Dometic air conditioner issue

[Robmawer]

I have an issue that just started with my Dometic A/C unit on our Sprinter travel trailer. As soon as the my rig is plugged in to a land line, the A/C unit starts. I climbed up on the roof and it seems the compressor is running and there was definitely heat coming from the unit (but no fan). The unit would run for about a minute and turn off. 3 or 4 minutes later, the cycle would start again. I went so far as to set the thermostat to the lowest position and the selector switch to furnace (to maybe trick the system into only turning on the furnace) and plug the rig land line in. No luck. Compressor kicked on then off then on then ......... I then raised the temperature and the heater kicked on (with the compressor still running). When I went to disconnect the landline (a 12 gage extension cord), the socket was noticeably warm indicating I was pulling some serious current.

Two things to note. 1) Everything worked perfectly about two months ago then the rig was sitting idle until now. 2) this all happened with the thermostat is in the off position and the fan is set to auto.

Not sure if this is a thermostat issue, a relay issue, or ????? I would appreciate and insight on what to trouble shoot. Right now, I've set the "air" breaker to off to protect the system.

[Bama Rambler]

If you have the square looking Single Zone LCD Thermostat, you can try initializing it. If that doesn't work you'll probably have to replace it.

• Make sure the Single Zone LCD thermostat is in the Off condition.
See page 4, “Quick Reference To Control Buttons”.
• Press the “+” button and, while holding it, also press and hold
the On/Off Mode button for three seconds. LCD will show ― ― .
Press the On/Off Mode button again to turn system off.
This completes the initialization.

[Toddupton]

Pull the red wire off the thermostat and see if it still does the same thing. If it does then most likely not the stat.

[russhd1997]

I would start at the thermostat and fault trace further if that's not the issue. The puzzling part is that you can't hear a fan running on the A/C unit. Any time that the compressor is running a fan should also be running to pull air through the condenser. The air flow through the condenser keeps the high pressure in check.

[Robmawer]

OK - I took Todd Uptons recommendation and removed the "red" wire (ID'd as 7.5v). When I turned the breaker back on, No change; same thing on top unit was running. I took a chance and removed the yellow wire from the thermostat (ID'd as Cool). As I removed it, I can hear a relay clicking in the "Stat". When I turned the breaker back on, silence. No A/C, pump, etc.

Is it same to assume, the problem is elsewhere (meaning not the thermostat)?

Also, does anyone see any issue with leaving the yellow (Cool) wire disconnected? We're going out in a few weeks for a weekend trip and short time RV service seems to be tougher to get here in So. Cal (meaning a 2-3 month waiting list for my local repair place). Seeing as we are into fall, the need for A/C is probably gone.

[Robmawer]

Thanks to everyone who responded to my problem so quickly

[Tiggerdad]

Did you try to spin the fan to see if it's locked up or stiff. Your AC will continue to short cycle until the fan comes on.

[Toddupton]

Originally Posted by Tiggerdad

Did you try to spin the fan to see if it's locked up or stiff. Your AC will continue to short cycle until the fan comes on.

It shouldn’t if the thermostat is not calling for it.

[mkekflyers]

Originally Posted by Bama Rambler

If you have the square looking Single Zone LCD Thermostat, you can try initializing it. If that doesn't work you'll probably have to replace it.

That cured some wired issues I had with one of mine. Kinda like a CNTRL+ALT+DEL on your PC...

Let us know if that works please...

[Tiggerdad]

Originally Posted by Toddupton

It shouldn’t if the thermostat is not calling for it.

While I agree there could be a number of issues and not related to whether the fan works or not BUT do we know whether or not the thermostat is calling for cooling? If it is, shouldn't it also call for the fan to run just prior to the compressor coming on?

I am confused why the compressor comes on when the OP plugs into AC. Is the thermostat on prior to plugging in?

Edit, I see where the OP did say the thermostat was off.

[Bama Rambler]

Apparently you have an older unit with an analog thermostat. The 7.5V terminal is just used to power the thermostat.

Dometic switches the negative wire in those installations, so removing the red wire only powered down the thermostat.

With the unit stopping when you pulled the yellow wire says that the relay in the thermostat is still supplying a ground to the yellow wire.

I suspect the thermostat is the problem.

[Robmawer]

Tiggerdad - Tried it both ways. Switch to off and furnace only ( with temp set at 50F). Unit still started as soon at it was plugged in).

[Robmawer]

Bama rambler. - thanks. That’s dropping my stress level a bit.

[Sealyn]

I had this happen and my problem was a stuck relay on the control board. It is located in the control box above the return air filter.

[Robmawer]

Sealyn - think i messed up my reply. What is the process for accessing the control box? Complete removal from the ceiling ?

[RnRushin]

Here is a single zone thermostat and control box
https://pantherrvproducts.com/dometi...furnace-white/

for the cost difference compared to thermostat alone might be worth it to replace both

[RnRushin]

The control box is most likely behind the filter. If you take the ceiling trim off is should be easy to find.

[whitewinghunter]

Like said above. I bet your control box is bad. Probably got water in it from the gasket needing snugged down.
It happened to me except my blower ran all the time, compressor would cycle on and off as needed but the blower always ran. Took old one off got the numbers off it called and ordered a new one. Installed and everything worked as should.

[Bama Rambler]

I'd think if a relay in the control box was stuck then removing the power to it wouldn't stop the compressor from running. However, if a relay in the thermostat is stuck, then removing power from it would stop the compressor.