91 beaver marquis no power to rear rooftop ac thermostat

[Glenn Van Shellenbeck]

Hello, I am new to the forum. My name is Glenn VanShellenbeck and I own a 1991 Beaver Marquis. This is my third coach. My first was a 1978 bluebird wanderlodge 33fc, my second was a 1984 MCI mc9 Marathon and I purchased this one a couple of years ago and it has performed well. I am about to take off on a trip to South Dakota and the rear a/c thermostat has no display. I switched thermostats between the front and rear and both thermostats work properly on the front unit. I do not seem to be getting 12v to the rear thermostat and the manuals show the 12v supply to the thermostats coming from the a/c relays. I have researched the manuals and found a section that talks about the a/c relays being located next to the 110v breaker box in the bedroom but I cannot find them. If anyone has any suggestions they would be much appreciated. Thank you in advance.

                                                                                                                                           Glenn VanShellenbeck

[Jason Worman]

Glenn,

I have a 91 Contessa, I looked high and low for the 12v relays and could not find them either. The paperwork I have shows them by 120v panel. You might look up front under hood and see if they are up front. I have found that some of the wiring makes no sense. Good luck and I'm glad that somebody  else on the forum owns a classic Beaver.

Jason  Worman

[John Fitzgerald]

Out of curiosity I took a quick look online and found that at least on some rooftop units there is a source of 12VDC power from the coach that goes to a terminal in the rooftop unit that in turn is fed down to the thermostat.  If you can look at your rooftop unit and see if there is a wiring diagram inside somewhere.  If the above is the case see if 12VDC is present up there where it should be.  If not there is probably a fuse or breaker issue on that branch.  I am an 'HVAC' guy in real life, but have zero experience with RV stuff other than the fact that my '95 Marquis rooftop AC units run.

[Fred Brooks]

   Glenn,
Found these in the schematics under coach assist. Seems circuit breaker #9 is the 12 volt source labeled furnace. Hope this helps, Fred

[Glenn Van Shellenbeck]

Thanks for all of the responses. I have seen several different threads on power to or from the a/c units but nothing that is a for sure. One other thing that I looked at after reading another thread about 12v power had to do with fantastic fans not having power. I also do not have working fans which leads me to believe that a transformer might be supplying 12v to fans and rear thermostat. All of the 12v circuit breakers are working. The furnace in the coach is gas and has its own thermostat, It seems to be working properly. Will keep looking. Thank you

[John Fitzgerald]

I don't see why there would be a need for a separate 12 VDC transformer when you have the house 12 VDC system there already.  Your inverter / converter is essentially a 12 VDC transformer.

[Eric Maclean]

Glen
Although the wiring diagrams for your coach shows the air conditioning relays near the 110 volt panel at some time in the evolution of these coaches the A/C controler ( relay box ) was relocated to the A/C ceiling unit .
Have you dropped the A/C ceiling unit to see if there is a control box there with thermostat wiring like the later Domestic
 Dou-therm units.
The wiring diagrams also show that the earlier units had a 20 amp inline fuse on the 110 volt feed at the A/C units.

The control box ( or relay box) looks like the picture in this link.

https://youngfartsrvparts.ca/products/3109407-001?variant=39250574180407&currency=CAD&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=CjwKCAjwlrqHBhByEiwAnLmYULznpvaNbkv3bR7uBEvuo0j7u_P-7xHEEnQyZ3IgJWutS4dvgCOMmRoCs2UQAvD_BwE

Hope this helps
Eric

[Glenn Van Shellenbeck]

Hi Eric, I have opened up the a/c unit and yes there is the control box that you referred to in there. There is an rj11 connector that runs from that unit to an extension and then to the rear thermostat. Do you know if the 12v power to the thermostat comes from that control box? I see a 3a plug-in style fuse on the board and assumed that was for 12v. I definitely have 110v Ac running to the compressor and a separate 110 line running to the control box but didn’t know if there was a transformer in the a/c control box that converted to a 12v dc to go to the thermostat. Thank you  Glenn

[Eric Maclean]

Glen
In my coach there is a 12 volt power source ( coach battery power ) up at the A/C unit which I believe comes from the DC fuse panel it is connected to the thermostat wiring and controls the relays in the control box at the A/C unit to control the compressor and fan high low functions yours my be different but I don't think they used a transformer in these systems.
There are several different control boxs and some of them are now obsolete .

https://www.google.com/search?sxsrf=ALeKk01Y76JfbZa9s3V9fuX2YZdFUkma4g:1626443971126&source=univ&tbm=isch&q=dometic+duo+therm+wiring&client=tablet-android-lenovo-rev2&fir=0KkrzXTfDI-CNM%252CahE1TRZzzQeFKM%252C_%253B_jC1ary9mLNuVM%252Cylw82xdD3ag9bM%252C_%253BvrTKqCCekh7HnM%252C1QqcETGY9j-PvM%252C_%253BswkowFYDgI1MRM%252CT_ToRxFr7u5ZNM%252C_%253BsJoDtArGEMSoLM%252C1QqcETGY9j-PvM%252C_%253BbHPiSflz_RKLWM%252C0ZKjBlcBxBvgjM%252C_%253BQXRgvWLBhTXHPM%252C0ZKjBlcBxBvgjM%252C_%253BRfw-bMrxPw-PbM%252CW3zhr-BKDoBVGM%252C_%253BMvZv-Fs1fF97ZM%252CW3zhr-BKDoBVGM%252C_%253B_kOeeXbNCKUfUM%252C5mQuM76fFAVjzM%252C_%253BYQhtlCRqk1-6qM%252CI2MDYZgTwn57bM%252C_%253BIxapVnlvinh4VM%252CcCXvvBKXeQf1CM%252C_%253BAHDrSE5qULz07M%252CMPSL_EywHiDomM%252C_&usg=AI4_-kQE4UoVJpIEVvNAca3QPkP5kz1IMg&sa=X&ved=2ahUKEwiZjpX53-fxAhVFXM0KHS8MCWYQ7Al6BAgHEDo&biw=1103&bih=690#imgrc=T5-0INPwnWO0LM

Hope this helps Eric

[Glenn Van Shellenbeck]

Fixed.   Thanks for all of the help guys. I found that the 12v supply from the circuit breaker panel in the rear of the bus had a break somewhere. I decided that the best solution was to find a new 12v supply. I found a live 12v wire in the rear tv cabinet and spliced it it. This wire is powered by cb2. After running it forward to the a/c box I had my 12v supply. The a/c now works although the connections on the end of the ribbon cable coming from the a/c unit seem to be somewhat problematic. I’m going to see if I can find replacement connectors and replace them. At least we have both a/c units. Thanks again for your help. Next is to repair the step so it retracts dependably. Glenn

[Fred Brooks]

    Glenn,
Glad to hear that you got it cooling. If I could make a suggestion, add a label to cb-2 stating it is also the 12 volt source to the rear air conditioner for the benefit of a future owner or technician. Have fun, Fred