Why Doesn't My New AC Thermostat Work?

[dbj216]

Hello Airstream Friends:

I need some help with a new Dometic 651916Hxxx 15k btu air conditioner we are installing. We are controlling it with a LCD SZ (single zone?) three wire thermostat. The instructions describe the 12v+ red/white wire, the 12- black ground wire and the orange "comms" wire to the thermostat. We did that. Not described in the instructions, but the wiring diagram shows a customer supplied 12+ volts to the red wire before the connector to the thermostat. We did that too.

Well, I "assumed" the ground for the AC was supplied by the 120v power connection to the AC somewhere in the AC inself. My romex wire to the AC has a solid ground to the body/chassis of the Airstream, and it has a solid 120v to the AC harness at our junction box.

We have no power to the thermostat when connected. The black ground wire is open, thus no completed 12v circuit. I can not find where the ground is supposed to be in the AC unit itself in the wiring diagram for our part number. The AC sits on a 1" thick rubber gasket so there is no metal to metal contact of the AC with the Airstream on the roof. The 4 bolts that squeeze the inside plate and AC to the roof do not touch any part of the trailer that is bare metal.

I'm hoping one of you Airstream AC replacement experts will straighten me out on how to power up the thermostat.

David

[dbj216]

Well, this must be a very boring thread. No one has helped yet, so I had to figure it out myself.

First, the Dometic install instructions are poor in my view. They are written for several different models of air conditioners. It is difficult to find what you need. And then the instructions aren't much help. Dometic gets a D in technical writing.

Looking at the wiring diagram, it shows we power the "control wiring" from the AC from the trailer 12v system. Okay, but where to connect? Then they talk about routing -12v also. We call this a ground wire in Colorado. Never seen it written as a -12v wire. After two afternoons trying to decipher instructions and wiring diagrams, we finally got the thermostat to power up.

Dometic sent us three long bolts to clamp the AC down to the roof. That won't work on an Airstream curved roof and interior skin. So we had to figure out a 4 bolt pattern.

Bottom line, we got the AC mounted and tested. Project finished.

David

[dbj216]

Another boring reply to a boring thread. We wanted to test the new AC to make sure all the features worked as described. It is a heat pump AC as many are now days. The LCD thermostat is goofy, but with enough taps, you can get the AC to run.

The Dometic instructions do say run a 20 amp 120V AC circuit protected by a known good 20 amp breaker. We did this, but as explained earlier, the ground wire on this circuit does not provide any 12v DC grounding. Both my 12v DC wiring and the 120v AC wiring is grounded to the Airstream chassis.

But since Dometic says 20 amp 120v AC is required to run the AC, why not try it. I've always believed a guy needs a 30 amp power input for the AC, otherwise the voltage will droop and the motor will get very warm.

I have one of those cheap voltmeters that plug into an outlet in the trailer. It read 125v AC with no load except the converter. So we turned on the AC fan and saw almost no voltage drop. Then we turned on the AC compressor to low and saw maybe 5 volt droop. Finally, we ran the AC compressor on high and experienced very little additional voltage droop. We were good.

Maybe modern AC units are more efficient and will run safely on a 20 amp circuit. Now I fully realize we aren't going to run a microwave, or a 1500 watt hair dryer or electric heater with the AC running.

Okay, all done whining.

David