No 12V to rooftop AC

[P J Johnson]

I do not have 12volts to the thermostat for my front AC, there is 120 ac volts to unit. I do not see a transformer at unit for step down to 12v so there must be another 12v source for my thermostat circuit. I have had shorts in 2 other 12v circuits (and repaired them) but can't see where I might find the thermostat source. Can anyone give me some help as to where I go from here?

[tncruiser]

the 12v should come from your housebattery/converter
check all fuses
brianj

[frankdamp]

There should be a basic 120-to-12 volt transformer some place similar to what a Stick house has for the doorbells.

[texnet]

I disagree with the transformer... I don't know of any RVs that would use this when they have 12vdc batteries installed :-) What chassis/MH and year MH do you have? That would help a lot! As tncrusier said.. check all fuses!

[jamesgr81]

The RV converter/battery output should have a 12 volt circuit running directly to the AC. Then a multiwire cable connects the comfort control center to the AC.

If it's an older non-digital setup with lever for temperature then the 12 volts could go to thermostat then up to AC.

Won't be a transformer unless it's an oddball installation, RV AC's generally use 12 volts DC for controls.

[Nuge]

The thermostat controls are internal on both of our rooftop A/C units.

[Airtool]

The 12 volts generally is supplied from the main house DC fuse panel to a control board located somewhere in the coach.The thermostat closes a relay that sends the 12 volts the the roof AC.
There are often relays located in the same area as the start and run capacitors that are in a covered area of the unit.
First check all fuses,Then look for the control circuit board, there is often an manual over ride switch on the boards to run the units in test mode. Generally there is a red LED that will light when in this mode.
If the LED lights you have power to the board.
Should the unit fail to start, the problem maybe on the roof with said relay.
NOW YOU MUST be very care full if you remove any covers. ALL AC OFF from the ac barker panel. Capacitors hold a hefty charge and are not to be taken lightly. You could fine yourself on the ground in a hurry.
ITS NOT THE Fall !! its that stop at the bottom the hurts!!!!
Best advice if you are not familiar with things like this just get some help.
The problem could be just a loose or corroded connection,a defective relay,defective capacitor(s),Motor starter or motor problems.

Please work safe do not spoil the day.
Dick Bounder Diesel

[P J Johnson]

Quote:

Originally Posted by texnet I disagree with the transformer... I don't know of any RVs that would use this when they have 12vdc batteries installed :-) What chassis/MH and year MH do you have? That would help a lot! As tncrusier said.. check all fuses!

It's a 2008 Hurricane 34y on a Workhorse chassis. I have found the power lead for the thermostat. It is an18ga red leading to a terminal block near the invertor. It is not fused at the 12v panel where all other 12v items are. I have a background in installing HVAC but that was in the outside world. I haven't spent much time in the 12v RV world

[Airtool]

PJ! having a background in HVAC puts you in good chance of fixing the AC.
Looked up the 34y,looks like a two slide toy hauler.
With two AC units,thinking one is working,is that correct?
That being the case you need to have a 50 amp service to operate both AC units and the 34y do not come with the 5500KW getset.
There should be a control panel to set the temperature somewhere for both the front and rear AC units.
Do you have a volt meter?
Think I will stop there and see your response before continuing.
This is a link to the info I'm using
http://www.thormotorcoach.com/media/...bbf1195523be60

Dick.

[P J Johnson]

Quote:

Originally Posted by Airtool PJ! having a background in HVAC puts you in good chance of fixing the AC. Looked up the 34y,looks like a two slide toy hauler. With two AC units,thinking one is working,is that correct? That being the case you need to have a 50 amp service to operate both AC units and the 34y do not come with the 5500KW getset. There should be a control panel to set the temperature somewhere for both the front and rear AC units. Do you have a volt meter? Think I will stop there and see your response before continuing. This is a link to the info I'm using http://www.thormotorcoach.com/media/...bbf1195523be60 Dick.

