Yet another home AC problem

Different place(IL this time), different problem:

Our Lux TX9100E thermostat looks like it's working fine, but the AC never turns on. Heat(gas) & fan both work. When you switch the thermostat to AC nothing happens. If you press the up/down temperature controls the display shows "OFF". I verified the breaker for the AC unit wasn't tripped, and reset it anyway. The wiring from the thermostat to the furnace is intact and not damaged.

What else can I test?

I think that you should check the relay & motor start capacitor on the compressor unit.

Good luck!

-- Pete

Check the disconnects at the units. There is a good chance they have fuses in them. Then check the voltage with a meter.

When you turn on the ac at the t'stat, and turn the temperature below the ambient, do you hear a little click from the t'stat?

I wonder if the reed switch (or whatever it is) is stuck.

In reply to paranoid_android74:

I'll check that out. Actually, I just remembered I have an identical thermostat on our other unit, so I'll swap them around and see if the problem follows.

Yes I believe it is a control issue. Tstat saying OFF in cool mode????

Your Tstat should have 4 wires, power, heat, cool, and fan.

Crossing power and heat will turn on heater

Crossing cool and power will turn on AC

Crossing fan and power turns on manual fan. (fan will run automatic from the heat or cool functions)

Thanks for the tips everyone. It turned out being an odd, yet simple problem...

When I swapped thermostats, the problem moved with the unit. The batteries looked original, and even though I didn't think they would cause this problem, I swapped them anyway. Still no luck.

In the mean time I connected the "good" thermostat onto the other system, and after a few minutes it started cooling down[Yay!]. While messing with the "bad" thermostat, I happened to wiggle it a bit, and the display flashed "Cool" as though it was going to work. Pressing on it firmly did, in fact, get it to turn on the AC and start working.

After both systems completed their cycle I swapped thermostats around again, and experienced the same symptoms when putting the "bad" unit back on it's original system. It seems there's a loose connection/bad solder joint causing the problem.

I've not pulled it back out yet to check the solder connections on the board - I needed to cool the place down first - but at this point if it's not something obvious I'll just replace it.