Roof Air won't start without fan running

[BurbMan]

I have a 15k BTU roof a/c unit in our 2008 TT, I can't find the OEM paperwork so not sure what brand it is.

The unit seems to work well and cools fine, but won't cycle without the fan running. Here are the symptoms:

1. If I set the t-stat to "cool" with the fan on "auto", nothing happens.

2. If I turn the fan to "on" first, and then select "cool" the compressor starts right up.

3. If I turn the fan back to "auto" while the compressor is running, it will continue to run until the t-stat cycles it off. However, it will not re-start with the fan off.

4. If I leave the fan in the "on" position, the compressor will cycle normally as commanded by the t-stat.

5. Makes no difference if fan is set to the "hi" or "lo" position.

I did replace the OEM t-stat with a digital model years ago, and everything worked fine until this season. Any ideas on what the issue is here? I am thinking a relay is bad somewhere in the roof unit but I don't have any experience diagnosing these units.

[wa8yxm]

Passing by a 2nd time a thought hit me.....

POLARITY

It may be you have two same color wires reversed Here is why

If you have the POWER wire hooked to a FAN terminal on the T-Stat then with the FAN on (The fan is hooked to the POWER terminal) everything has power.. but with the fan off. No power save internal battery power to the comuter and display.

HOW TO FIGURE THAT OUT.. Well the diagrams for the A/C would help but a voltmeter is teh tool of the day.

[BurbMan]

:S :S :S :S :S :S

OK...so I changed the two AAA batteries in the thermostat and everything is working normally again...I never would have thought that a low battery condition would cause an odd problem like this.

Thanks to all for the great advice!

[BurbMan]

Thanks guys, I will replace the stat and report back. FWIW, it's NOT wired wrong, it has worked flawlessly for 9 years....

I do agree with the fan noise comment though....I'm a light sleeper and the a/c was waking me up at night when it cycled on, so leaving the fan on "lo" becomes white noise...

We're doing some driveway camping this week, our youngest is home visiting from Europe and we are short on space due to remodeling in progress. My concern was having the fan running 24/7 instead of cycling off and getting a rest. I usually leave the fan on "auto" during the day and then on "lo" at night to provide some white noise.

[MrWizard]

I agree the thermostat is failing

[wa8yxm]

Initally I suspected something else but reading the directions I agree the T-Stat, likely wired wrong.

But here is a suggestion: FORGET AUTO, always run fan on HIGH..

WHY: Well Roof top A/C's are often a bit.. er.. noisy, but your mind will swiftly filter out the constant drone of the high speed fan... Leaving you much "Quiter" at least you will think it is.

But cycling it on and off means your mind cycles its filters as well so you hear trhe noise more.

[dougrainer]

RVP(Coleman) and Dometic do not have any such relay. NOW, if you have a Carrier brand(orphan), they may have such a control system. But, odds are it is your aftermarket tstat, even tho it has worked for years. It may be defective. Doug

[DownTheAvenue]

I bet it is the thermostat! If you replaced the OEM with one of the battery household digital models, then they are cheap to replace. First check the batteries, although I doubt that is the problem.