Right on the model but I do have a Onan 55. It was during my monthly run that I noticed the front AC didn't work. Rear AC ,in garage, is indpendent and works with attached controls. Front wall thermostat works both AC and furnace. It's the red power lead to the thermostat that is not "hot". I have traced it to a block near the invertor. It's not in the 12v fuse panel. So I've assumed that it's power comes from the invertor and therefore is only powered with Gen running or the proper shorepower. Curious it isn't fused that I can see and wonder way it's "open" now. I've had two "shorts" in my 12v system that I have fixed but this is first "open" unless I"m missing a fuse somewhere.

[Airtool]

Well we are on the same page for the most part.
By the way do you have a meter??
OK ! just check and see if the furnace will operate. Then you know if power is getting to the controller. Got a feeling the furnace its not going to fire up from your info.,but it will prove there is no power to the controller. Then you need to check where the 12 volts are lost.
If there is 12 volts at the inverter, is the wire open between there and the circuit board?

Can you get to the controller? meaning can you remove it and see if there is power to the connector. If you have 12 volts at the input, is there a fuse right on the board ?
Fuse on the board OK or not present,when the Thermostat is set to a point that would cause the AC or heat to come on ---Is there power out showing up at a connection or wire that is not normally powered?
This wire(s) should be the control voltage for the AC and / or Furnace. Power not present at some point when controller calls for heat or cooling.
No power could be a defective circuit board.
That will keep you busy for a bit.

Dick.

[P J Johnson]

Quote:

Originally Posted by Airtool Well we are on the same page for the most part. By the way do you have a meter?? OK ! just check and see if the furnace will operate. Then you know if power is getting to the controller. Got a feeling the furnace its not going to fire up from your info.,but it will prove there is no power to the controller. Then you need to check where the 12 volts are lost. If there is 12 volts at the inverter, is the wire open between there and the circuit board? Can you get to the controller? meaning can you remove it and see if there is power to the connector. If you have 12 volts at the input, is there a fuse right on the board ? Fuse on the board OK or not present,when the Thermostat is set to a point that would cause the AC or heat to come on ---Is there power out showing up at a connection or wire that is not normally powered? This wire(s) should be the control voltage for the AC and / or Furnace. Power not present at some point when controller calls for heat or cooling. No power could be a defective circuit board. That will keep you busy for a bit. Dick.

Thanks for y'alls input. I have run a new power lead to the wall thermostat and I'm back in business. Why I lost that lead in the first place is a mystery but the wiring in this Hurricane 34y has been a b*#$h. This bus has alot of good qualities but the wiring has left much to be desired. and yes I don't go anywhere without my meter and other tools.

[Airtool]

PJ ! The Wire used today is copper clad steel. It carries power and signals,But will brake Very easy. Steel does not flex much.
Its one of those bottom line items,saving money by the builders and they are at a point where I just say GIVE ME A BRAKE !!! is the 50 to 100 $ you save worth it.
Not to bear bad news,but you may need to slowly change out at least most of the 12 volt wiring. Hope the 110 volt wiring has a better copper content.
Glad you got the AC up and running,had a feeling you could with your background.
Dick.

[P J Johnson]

Quote:

Originally Posted by Airtool PJ ! The Wire used today is copper clad steel. It carries power and signals,But will brake Very easy. Steel does not flex much. Its one of those bottom line items,saving money by the builders and they are at a point where I just say GIVE ME A BRAKE !!! is the 50 to 100 $ you save worth it. Not to bear bad news,but you may need to slowly change out at least most of the 12 volt wiring. Hope the 110 volt wiring has a better copper content. Glad you got the AC up and running,had a feeling you could with your background. Dick.

Thanks Dick,
Ya don't ya know. as they say in the U P. Lets hope it doesn't come to that. I thought rewiring a house was a pain. Finding routes for new wire in a MH is a real experience. take care P J