A/C runs "forever".

[Needaclue]

I'm thinking I have a bad thermostat. Can you confirm?

I have a 90s vintage Robertshaw digital programable thermostat. I have the A/C temp set at 78 deg. The A/C will come on and stay on for what seems like forever - up to 3 hours. If I let it go it will take the room temp down to 72-73 degrees before it shuts off. The strange thing is the temp indication on the display will rise up to 80 deg then slowly fall back to 77 deg before it shuts off.

Using two indoor outdoor thermometers, I taped the probe of one one inch below the case of the thermostat. I inserted the other probe inside the thermostat case. Sure enough, when the A/C turned on the temp inside the thermostat immediately started rising and stabilized at 81 deg. The temp on the outside of the case slowly fell to 73 deg. It appears something is creating heat in the thermostat making the A/C stay on.

I blew it out with compressed air and adjusted the "temperature variance thing" with no luck. Bad t-stat? What do you think?

[Hube]

Make sure the wall cavity area that the T stat is mounted on is well insulated, so it is not influenced by any air from above or below.

[Needaclue]

Couldn't get into the wall to see how well its insulated so I pulled the thermostat away from the wall and redid the test. Same result. Temp inside thermostat started rising when A/C came on.

[Hube]

So now that you are sure that the T stat is not picking up heat from the wall cavity means it is definitely a fault in the T stat.
Since you say it is a 90's "vintage" t stat, Maybe its days of service are over.(worn out)
Best to just bite the bullet and spring for an newer one.

[HayZee518]

inside the stat is a small resistor which is heating up creating the inside temperature difference. called the heat anticipator it is used to anticipate the rise or fall of temperature as the setpoint is reached. try and see if the setting matches the millivolt current on the circuit it's used in.

[Needaclue]

Sorry. Thought I answered this long ago. Bought a new T-stat and all is well now. Got one of those fancy touch pad type (Lux). Working great. Thanks for the input.