Central A/C unit won't stop running

[AppMathGuy]

As of last night, our central A/C unit keeps running continuously. I've tried all thermostat settings/combinations that should stop it, but nothing works. The *only* action that stops it is tripping its circuit breaker.

I've tried
1. setting the thermostat in 'neutral' i.e. neither 'warm' nor 'cool'
2. setting the target temperature extremely high, near 90 degrees (the internal temperature was at 70 degrees, after a night of continuous cooling!)
3. setting the fan (to circulate air within the house) to 'auto'
4. setting the fan to 'on'

As noted earlier, none of these actions at the thermostat stops the air conditioner from working. I thought I understood the basic logic behind thermostats - either action 1 or 2 should have stopped the A/C ?

Help, please...

[hiltontech]

Did you try turning the t-stat off, if so did that work for you?

[DNT1]

I am not sure how adept you are at electrical, so the first thing is to be careful and not attempt anything that you are not comfortable with. Obviously 240 volts can kill you dead in about two seconds. Now that the disclaimer portion is over Turn off all power to the unit and take off the eletrical controls access panel. Take a real close look at the contactor. I am betting that we have had a contactor points meltdown event in other words the silly thing has melted together and cannot break the contacts to cut off the compressor and condensor fan. Post back your visual results of the contactor condition for further assistance. Note: leave the power and access panel off if the contactor looks good we will do a bit of testing at the thermostat wire connections, you do have a multimeter right?

[AppMathGuy]

I *think* I did. When I don't set the thermostat to either 'warm' or 'cool', that's shutting it off, right?

[AppMathGuy]

DNT1:
Will do, but it may take a while. Bear with me.

...and yes, I have a multimeter. Since I'm a devout coward :), and tend to triple-check things, I should be reasonably safe from frying myself with 240 volts. But - seriously - thanks for the warning.

[DNT1]

Could be a thermostat problem but they rarely malfunction in this way, you could try opening the thermostat cover and removing the red wire see if the unit shuts off then.

[AppMathGuy]

DNT1:
I think I just got religion...

I removed the thermostat cover, tried to blow off all the dust I could see, and rotated the temperature control a couple of times from 55 degrees to 95 degrees and back, on the off-chance that this might shake loose any contacts that were frozen shut. Set the thermostat to 'off', and the target temperature to 95 degrees, then went back downstairs and threw the circuit_breaker for the A/C unit to 'on', and voila - the air conditioner stayed off, like it was supposed to.

I set the thermostat to 'cool', slowly decreased the target temperature, and the A/C began working when I went below the room temperature. So I'm back in business, at least for now.

(You realize all this was the equivalent of a guy getting out of his car when the engine won't start, lifting the hood, kicking the tires and then trying to start it again, and it fires right up. Oh, well - if the problem shows up again, at least I have a couple of sensible suggestions to follow.)

Thanks, everybody, for your help. If you or your brood have any math problems you'd like solved, let me know...