Dometic AC problem...

[MTGunner]

Our AC $oes not operate. This is in our 2009 Unleashed 27SBU, Dometic AC model number 3107541.009 which is an older model. He fan works on low alone but the compressor does no5 start. Thanks fo4 the input. MTG

[Bama Rambler]

That should have an analog thermostat. What all have you tried to get it to run?

If you haven't tried it, remove the front cover of the thermostat and jump a wire form the green wire to the yellow wire and see if the compressor starts. If it starts then that tells you that the thermostat is the problem.

[MTGunner]

Jumped yellow to green with no effect. The capacitor tests good. The fan will run on low only.

[Bama Rambler]

If you jumped yellow to green and the compressor didn't kick on then the thermostat isn't your problem. It could be a relay in the control box above the A/C grill, but it also could be the compressor.

[Mr Towed]

Originally Posted by Bama Rambler

That should have an analog thermostat. What all have you tried to get it to run?

If you haven't tried it, remove the front cover of the thermostat and jump a wire form the green wire to the yellow wire and see if the compressor starts. If it starts then that tells you that the thermostat is the problem.

Just adding to this because I did one last search before I posted about a similar problem.

According to this I will take the yellow, which is the "cool" lead and jump it to green, which appears to be ground, this should get the compressor to kick on.

Is there any requirement that the fan also be on or anything else that will prevent the compressor from coming on? Thought I had read something along those lines in another discussion but I can't find that now.

I am suspecting the thermostat because the compressor seemed to momentarily be attempting to kick on as I moved the setpoint potentiometer past a certain spot, not just below a certain level, almost like it was only making contact at a specific spot along its travel.

Thanks, -Jeremy

[rockfordroo]

This probably won't help, but:

On my thermostat, you'd short red to yellow to start the compressor. Red to green for hi speed fan, red to gray for low speed fan.

But Dometics are wired funny. Switched ground instead of switched positive.

[Bama Rambler]

There's nothing in the A/C or thermostat that prevents the compressor from running if the fan isn't running. You don't want it running very long, but it's ok to run it for testing.

[Mr Towed]

Originally Posted by rockfordroo

This probably won't help, but:

On my thermostat, you'd short red to yellow to start the compressor. Red to green for hi speed fan, red to gray for low speed fan.

But Dometics are wired funny. Switched ground instead of switched positive.

Thanks, I actually saw another post where you posted that diagram but it seems to apply to coleman, if i'm not mistaken, and like you said, it appears that dometic/duo-therm have the 7.5 volt control / switched ground thing going. I even heard that it could damage the thermostat (if it's not busted already) to give 12Volts to certain leads.

[Mr Towed]

Originally Posted by Bama Rambler

There's nothing in the A/C or thermostat that prevents the compressor from running if the fan isn't running. You don't want it running very long, but it's ok to run it for testing.

Thanks, I will try it then. just want to see if the compressor will actually spin up or if it's locked up or something.

[Mr Towed]

So i got a chance to jump the yellow wire to ground the other day and it got the compressor to start up. I ordered a new thermostat and installed it yesterday. Our A/C now operates like it should, turning on an off as you slide the temperature adjustment up and down, kicking on, right when it was set below the ambient temperature.

The new thermostat is branded "Dometic" where the old one was "Duo-Therm, by Dometic" Otherwise the casing is exactly the same besides just looking like a purer white that the old one. Internally there actually is a very significant contrast, the newer version seemingly much simpler, perhaps it is a solid state board or something.

In any event it works now so thanks for the help troubleshooting!

[Bama Rambler]

Thanks for coming back and letting us know what you found and how you fixed it. I'm glad that a new thermostat was all that was called for.