8M%252C_%253BMQYZflyRCKZ9TM%252CDXDSzuNmIZYcTM%252C_%253BYFbFinVyzZMcPM%252CtJH03Tz1_g-DtM%252C_%253BYRbu1X-23wsufM%252ClrokvToSln8faM%252C_%253BEwNSF8coq8os9M%252CWytUIwpciEyMGM%252C_&usg=AI4_-kSo6zE9sXEVemVuK_DT0gl1TgkTHA&sa=X&ved=2ahUKEwibobrtien2AhWQmmoFHfknAkoQ9QF6BAhSEAE#imgrc=EWQc7uQNBQDthM