Robertshaw mechanical thermostat

When I thought this was a mercury switch, before I read it a second time, I thought it likely a wire to the swtich had broken where it flexed. Even good wire could break after 45 years. But a reed switch changes the odds towards a failed switch. If you didn't do it nicely like thixs, you absolutely should turn the AC on manually in some way.

What happens if you use measure the voltage across the reed swithc when the temp and setting calls for AC. It should be zero or teeny-weeny if the switch is good. But 12v if the switch is no good at all. Rotate the setting to as low as it goes and make sure the vom registers the same.

Or short the right two contacts of the switch with a screwdriver blade and a) see if ther'e s a spark, b) if the AC goes on.

YOu coudl run aq couple wires behind the switch across the AC terminals with a toggle or slide switch at the end, wnad wne it doesn't go on when it should, you could turn it on manaully and be sure that the AC is really working.

Bear in mind that my oil furnace takes a minute or two or more to turn on aftr it's powered up, and even then, it's only the burner that starts right away, the ciruclation fan doesn't start until the burner gets hot enough (and that fan continues after the burner turns off. The first half of this effect is probably less if it's already powered on and the thermostat sets it on, .

But I don't remember or never knew if AC starts promptly when the thermostat tells it to.

FWIW

Thanks very much for the reply. Yes, my conclusion was that while a mercury bulb could last forever, a reed switch is going to wear out eventually. From contact pitting I would think, or perhaps an arm would break.

I haven't done that yet, but will give it a try if I can get the front cover off - it's not clear how that's supposed to happen. Probably just a friction fit.

Yes, the blower motor starts immediately when the thermostat switches. I really don't think there's a problem with the AC itself.

Yeah, but if I'm going to replace it, I think I'll just get a "modern" one at Home Depot for less money. I don't need it to be programmable, but I would like to be able to adjust the hysteresis.

I have a variety of Reed Switches for sale for classic arcade games, but I expect it would cost less to just replace the thermostat than buying and shipping a NOS reed switch...