Central AC Won't Respond to Thermostat

RandyR · #1 IP: 63.25.11.131 August 6th, 2005, 03:04 PM

HI, I have a 2.5 ton Ruud central unit (upstairs) which will not respond in any way to the thermostat: neither fan nor compressor-condensing unit will run or do anything. Unit is about 5 years old.
I have a Fluke meter and have accomplished some pretty serious auto repairs, but haven't much experience with central AC units. If someone could offer suggestions where to start looking, I might be able to find the problem.
I have checked the breakers and the outdoor disconnect for the condensing unit already.
Thanks

**Mr T** · #2 IP: 24.210.203.152 August 6th, 2005, 03:41 PM

Check the breakers for the indoor unit. THere may be a in site disconnect/fuse/switch near the indoor unit too. Check for any status LED's in your unit. If all checks out ok. Check doors for safety switches (especialy around your blower). Then check for voltage (about 24V) to your thermostat/coming from the transformer.

RandyR · #3 IP: 63.25.144.38 August 6th, 2005, 06:31 PM

HI; OK I will check the thermostat for 24v and get back to you.
When I removed the front door of the air-handling unit, I saw a circuit board; I hate those things; it's probably bad and costs $$$.
Anyway, more later, and thanks!

**Mr T** · #4 IP: 24.210.203.152 August 6th, 2005, 07:01 PM

Did you check your indoor breakers too? there will be 2 sets of breakers for your unit...indoor+outdoor. THe voltage at your thermostat will be AC voltage. Forgot to mention that earlier.

RandyR · #5 IP: 63.25.44.228 August 6th, 2005, 08:17 PM

Hi; OK, I have 24 VAC at the thermostat (the thermostat is a simple thing with a mercury switch, not a complex electronic, programmable (although I am planning to get 2 of those).
My unit has 2, 220VAC breakers in the electrical panel: one for the condensor unit (outdoor unit) and one for the blower (indoor) unit. Additionally, there is a disconnect at the outdoor unit: pull out the handle, and it is disconnected.
Next step?

**Mr T** · #6 IP: 24.210.203.152 August 6th, 2005, 09:48 PM

Pull out your manual and see if your unit utilizes error codes.
If not, pull out your manual and see if there is a schematic. Trace out your fan relay see if anything is getting to it.

RandyR · #7 IP: 63.25.144.149 August 7th, 2005, 12:34 PM

OK. this afternoon I verified: the outdoor (compressor-condenser unit) will run perfectly if I engage the starter plunger manually. I cleaned the dust and bugs out of it while I had the cover off. Saw the nice Copeland scroll compressor; everything else looked OK.
I tried jumpering the red-yellow and red-green control wires at the thermostat; nothing.
I checked the red-yello again for 24v, and, no voltage (with the Fluke meter in the correct range this time).
OK, the thermostat control circuit has lost its power supply; I assume it is located in the front of the blower cabinet; I will check it out tonight, after the attic cools off.
Update later, Randy

RandyR · #8 IP: 63.25.240.255 August 7th, 2005, 07:38 PM

Turns out there is a little switch, float operated, on the edge of the overflow pan the indoor unit sits in; this switch is supposed ot turn the unit off if the pan fills with water (condensate) before it ruins the ceiling. This switch was poorly installed, had come loose and was sitting there, tripped; hence, no A/C.
Simple problem; did not cost anything to get the unit going (Phew!), and I learned some things about my units.

Thanks Mr. T for the assistance. Remember: I am only an amateur. If you have a problem with a gas-turbine engine or piston engine of 2000-12000 horsepower, or a gas compressor, just let me know; those I know.

**Mr T** · #9 IP: 24.210.203.152 August 7th, 2005, 08:09 PM

Glad to hear it's fixed

Sorry, my engine is 13000 HP... but thanks anyways!

nuwave42 · #10 IP: 96.25.180.153 June 30th, 2009, 10:43 AM

Hello,

I have somewhat of a similar problem. I have a RUUD 2 ton A/C unit (upstairs). The compressor nor the fan will come on, so no a/c upstairs. When the thermostat is turned on to cool, the compressor will make a humming noise approximately every 30 - 45 secs, like it's trying to kick on. In my troubleshooting I have replaced the "contactor" and the "delay on timer switch" to no avail. I'm not an a/c person so I'm at the end of my limited expertise. I'm wondering now is there anything else that can be done? Is there any hope? My system specs are as follows: A/C Unit info: Model - UAKA-024JAZ, Volts - 208-230, Compressor: R.I.A. 12.2/12.2, HP watts 1/6. Compressor: copeland, model: ZR23K1-PVF-230, oil: 26oz .77L, LRA block 62.5. I hope that I provided enough info, and thanks in advance for any help you can provide.